The Basic Principles of Rolling and Pass Design

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Principles of Plasma Discharges and Materials Processing第3章

CHAPTER3ATOMIC COLLISIONS3.1BASIC CONCEPTSWhen two particles collide,various phenomena may occur.As examples,one or both particles may change their momentum or their energy,neutral particles can become ionized,and ionized particles can become neutral.We introduce the funda-mentals of collisions between electrons,positive ions,and gas atoms in this chapter, concentrating on simple classical estimates of the important processes in noble gas discharges such as argon.For electrons colliding with atoms,the main processes are elastic scattering in which primarily the electron momentum is changed,and inelas-tic processes such as excitation and ionization.For ions colliding with atoms,the main processes are elastic scattering in which momentum and energy are exchanged, and resonant charge transfer.Other important processes occur in molecular gases. These include dissociation,dissociative recombination,processes involving negative ions,such as attachment,detachment,and positive–negative ion charge transfer,and processes involving excitation of molecular vibrations and rotations. We defer consideration of collisions in molecular gases to Chapter8.Elastic and Inelastic CollisionsCollisions conserve momentum and energy:the total momentum and energy of the colliding particles after collision are equal to that before collision.Electrons and fully stripped ions possess only kinetic energy.Atoms and partially stripped ions have internal energy level structures and can be excited,de-excited,or ionized, Principles of Plasma Discharges and Materials Processing,by M.A.Lieberman and A.J.Lichtenberg. ISBN0-471-72001-1Copyright#2005John Wiley&Sons,Inc.43corresponding to changes in potential energy.It is the total energy,which is the sum of the kinetic and potential energy,that is conserved in a collision.If the internal energies of the collision partners do not change,then the sum of kinetic energies is conserved and the collision is said to be elastic.Although the total kinetic energy is conserved,kinetic energy is generally exchanged between particles.If the sum of kinetic energies is not conserved,then the collision is inelas-tic.Most inelastic collisions involve excitation or ionization,such that the sumof kinetic energies after collision is less than that before collision.However,super-elastic collisions can occur in which an excited atom can be de-excited by acollision,increasing the sum of kinetic energies.Collision ParametersThe fundamental quantity that characterizes a collision is its cross section s(v R), where v R is the relative velocity between the particles before collision.To deﬁne this,we considerﬁrst the simplest situation shown in Figure3.1,in which aﬂux G¼n v of particles having mass m,density n,andﬁxed velocity v is incident on a half-space x.0of stationary,inﬁnitely massive“target”particles having density n g.In this case,v R¼v.Let d n be the number of incident particles per unit volume at x that undergo an“interaction”with the target particles within a differential distanced x,removing them from the incident beam.Clearly,d n is proportional to n,n g,and d x for infrequent collisions within d x.Hence we can writed n¼Às nn g d x(3:1:1)where the constant of proportionality s that has been introduced has units of area and is called the cross section for the interaction.The minus sign denotes removal from the beam.To deﬁne a cross section,the“interaction”must be speciﬁed,for example,ionization of the target particle,excitation of the incident particle to a given energy state,or scattering of the incident particle by an angle exceeding p=2.Multiplying(3.1.1)by v,weﬁnd a similar equation for theﬂux:d G¼Às G n g d x(3:1:2) FIGURE3.1.Aﬂux of incident particles collides with a population of target particles in the half-space x.0.44ATOMIC COLLISIONSFor a simple interpretation of s,let the incident and target particles be hard elastic spheres of radii a1and a2,and let the“interaction”be a collision between the spheres.In a distance d x there are n g d x targets within a unit area perpendicular to x.Draw a circle of radius a12¼a1þa2in the x¼const plane about each target.A collision occurs if the centers of the incident and target particles fall within this radius.Hence the fraction of the unit area for which a collision occurs is n g d x p a212.The fraction of incident particles that collide within d x is thend G G ¼d nn¼Àn g s d x(3:1:3)wheres¼p a212(3:1:4)is the hard sphere cross section.In this particular case,s is independent of v.Equation(3.1.2)is readily integrated to give the collidedﬂuxG(x)¼G0(1ÀeÀx=l)(3:1:5) with the uncollidedﬂux G0eÀx=l.The quantityl¼1n g s(3:1:6)is the mean free path or the decay of the beam,that is,the distance over which the uncollidedﬂux decreases to1=e of its initial value G0at x¼0.If the velocity of the beam is v,then the mean time between interactions ist¼lv(3:1:7)Its inverse is the interaction or collision frequencyn;tÀ1¼n g s v(3:1:8)and is the number of interactions per second that an incident particle has with the target particle population.We can also deﬁne the collision frequency per unit density,which is called the rate constantK¼s v(3:1:9)3.1BASIC CONCEPTS45and,trivially,from (3.1.8)and (3.1.9)n ¼Kn g(3:1:10)Differential Scattering Cross SectionLet us consider only those interactions that scatter the particles by u ¼908or more.For hard spheres,taking the angle of incidence equal to the angle of reﬂection,the 908collision occurs on the x ¼458diagonal (see Fig.3.2),therefore having a cross section s 90¼p a 2122,(3:1:11)which is a factor of two smaller than (3.1.4).Of course,multiple collisions at smaller angles (radii larger than a 12=ﬃﬃﬃ2p )also eventually scatter incident particles through 908.This indeterminacy indicates that a more precise way of determining the scat-tering cross section is required.For this purpose we introduce a differential scatter-ing cross section I (v ,u ).Consider a beam of particles incident on a scattering center (again assumed ﬁxed),as shown in Figure 3.3.We assume that the scattering force is symmetric about the line joining the centers of the two particles.A particle incident at a distance b off-center from the target particle is scattered through an angle u ,as shown in Figure 3.3.The quantity b is the impact parameter and u is the scattering angle (see also Fig.3.2).Now,ﬂux conservation requires that for incoming ﬂux G ,G 2p b d b ¼ÀG I (v ,u )2p sin u d u (3:1:12)FIGURE 3.2.Hard-sphere scattering.46ATOMIC COLLISIONS3.1BASIC CONCEPTS47FIGURE3.3.Deﬁnition of the differential scattering cross section.that is,that all particles entering through the differential annulus2p b d b leave through a differential solid angle d V¼2p sin u d u.The minus sign is because an increase in b leads to a decrease in u.The proportionality constant is just I(v,u), which has the dimensions of area per steradian.From(3.1.12)we obtainI(v,u)¼bsin ud bd u(3:1:13)The quantity d b=d u is determined from the scattering force,and the absolute value is used since d b=d u is negative.We will calculate I(v,u)for various potentials in Section3.2.We can calculate the total scattering cross section s sc by integrating I over the solid angles sc¼2p ðpI(v,u)sin u d u(3:1:14)It is clear that s sc¼s for scattering through any angle,as deﬁned in(3.1.2).It is often useful to deﬁne a different cross sections m¼2p ðp(1Àcos u)I(v,u)sin u d u(3:1:15)The factor(1Àcos u)is the fraction of the initial momentum m v lost by the incident particle,and thus(3.1.15)is the momentum transfer cross section.It is s m that is appropriate for calculating the frictional drag in the force equation(2.3.9).For asingle velocity,we would just have n m¼s m v,where s m is generally a function of velocity.In the macroscopic force equation(2.3.15),n m must be obtained by aver-aging over the particle velocity distributions,which we do in Section3.5.We illustrate the use of the differential scattering cross section to calculate thetotal scattering and momentum transfer cross sections for the hard-sphere modelshown in Figure3.2.The impact parameter is b¼a12sin x,and differentiating, d b¼a12cos x d x,so thatb d b¼a212sin x cos x d x¼12a212sin2x d x(3:1:16)From Figure3.2the scattering angle u¼pÀ2x,such that(3.1.16)can be written asb d b¼À1a212sin u d u(3:1:17)48ATOMIC COLLISIONSSubstituting(3.1.17)into(3.1.13),we haveI(v,u)¼14a212(3:1:18)Using the deﬁnitions of s sc and s m in(3.1.14)and(3.1.15),respectively,weﬁnds sc¼s m¼p a212(3:1:19) for hard-sphere collisions.In general,s sc=s m for other scattering forces.For electron collisions with atoms the electron radius is negligible compared to the atomic radius so that a12%a,the atomic radius.Although the value of a% 10À8cm gives s sc¼s m%3Â10À16cm2,which is reasonable,it does not capture the scaling of the cross section with speed.In the following sections of this chapter,we consider collisional processes in more detail.Except for Coulomb collisions,we conﬁne our attention to electron–atom and ion–atom processes.After a discussion of collision dynamics in Section3.2,we describe elastic collisions in Section3.3and inelastic collisions in Section3.4.We reserve a discussion of some aspects of inelastic collisions until Chapter8,in which a more complete range of atomic and molecular processes is considered.In Section3.5,we describe the averaging over particle velocity distri-butions that must be done to obtain the collisional rate constants.Experimental values for argon are also given in Section3.5;these are needed for discussing energy transfer and diffusive processes in the succeeding chapters.A more detailed account of collisional processes,together with many results of experimental measurements,can be found in McDaniel(1989),McDaniel et al.(1993),Massey et al.(1969–1974),Smirnov(1981),and Raizer(1991).3.2COLLISION DYNAMICSCenter-of-Mass CoordinatesIn a collision between projectile and target particles there is recoil of the target as well as deﬂection of the projectile.In fact,both may be moving,and,in the case of like-particle collisions,not distinguishable.To describe this more complicated state,a center-of-mass(CM)coordinate system can be introduced in which projec-tiles and targets are treated equally.Without loss of generality,we can transform to a coordinate system in which one of the particles is stationary before the collision. Hence,we consider a general collision in the laboratory frame between two particles having mass m1and m2,position r1and r2,velocity v1and v2;0,and scattering angle u1and u2,as shown in Figure3.4a.We assume that the force F acts along the line joining the centers of the particles,with F12¼ÀF21.3.2COLLISION DYNAMICS49The center-of-mass coordinates may be deﬁned by the linear transformationR ¼m 1r 1þm 2r 2m 1þm 2(3:2:1)andr ¼r 1Àr 2(3:2:2)with the accompanying CM velocityV ¼m 1v 1þm 2v 2m 1þm 2(3:2:3)and the relative velocityv R ¼v 1Àv 2(3:2:4)v 2´m 1m R center(a )(b )FIGURE 3.4.The relation between the scattering angles in (a )the laboratory system and (b )the center-of-mass (CM)system.50ATOMIC COLLISIONSThe force equations for the two particles are:m1_v1¼F12(r),m2_v2¼F21(r)¼ÀF12(r)(3:2:5) Adding these equations we get the result for the CM motion that_V¼0,such that the CM moves with constant velocity throughout the collision.Now dividing theﬁrst of (3.2.5)by m1and the second by m2,and using the deﬁnition in(3.2.4)we havem R_v R¼F12(r)(3:2:6) which is the equation of motion of a“ﬁctitious”particle with a reduced massm R¼m1m2m1þm2(3:2:7)in aﬁxed central force F12(r).Theﬁctitious particle has mass m R,position r(t), velocity v R(t),and scattering angle Q,as shown in Figure3.4b.This result holds for any central force,including the hard-sphere,Coulomb,and polarization forces that we subsequently consider.If(3.2.6)can be solved to obtain the motion,includ-ing Q,then we can transform back to the laboratory frame to get the actual scattering angles u1and u2.It is easy to show from momentum conservation(Problem3.2)thattan u1¼sin Q(m1=m2)(v R=v0R)þcos Q(3:2:8a)andtan u2¼sin Qv R=v0RÀcos Q(3:2:8b)where v R and v0R are the speeds in the CM system before and after the collision, respectively.For an elastic collision,the scattering force can be written as the gradient of a potential that vanishes as r¼j r j!1:F12¼Àr U(r)(3:2:9) It follows that the kinetic energy of the particle is conserved for the collision in the CM system.Hence v0R¼v R,and we obtain from(3.2.8)thattan u1¼sin Q1=m2þcos Q(3:2:10)3.2COLLISION DYNAMICS51and,using the double-angle formula for the tangent,u2¼1(pÀQ)(3:2:11) For electron collisions with ions or neutrals,m1=m2(1and we obtain m R%m1 and u1%Q.For collision of a particle with an equal mass target,m1¼m2,we obtain m R¼m1=2and u1¼Q=2.Hence for hard-sphere elastic collisions against an initially stationary equal mass target,the maximum scattering angle is908.Since the same particles are scattered into the differential solid angle 2p sin Q d Q in the CM system as are scattered into the corresponding solid angle 2p sin u1d u1in the laboratory system,the differential scattering cross sections are related byI(v R,Q)2p sin Q d Q¼I(v R,u1)2p sin u1d u1(3:2:12)where d Q=d u1can be found by differentiating(3.2.10).Energy TransferElastic collisions can be an important energy transfer process in gas discharges,and can also be important for understanding inelastic collision processes such as ioniz-ation,as we will see in Section3.4.For the elastic collision of a projectile of mass m1 and velocity v1with a stationary target of mass m2,the conservation of momentum along and perpendicular to v1and the conservation of energy can be written in the laboratory system asm1v1¼m1v01cos u1þm2v02cos u2(3:2:13)0¼m1v01sin u1Àm2v02sin u2(3:2:14)1 2m1v21¼12m1v012þ12m2v022(3:2:15)where the primes denote the values after the collision.We can eliminate v01and u1 and solve(3.2.13)–(3.2.15)to obtain1 2m2v022¼12m1v214m1m2(m1þm2)2cos2u2(3:2:16)Since the initial energy of the projectile is12m1v21and the energy gained bythe target is12m2v022,the fraction of energy lost by the projectile in the laboratory52ATOMIC COLLISIONSsystem isz L¼4m1m2(m12)cos2u2(3:2:17) Using(3.2.11)in(3.2.17),we obtainz L¼2m1m2(m1þm2)2(1Àcos Q)(3:2:18)where Q is the scattering angle in the CM system.We average over the differential scattering cross section to obtain the average loss:k z L l Q¼2m1m2(m1þm2)2Ð(1Àcos Q)I(v R,Q)2p sin Q d Q ÐI(v R,Q)2p sin Q d Q¼2m1m2 (m1þm2)2s ms sc(3:2:19)where s sc and s m are deﬁned in(3.1.14)and(3.1.15).For hard-sphere scattering of electrons against atoms,we have m1¼m(electron mass)and m2¼M(atom mass),and s sc¼s m by(3.1.19),such that k z L l Q¼2m=M 10À4.Hence electrons transfer little energy due to elastic collisions with heavy particles,allowing T e)T i in a typical discharge.On the other hand,for m1¼m2,we obtain k z L l Q¼12,leading to strong elastic energy exchange among heavy particles and hence to a common temperature.Small Angle ScatteringIn the general case,(3.2.6)must be solved to determine the CM trajectory and the scattering angle Q.We outline this approach and give some results in Appendix A. Here we restrict attention to small-angle scattering(Q(1)for which the ﬁctitious particle moves with uniform velocity v R along a trajectory that is practi-cally unaltered from a straight line.In this case,we can calculate the transverse momentum impulse D p?delivered to the particle as it passes the center of force at r¼0and use this to determine Q.For a straight-line trajectory,as shown in Figure3.5,the particle distance from the center of force isr¼(b2þv2R t2)1=2(3:2:20)where b is the impact parameter and t is the time.We assume a central force of the form(3.2.9)withU(r)¼C(3:2:21)3.2COLLISION DYNAMICS53where i is an integer.The component of the force acting on the particle perpendicu-lar to the trajectory is (b =r )j d U =d r j .Hence the momentum impulse isD p ?¼ð1À1b r d U d r d t (3:2:22)Differentiating (3.2.20)to obtaind t ¼r v R d r(r 2Àb 2)1=2substituting into (3.2.22),and dividing by the incident momentum p k ¼m R v R ,we obtainQ ¼D p ?p k ¼2b m R v R ð1b d U d r d r (r 22)(3:2:23)The integral in (3.2.23)can be evaluated in closed form (Smirnov,1981,p.384)to obtainQ ¼AW R b (3:2:24)where W R ¼12m R v 2R is the CM energy andA ¼C ﬃﬃﬃﬃp p G ½(i þ1)=2 (3:2:25)FIGURE 3.5.Calculation of the differential scattering cross section for small-angle scattering.The center-of-mass trajectory is practically a straight line.54ATOMIC COLLISIONSwith G ,the Gamma function.ÃInverting (3.2.24),we obtainb ¼A W R Q1=i (3:2:26)and differentiating,we obtaind b ¼À1i A W R 1=i d Q Q (3:2:27)Substituting (3.2.26)and (3.2.27)into (3.1.13),with sin Q %Q ,we obtain the differ-ential scattering cross section for small angles:I (v R ,Q )¼1i A W R 2=i 1Q 2þ2=i (3:2:28)The variation of s ,n ,and K with v R are determined from (3.2.28)and the basic deﬁnitions in Section 3.1.If (3.2.28)is substituted into (3.1.14)or (3.1.15),then we see that a scattering potential U /r Ài leads to s /v À4=i R and n /K /v À(4=i )þ1R .These scalings are summarized in Table 3.1for the important scattering processes,which we describe in the next section.3.3ELASTIC SCATTERINGCoulomb CollisionsThe most straightforward elastic scattering process is a Coulomb collision between two charged particles q 1and q 2,representing an electron–electron,electron–ion,or ion–ion collision.The Coulomb potential is U (r )¼q 1q 2=4pe 0r such that i ¼1and TABLE 3.1.Scaling of Cross Section s ,Interaction Frequency n ,and Rate Constant K ,With Relative Velocity v R ,for VariousScattering Potentials UProcessU (r )s n or K Coulomb1/r 1/v R 41/v R 3Permanent dipole1/r 21/v R 21/v R Induced dipole1/r 41/v RConst Hard sphere 1/r i ,i !1Const v RÃG (l )¼(l À1)!¼l G (l À1)with G (1=2)¼ﬃﬃﬃﬃp p .3.3ELASTIC SCATTERING 55we obtainA¼C¼q1q2 4pe0from(3.2.25).Using this in(3.2.28),weﬁndI¼b0Q2(3:3:1)whereb0¼q1q240W R(3:3:2)is called the classical distance of closest approach.The differential scattering cross section can also be calculated exactly,which we do in Appendix A,obtaining the resultI¼b04sin(Q=2)2(3:3:3)However,due to the long range of the Coulomb forces,the integration of I oversmall Q(large b)leads to an inﬁnite scattering cross section and to an inﬁnitemomentum transfer cross section,such that an upper bound to b,b max,must beassigned.This is done by setting b max¼l De,the Debye shielding distance for a charge immersed in a plasma,which we calculated in Section2.4.For momentumtransfer,the dependence of s m on l De is logarithmic(Problem3.5),and the exact choice of b max(or Q min)makes little difference.For scattering,s sc pl2De, which is a very large cross section that depends sensitively on the choice of b max. However,we are generally not interested in scattering through very small angles, which do not appreciably affect the discharge properties.The cross section for scattering through a large angle,say Q!p=2,is of more interest.There are two processes that lead to a large scattering angle Q for a Coulombcollision:(1)a single collision scatters the particle by a large angle;(2)the cumu-lative effect of many small-angle collisions scatters the particle by a large angle.Thetwo processes are illustrated in Figure3.6;the latter process is diffusive and,as wewill see,dominates the former.To estimate the cross section s90(sgl)for a single large-angle collision,we inte-grate(3.3.3)over solid angles from p=2to p to obtain(Problem3.6)s90(sgl)¼14p b2(3:3:4)To estimate s90(cum)for the cumulative effect of many collisions to produce a p=2deﬂection,weﬁrst determine the mean square scattering angle k Q2l1for a 56ATOMIC COLLISIONSsingle collision by averaging Q 2over all permitted impact parameters.Since the col-lisions are predominantly small angle for Coulomb collisions,we can use (3.2.24),which is Q ¼b 0=b .Hencek Q 2l 1¼1p b 2max ðb max b min q 1q 24pe 0W R 22p b d b b 2(3:3:5)The integration has a logarithmic singularity at both b ¼0and b ¼1,which is cut off by the ﬁnite limits.The singularity at the lower limit is due to the small-angle approximation.Setting b min ¼b 0=2is found to approximate a more accurate calcu-lation.The upper limit,as already mentioned,is b max ¼l De .Using these values and integrating,we obtaink Q 2l 1¼2p b 20p b 2max ln L (3:3:6)where L ¼2l De =b 0)1.The number of collisions per second,each having a cross section of p b 2max orsmaller,is n g p b 2max v R ,where n g is the target particle density.Since the spreadingof the angle is diffusive,we can then writek Q 2l (t )¼k Q 2l 1n g p b 2max v R tSetting t ¼t 90at k Q 2l ¼(p =2)2and using (3.3.6),we obtain (see also Spitzer,1956,Chapter 5)n 90¼t À190¼n g v R 8p b 20lnLFIGURE 3.6.The processes that lead to large-angle Coulomb scattering:(a )single large-angle event;(b )cumulative effect of many small-angle events.3.3ELASTIC SCATTERING 57Writing n90¼n g s90v R,we see thats90¼8p b 2ln L(3:3:7)Although L is a large number,typically ln L%10for the types of plasmas we are considering.Comparing s90(sgl)to s90,we see that due to the large range of the Coulomb ﬁelds,the effective cross section for many small-angle collisions to produce a root mean square(rms)deﬂection of p=2is larger by a factor(32=p2)ln L. Because of this enhancement,it is possible for electron–ion or ion–ion particle col-lisions to play a role in weakly ionized plasmas(say one percent ionized).Another important characteristic of Coulomb collisions is the strong velocity dependence. From(3.3.2)we see that b0/1=v2R.Thus,from(3.3.4)or(3.3.7)s90/1v4R(3:3:8)such that low-velocity particles are preferentially scattered.The temperature of the species is therefore important in determining the relative importance of the various species in the collisional processes,as we shall see in subsequent sections.Polarization ScatteringThe main collisional processes in a weakly ionized plasma are between charged and neutral particles.For electrons at low energy and for ions scattering against neutrals, the dominant process is relatively short-range polarization scattering.At higher energies for electrons,the collision time is shorter and the atoms do not have time to polarize.In this case the scattering becomes more Coulomb-like,but with b max at an atomic radius,inelastic processes such as ionization become important as well.The condition for polarization scattering is v R.v at,where v at is the charac-teristic electron velocity in the atom,which we obtain in the next section.Because of the short range of the polarization potential,we need not be concerned with an upper limit for the integration over b,but the potential is more complicated.We determine the potential from a simple model of the atom as a point charge of valueþq0,sur-rounded by a uniform negative charge sphere(valence electrons)of total chargeÀq0,such that the charge density is r¼Àq0=43p a3,where a is the atomic radius.An incoming electron(or ion)can polarize the atom by repelling(or attracting) the charge cloud quasistatically.The balance of forces on the central point charge due to the displaced charge cloud and the incoming charged particle,taken to have charge q,is shown in Figure3.7,where the center of the charge cloud and the point charge are displaced by a distance d.Applying Gauss’law to a sphere 58ATOMIC COLLISIONSof radius d around the center of the cloud,4pe0d2E ind¼Àq0d3 awe obtain the induced electricﬁeld acting on the point charge due to the displaced cloudE ind¼Àq0d 4pe0a3The electricﬁeld acting on the point charge due to the incoming charge isE appl¼q 4pe0rFor force balance on the point charge,the sum of theﬁelds must vanish,yielding an induced dipole moment for the atom:p d¼q0d¼qa3r2(3:3:9)The induced dipole,in turn,exerts a force on the incoming charged particle:F¼2p d q4pe0r3^r¼2q2a34pe0r5^r(3:3:10)FIGURE3.7.Polarization of an atom by a point charge q.3.3ELASTIC SCATTERING59Integrating F with respect to r,we obtain the attractive potential energy:U(r)¼Àq2a38pe0r4(3:3:11)The polarizability for this simple atomic model is deﬁned as a p¼a3.The relative polarizabilities a R¼a p=a30,where a0is the Bohr radius,for some simple atoms and molecules are given in Table3.2.The orbits for scattering in the polarization potential are complicated(McDaniel, 1989).As shown in Figure3.8,there are two types of orbits.For impact parameter b.b L,the orbit has a hyperbolic character,and for b)b L,the straight-line trajec-tory analysis in Section3.2can be applied(Problem3.7).For b,b L,the incoming particle is“captured”and the orbit spirals into the core,leading to a large scattering angle.Either the incoming particle is“reﬂected”by the core and spirals out again,or the two particles strongly interact,leading to inelastic changes of state.The critical impact parameter b L can be determined from the conservation of energy and angular momentum for the incoming particle having mass m and speed v0,with the mass of the scatterer taken to be inﬁnite for ease of analysis.In cylindrical coordinates(see Fig.3.8a),we obtain1 2m v2¼12m(_r2þr2_f2)þU(r)(3:3:12a)m v0b¼mr2_f(3:3:12b)TABLE3.2.Relative Polarizabilities a R5a p/a03ofSome Atoms and Molecules,Where a0is the Bohr RadiusAtom or Molecule a RH 4.5C12.N7.5O 5.4Ar11.08CCl469.CF419.CO13.2CO217.5Cl231.H2O9.8NH314.8O210.6SF630.Source:Smirnov(1981).60ATOMIC COLLISIONSAt closest approach,_r¼0and r ¼r min .Substituting these into (3.3.12)and elimi-nating _f ,we obtain a quadratic equation for r 2min:v 20r 4min Àv 20b 2r 2min þa p q 240m¼0Using the quadratic formula to obtain the solution for r 2min ,we see that there is noreal solution for r 2min when(v 20b 2)2À4v 20a p q 20 0Choosing the equality at b ¼b L ,we solve for b L to obtains L ¼p b 2L ¼pa p q 2e 0 1=21v 0(3:3:13)which is known as the Langevin or capture cross section.If the target particle has a ﬁnite mass m 2and velocity v 2and the incoming particle has a mass m 1and velocity v 1,then (3.3.13)holds provided m is replaced by the reduced mass m R ¼m 1m 2=(m 1þm 2)and v 0is replaced by the relative velocity v R ¼j v 1Àv 2j .We (a )(b )FIGURE 3.8.Scattering in the polarization potential,showing (a )hyperbolic and (b )captured orbits.3.3ELASTIC SCATTERING 61。

aufbau principle英语解释

aufbau principle英语解释The Aufbau principle is a fundamental rule in chemistry that dictates the order in which electrons fill energy levels and sublevels in an atom. This principle helps us understand the electronic structure of atoms and predict their chemical behavior. In this document, we will explore the Aufbau principle in depth, discussing its origins, applications, and implications for chemical bonding.Origins of the Aufbau PrincipleThe Aufbau principle, which translates to "building up" in German, was first proposed by the German physicist Arnold Sommerfeld in the early 20th century. Sommerfeld's research on atomic structure and spectral lines led him to develop a theoretical framework for understanding the arrangement of electrons in atoms. The Aufbau principle was later refined and popularized by the Danish physicist Niels Bohr and the Austrian physicist Wolfgang Pauli.Basic Principles of the Aufbau PrincipleThe Aufbau principle is based on two important principles of quantum mechanics: the Pauli exclusion principle and Hund's rule. The Pauli exclusion principle states that no two electrons inan atom can have the same set of quantum numbers, which means that each electron must occupy a unique orbital within a sublevel. Hund's rule dictates that electrons fill orbitals of equal energy singly before pairing up, in order to minimize repulsion and stabilize the atom.The Aufbau principle can be summarized in three main points:1. Electrons fill the lowest energy levels and sublevels first before filling higher energy levels.2. Each sublevel has a maximum number of electrons it can hold, according to the formula 2n^2 (where n is the principal quantum number).3. Electrons fill orbitals within a sublevel according to Hund's rule, filling them singly before pairing up.Applications of the Aufbau PrincipleThe Aufbau principle has numerous applications in chemistry, particularly in predicting the electronic configuration of atoms and ions. By following the Aufbau principle, we can determine the distribution of electrons in an atom's energy levels and sublevels, which in turn influences the atom's chemical properties and reactivity.One key application of the Aufbau principle is the construction of electron configurations for the elements in the periodic table. By arranging electrons in the order of increasing energy levels and sublevels, we can predict the properties of elements and their chemical behavior. This information is crucial for understanding trends in the periodic table and explaining the similarities and differences between elements.Implications for Chemical BondingThe Aufbau principle also plays a significant role in understanding chemical bonding and molecular structure. The distribution of electrons in an atom's orbitals affects its ability to form bonds with other atoms and participate in chemical reactions.For example, the electron configuration of an atom determines its valence electrons, which are the electrons involved in bonding. Elements with similar electron configurations tend to exhibit similar chemical behavior and form similar types of bonds. This is why elements in the same group of the periodic table have similar properties and tend to form similar compounds.In summary, the Aufbau principle is a fundamental concept in chemistry that governs the arrangement of electrons in atomsand influences their chemical properties. By understanding the principles of electron filling order, we can predict the behavior of atoms and molecules and make informed decisions about chemical reactions and bonding. The Aufbau principle continues to be a cornerstone of modern chemistry and a key tool for scientists in their study of the microscopic world.。

密立根油滴实验数据处理

密立根油滴实验数据处理密立根油滴实验数据处理罗泽海摘要：本文主要讨论了大学物理实验中的密立根油滴实验数据处理。

其中主要讲解了MOD-8型密立根油滴实验仪的使用及其实验实验事项、密立根油滴实验的基本原理，重点介绍密立根油滴实验平衡测量的数据处理，实验数据处理过程由的数值计算和图形绘制来实现，通过运用microsoft excel图表对数据处理，计算出电荷e的实验值幷与理论值进行比较，作出实验误差小结个人预见。

关键词：油滴实验数据处理个人预见Dense grain root oil drops experimental data processingLuozehaiAbstract: This paper discusses the physics experiment Millikan oil drop experiment data proce-ssing. Mainly explained MOD-8 type Millikan oil drop experiment and the experiment using the experimental instrument matters, Millikan oil drop experiment of the basic principles, focusing on balance Millikan oil drop experiment measurement data processing, data processing process from the numerical computation and graphics rendering to achieve, through the use of microsoft excel chart of data processing to calculate the charge e of the experimental data are compared with thetheoretical value Bing, individuals predicted to experimental error summary.Key words：Oil Drop Experiment；Data Processing;Individual predicted目录第1章绪论电荷有两个基本特征：一是遵循守恒定律；二是具有量子性。

(2021年整理)研究生机械工程专业英语考试必背单词

(完整版)研究生机械工程专业英语考试必背单词编辑整理：尊敬的读者朋友们：这里是精品文档编辑中心，本文档内容是由我和我的同事精心编辑整理后发布的，发布之前我们对文中内容进行仔细校对，但是难免会有疏漏的地方，但是任然希望（(完整版)研究生机械工程专业英语考试必背单词）的内容能够给您的工作和学习带来便利。

同时也真诚的希望收到您的建议和反馈，这将是我们进步的源泉，前进的动力。

本文可编辑可修改，如果觉得对您有帮助请收藏以便随时查阅，最后祝您生活愉快业绩进步，以下为(完整版)研究生机械工程专业英语考试必背单词的全部内容。

（完整版)研究生机械工程专业英语考试必背单词编辑整理:张嬗雒老师尊敬的读者朋友们：这里是精品文档编辑中心，本文档内容是由我和我的同事精心编辑整理后发布到文库，发布之前我们对文中内容进行仔细校对，但是难免会有疏漏的地方，但是我们任然希望 (完整版）研究生机械工程专业英语考试必背单词这篇文档能够给您的工作和学习带来便利.同时我们也真诚的希望收到您的建议和反馈到下面的留言区，这将是我们进步的源泉,前进的动力。

本文可编辑可修改,如果觉得对您有帮助请下载收藏以便随时查阅,最后祝您生活愉快业绩进步，以下为 <(完整版)研究生机械工程专业英语考试必背单词> 这篇文档的全部内容。

单词Lesson 1Gear 齿轮, 传动装置Bearing 轴承Cams 凸轮Cams and followers 凸轮和从动件Couple 力偶mechanics 力学statics 静力学,静止状态dynamics动力学,原动力，动力特性constraint forces 约束力applied forces 作用力Electric , magnetic, and gravitational forces 电,磁，重力mating surface 啮合表面,配合表面,接触面meshing 啮合，咬合，钩住meshing teeth 啮合齿journal bearing 滑动轴承，向心滑动轴承metal-to-metal contact 金属 - 金属接触Overheating 过热failure 失效flaking 薄片，表面剥落，压碎Spall 削，割，剥落,脱皮noise 噪音rough motion运动不精确inertia惯性particle 质点rigid body刚体deformable可变形的，应变的deformable Body 变形体Scalar 数量的，标量的Vectors矢量Density密度Mass质量Displacement位移Velocity速度Acceleration加速度Moment力矩，弯矩Momentum动量，冲量Lesson 2 Compressive压缩的，有压力的Turning 车削Rectilinear直线的micrometer 千分尺又称螺旋测微器Power hacksaws 弓锯床Shaper牛头刨床Thread 螺纹Work：功muscular action肌肉动作mechanical motion机械运动stretch an object拉伸对象tensile force：拉力in tension：受拉compressive force：压力torsional force:扭力torque：扭矩shearing force ：剪切力twist an object扭曲对象Slide滑,脱落Slip滑动，滑移in compression受压turning of a part对一个零件进行车削加工wedging action：楔入作用chip ：切屑centers of the lathe车床的顶尖lathe dog车床夹头centrifugal force ：离心力grinding wheel ：磨削砂轮bonding agent ：粘合剂abrasive particle：磨料颗粒centrifuge-type machines离心式机械Centrifuge离心机，离心作用Centrifugal force principles离心力原理centripetal force ：向心力rotary motion：回转运动rectilinear motion:直线运动hand tool手工工具power tool动力工具feed：进给shaping：采用牛头刨床（shaper）进行刨削加工power saw：弓锯床，弓式锯床the screw of a micrometer 意为“千分尺中的螺杆"harmonic and intermittent motion ：谐和运动和间歇运动simple harmonic motion ：简谐运动return stroke：快速回程shaper ram：刨床滑枕Pulley滑轮Screw螺丝钉Belt带Link链Lesson 3Interactive互相作用的Iterative重复的, 反复的, 迭代的Pinpoint 精确地定位或确认Equilibrium 平衡，均衡Tractable 易于处理或操作的Order of magnitude 数量级Ideally理想的情况下so as to为了with any precision很少精确idealize理想化idealization 理想化strength of materials材料力学Dynamics动力学Approximations近似值be inherent in为、、、所固有，是、、、的固有性质Render提出，给予，描绘degrading the result使结果降级pertinent有关prohibitive令人望而却步Influx流入,注入，涌进，汇集Lesson 5Sprocket链轮snap ring 卡环Universal joints万向联轴器Self-aligning bearing 调心轴承,球面轴承, 自位轴承Dry ice干冰Shot-peening喷丸硬化处理Pin销Key键Spline花键Couplings联轴器nondriving wheel非驱动轮idler gear空转齿轮,换向齿轮be subjected to承受Fluctuate变动,波动，起伏alternating bending stress交变弯曲应力deflections挠度lateral shaft deflection横向轴的挠度angular deflection角偏转non—self- aligning bearings非自调心轴承Torsional deflection扭转变形critical speed临界速度Attachment of the hub毂的附件Keyway键槽Axial轴向Circumferential圆周方向Positioning定位Retaining固定retaining ring定位环hub—to-shaft attachments轮毂与轴之间的连接interference fit过盈配合hub bore毂孔bending moment弯矩cold—rolling冷轧relative slope相对倾斜Journal轴颈plain bearing 滑动轴承Lesson 6Clutch 离合器Brake 制动器Friction 摩擦Chain 链，链条Timing belt 同步带Belt drive 带传动coefficient of friction摩擦系数rayon人造纤维timing belt同步带V—belt drive V带传动Foregoing在前的，前述的fatigue life疲劳寿命power transmitted电力传输rotatable shaft可以转动的轴，从动轴rotating shaft转动轴，主动轴input shaft输入轴output shaft输出轴unloaded state空载状态Rotor转子rotational inertia转动惯量torque capacity 扭矩容量kinetic and potential energy动能和势能provision 规定thermal capacity 热容量thermal stress热应力thermal distortion热变形rubbing velocity摩擦速度Lining内衬,衬套empirical value经验值Chain drives链传动gear drives齿轮传动speed ratio速比shaft separation distance轴间隔距离arbitrary center distance任意的中心距positive （no slip) drive强制（无滑动）传动synchronized motion同步运动conveyor systems, farm machinery, textile machinery传送带系统，农用机械，纺织机械chain loop链环40-kW power ratings ：40千瓦的额定功率Lesson 9Ceramic bearing 陶瓷轴承Silicon硅Titanium 钛Adherence 粘附，附着gas turbine engines 燃气涡轮发动机liquid lubricant液体润滑剂Exploit利用，发挥,使用Tribological 摩擦学的ceramic rolling bearing陶瓷滚动轴承thermo-mechanical热机械Tool steel工具钢Aeroengine航空发动机practical temperature limit 实际的温度上限virtual exclusion虚拟排斥hot pressed 热压hot isostatically pressed 热等静压的silicon nitride Si3N4rolling contact fatigue滚动接触疲劳low fracture toughness低的断裂韧性coefficient of thermal expansion热膨胀系数thermal conductivity导热系数thermal diffusivity热扩散系数，温度扩散率oxidation resistance抗氧化性Hertzian contact stresses 赫兹接触应力Solid lubricant固体润滑剂synthetic lubricant合成润滑剂unconventional lubricant非常规润滑剂boundary lubrication边界润滑wear resistance耐磨性tribo-chemical film摩擦化学膜Shear剪切，切断heat stable热稳定Imperative命令，绝对必要的，必不可少的Lesson 14Melting point熔点Specific heat比热Specific gravity比重Shrink fit 冷缩配合，收缩配合thermal conductivity热导率,导热率thermal expansion热膨胀corrosion resistance耐蚀性reduce inertial force减小惯性力Substitution 替换recrystallization temperature再结晶温度Annealing退火heat treating热处理hot working热加工minor 微小的surface roughness 表面粗糙度Metallurgical冶金学的Titanium钛thermal gradient热梯度relative expansion相对膨胀glass-to—metal seals玻璃—金属密封件Shrink fit冷缩配合，收缩配合Deterioration恶化，变质，退化Degradation降解，老化，退化petroleum 石油elevated temperature高温Alkalis碱oxygen， moisture, pollution， and acid rain氧气，湿气，污染和酸雨Nonferrous metals, stainless steels, and nonmetallic materials，有色金属,不锈钢，和非金属材料cast iron铸铁chromium铬protective film保护膜Lesson 28Basic size基本尺寸Deviation偏差Interchangeable互换性Interchangeability互换性Unilateral， bilateral, and limit forms。

courage

The Strongest EnemyI remember last time I took part in the English recitation contest, the whole process was loaded with tension, anxiety and stress. Especially in the final contest, all the worries of forgetting the words, not reciting fluently and emotively attacked me, which flourished the only thought of giving up. However, the last minute before I stepped onto the stage, I reassured myself with the belief of persistence and perseverance bearing in my mind and conquered the cowardice. I succeeded! I was the person who possess the precious courage.My dictionary defines courage as the ability to do something dangerous or face pain or opposition, which I quite disagree with. In my mind, people who are courageous own the ability to overcome themselves when encountering either physical or psychological difficulties. Undoubtedly, the strongest enemy is self, who may block our way, stick our mind and produce a lot of excuses for us to leave away. Only by conquering it can we show the real side and make continuous progress by reassessing ourselves on the ground.Courage no longer belongs to the great men, or the revolutionists who lead people to the new world in the cost of their lives, nor belongs to people who have experienceddangerous adventures at the edge of life and death. Courage can be seen everywhere and everytime, for example, when a baby starts to learn to walk and cautiously explores his/her steps, when the kids learn to ride a bike and repetitiously practise after many times’ falling and tumbling off the bike, when a teenager shows her first piano performance or makes a speech in front of many audience with tension and excitement rolling in her innocent heart, or when an adult makes his/her attempt to pursure the harmony between family and work for fear of losing one of them.Someone may doubt that if people dare to rob a bank or commit suicide, can we define them as courageous people? Maybe the spirit that supports their behaviors is something relating to courage, but they have not used it in a proper way or even have wasted the flow of brave emotions in their mind. On the other hand, courage is judged by the basic principles and justice of the society and common pratice of human beings. Thus, when trapped in troubles, what we need to do is take courage to face the reality and think positively about the measures for future actions instead of escaping from the truth and solving the problems improperly and illegally to show how brave we are!An ancient Greek dramatist once said, “ we know the good, we apprehend it clearly, but we can’t bring it to achievement. To persevere, trusting in what hopes he has, is courage in a man.”That’s true; courage is the capacity to trust hopes together with the premise of overcoming the negative soul and developing the positive. That’s why life can be whatever we want it to be, just depending on the view we take.Life can never be perfect. It has weak sides. However, whatever is going on, just believe the truth, take courage, face the reality and then what comes in the end is always the eternal beauty.。

基本原理英文

Basic PrinciplesIntroductionIn this document, we will explore the fundamental principles that form the basis of various systems and technologies. Understanding these basic principles is crucial for gaining a deep insight into the functioning of different processes and mechanisms. By delving into these principles, we can lay a solid foundation for comprehending complex concepts and designing innovative solutions.Principles of PhysicsPhysics, the fundamental science that studies matter, energy, and their interactions, is built upon several key principles. These principles serve as the building blocks for understanding the behavior of the physical world.1.Newton’s Laws of Motion: Newton’s three laws of motion describethe relationship between the motion of an object and the forces acting upon it.They form the foundation of classical physics and are still widely used today.2.Principle of Conservation of Energy: This principle states thatenergy cannot be created or destroyed but can only be transformed from one form to another. It underlies various concepts such as work, potential andkinetic energy, and the law of conservation of energy.3.Principle of Conservation of Mass: The principle of conservation ofmass states that the total mass of a closed system remains constant over time, regardless of any physical or chemical changes that may occur within thesystem.Principles of ElectronicsElectronics deals with the study and application of electrical circuits and devices. The following principles form the backbone of electronic systems:1.Ohm’s Law: Ohm’s law states that the current flowing through aconductor is directly proportional to the voltage applied across it and inversely proportional to its resistance. This law is essential for understanding andanalyzing circuit behavior.2.Kirchhoff’s Laws: Kirchhoff’s laws are fundamental principles usedto analyze electrical circuits. They include the Kirchhoff’s Voltage Law (KVL), which states that the sum of voltages around any closed loop in a circuit is zero, and the Kirchhoff’s Current Law (KCL), which states that the sum of currents entering a junction in a circuit is equal to the sum of currents leaving thejunction.3.Semiconductor Principles: The behavior of semiconductor materialsforms the basis of modern electronics. Principles such as P-N junctions,transistors, and diode characteristics are essential for understandingelectronic devices and circuits.Principles of CommunicationCommunication systems play a vital role in connecting people and transmitting information. The following principles underpin the design and operation of various communication technologies:1.Modulation: Modulation is the process of modifying a carrier signalto encode information. Principles such as amplitude modulation (AM),frequency modulation (FM), and phase modulation (PM) allow thetransmission of data through different signaling techniques.2.Signal-to-Noise Ratio (SNR): The SNR is a measure of the ratiobetween the power of a signal and the noise corrupting it. Maintaining a high SNR is crucial for ensuring reliable and efficient communication.3.Error Detection and Correction: Communication systems oftenemploy techniques such as parity checks and checksums to detect and correct errors introduced during transmission.Principles of Control SystemsControl systems are widely used in various applications to regulate and manage processes. Understanding the following principles is essential for designing and analyzing control systems:1.Feedback Control: Feedback control systems utilize feedback loopsto measure the output of a system, compare it to the desired setpoint, andadjust the system’s inputs accordingly. This principle enables the system tomaintain stability and achieve desired performance.2.Proportional-Integral-Derivative (PID) Control: PID control is awidely used control algorithm that adjusts system inputs based on theproportional, integral, and derivative terms. It provides excellent stability and responsiveness in a variety of control applications.3.System Modeling: Creating mathematical models of physical systemsallows engineers to analyze and design control systems effectively. Principles such as transfer functions and state-space modeling aid in understanding and predicting system behavior.ConclusionUnderstanding the basic principles underlying various systems and technologies provides a solid foundation for further exploration and application. Whether it be physics, electronics, communication, or control systems, these principles form the backbone of their respective fields. By grasping these principles, we can gain a deeper insight into the mechanisms at play and use this knowledge to innovate and solve complex problems.。

新世纪大学英语综合教程第二版第一册全套教材

in general
for the most part; commonly, usually 一般说来
anticipation n.
the feeling one has when expecting sth. pleasant to happen 期望；预期
discovery n.
a fact or thing that has been discovered 被发现的事实、事物
happening or existing before the one mentioned 在先的，在前的，以前的
patience n.
[U] the ability to accept pain, trouble, anything that causes annoyance, without complaining or losing one’s self-control 忍痛的能力，忍受力，耐性
diverse a.
different (from each other); showing variety 各不相同的，各种各样的
relationship n.
[(between, with)] a friendship or connection between people [常与between 或with连用]友谊；关系
— W. B. Yeats
Click Picture
Interpretation: Instead of stuffing students with knowledge, real education aims at bringing out what is inside students and cultivating their potential.

motion

motionMotion: Understanding the Basics and ApplicationsIntroductionMotion is a fundamental concept in various fields, including physics, engineering, and computer science. At its essence, motion refers to the movement of an object or a system of objects in relation to a frame of reference. This document aims to provide an overview of the basic principles of motion and explore its applications in different domains.I. Newtonian Mechanics and MotionMotion, as understood in classical physics, is primarily governed by the laws of motion formulated by Sir Isaac Newton. These laws describe how the motion of an object changes in response to forces acting upon it. The three laws of motion can be summarized as follows:1. Law of Inertia: An object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and direction unless acted upon by an external force.2. Law of Acceleration: The acceleration of an object is directly proportional to the net force exerted on it and inversely proportional to its mass.3. Law of Action and Reaction: For every action, there is an equal and opposite reaction.Understanding these laws is crucial for predicting and analyzing the motion of objects in a wide range of scenarios, from everyday movements to celestial mechanics.II. Types of MotionMotion can be classified into various types based on different criteria. Some common types of motion include:1. Linear Motion: This type of motion refers to the movement of an object along a straight line. Examples include themotion of a car on a straight road or a ball rolling down a slope.2. Circular Motion: Circular motion involves the movement of an object along a circular path. A simple example is the motion of a satellite orbiting the Earth.3. Oscillatory Motion: Oscillatory motion is characterized by repeated back-and-forth or to-and-fro motion about a fixed point or equilibrium position. Simple pendulums and springs undergoing compression and expansion are examples of oscillatory motion.4. Rotational Motion: Rotational motion occurs when an object spins or rotates about a fixed axis. The spinning of a top or a spinning wheel are examples of rotational motion.5. Projectile Motion: Projectile motion describes the motion of an object launched into the air at an angle, following a curved trajectory. A perfect example is the motion of a thrown ball.III. Applications of MotionUnderstanding motion has significant practical applications across multiple fields. Some notable applications include:1. Robotics: Motion planning and control are crucial in the field of robotics. Robots need to precisely navigate their surroundings and perform tasks, such as assembly or movement, based on predefined algorithms.2. Animation and Gaming: The realistic animation of characters and objects in movies, video games, and virtual reality environments heavily relies on understanding and replicating natural motion. Accurate physics-based simulations are key to providing immersive virtual experiences.3. Transportation: The study of motion plays a vital role in the design, analysis, and optimization of transportation systems. Understanding the principles of motion helps engineers develop efficient vehicles and transportation networks, leading to improved safety and reduced energy consumption.4. Sports Science: Analyzing and enhancing human motion in sports is essential for improving performance, preventing injuries, and optimizing training programs. Motion capture technology allows coaches and athletes to analyzebiomechanics and technique, leading to better results on the field.ConclusionMotion is a fundamental concept across various disciplines and has a wide range of applications. Understanding the principles of motion, as described by Newtonian mechanics, enables us to predict and manipulate the movements of objects and systems. From robotics to animation, transportation to sports science, motion plays a vital role in advancing technology and improving our understanding of the world around us.。

面食的原理其实很简单英文

面食的原理其实很简单英文Here is an explanation of the basic principles behind making noodles in simple English:The ingredients for noodles are quite simple - mainly flour and water. The key is the ratio of flour to water.More flour makes the dough less sticky and easier to handle. More water makes the dough soft and stretchy. Finding the right ratio is important for good noodle texture.Kneading the dough mixes the ingredients and allows the gluten in the flour to develop. Gluten gives noodles their chewy texture. Kneading makes the gluten strands link together.Resting the dough allows gluten to relax. This prevents noodles from becoming hard. A rest of 30 minutes is common.Rolling and folding stretches the dough. This aligns the gluten strands to make the texture smoother. Stretching also makes noodles loose their springiness.Cutting the dough into strands shapes the noodles. Thicker strands make flatter noodles like fettuccine. Thinner strands make long noodles like spaghetti.Cooking the noodles in hot water denatures the gluten. This sets the noodle structure. Cooking time affects noodle texture - al dente or fully soft.So in summary, good noodle texture relies on ingredient ratios, kneading, resting, stretching and controlled cooking. Varying these gives different types of noodle. The principles are quite simple.。

足球训练英语作文

足球训练英语作文Football is a sport that has captivated the hearts and minds of people around the world. For many, the thrill of the game, the camaraderie of the team, and the sense of accomplishment that comes with achieving victory are all part of what makes football such a beloved pastime. At the heart of this passion is the rigorous training that players undergo to hone their skills and prepare for the challenges of the game.The journey of a football player begins with the fundamentals of the sport. From the moment they step onto the pitch, players must learn the basic techniques of ball control, passing, shooting, and positioning. This foundation is essential for building the skills and instincts that will serve them well throughout their careers.One of the key aspects of football training is the development of physical fitness. Players must possess a high level of cardiovascular endurance, strength, and agility to compete at the highest levels of the game. This requires a comprehensive training regimen that incorporates a variety of exercises and drills.Cardio training is a crucial component of a footballer's fitness routine. This may involve running, sprinting, and interval training to improve the player's ability to sustain high-intensity efforts over the course of a match. Strength training, on the other hand, focuses on building the muscular power and explosiveness needed for tasks such as jumping, tackling, and accelerating.Agility drills are another important aspect of football training. These exercises are designed to improve a player's ability to change direction quickly, react to sudden movements, and maintain balance and control in tight spaces. Examples of agility drills include ladder drills, cone drills, and plyometric exercises.In addition to physical conditioning, football players must also develop their technical skills. This involves extensive practice with the ball, focusing on areas such as first touch, passing, shooting, and dribbling. Players may engage in individual drills, small-sided games, and scrimmages to hone their technical abilities and improve their decision-making under pressure.One of the most challenging aspects of football training is the development of tactical awareness. Players must learn to read the game, anticipate the movements of their teammates and opponents, and make split-second decisions that contribute to the team's overallstrategy. This requires a deep understanding of the principles of play, as well as the ability to adapt to changing game situations.Tactical training may involve video analysis, where players review footage of their own performances and those of their opponents to identify strengths, weaknesses, and areas for improvement. Coaches may also use tactical exercises and small-sided games to challenge players' decision-making and problem-solving skills.Mental preparation is another crucial component of football training. Players must develop the mental toughness and resilience to handle the pressures and challenges of the game. This may involve techniques such as visualization, goal-setting, and self-talk, as well as the development of strong communication and leadership skills.Throughout the training process, players must also learn to work effectively as a team. This involves developing strong interpersonal skills, the ability to communicate effectively, and a willingness to put the team's goals ahead of individual interests. Teamwork is essential for success in football, as the sport requires the coordinated efforts of all players on the pitch.Finally, it is important to recognize the role of recovery and rest in the training process. Football is a physically and mentally demanding sport, and players must take the time to recover, both physically andmentally, to prevent injuries and maintain peak performance. This may involve activities such as stretching, foam rolling, ice baths, and mindfulness practices.In conclusion, the training process for a football player is a multifaceted and challenging endeavor. It requires a dedication to physical fitness, technical skill development, tactical awareness, mental preparation, and teamwork. Through this rigorous training, players can unlock their full potential and contribute to the success of their team on the pitch. The journey of a football player is one of constant growth and improvement, and the rewards of this dedication can be seen in the excitement and joy of the game.。

介绍科学博物馆英语作文初中

介绍科学博物馆英语作文初中Science museums are a fascinating blend of education and entertainment, offering visitors a chance to explore the wonders of the natural world, scientific achievements, and technological innovations. In this essay, I will introduce the concept of a science museum and discuss its importance in fostering a love for science among students.The first thing to note about science museums is their interactive nature. Unlike traditional museums where exhibits are often behind glass and off-limits, science museums encourage hands-on learning. This approach is particularly effective for younger audiences, such as middle school students, who are naturally curious and eager to experiment.Upon entering a science museum, one might be greeted by a variety of exhibits. These can range from displays on the human body and its functions to exhibits on space exploration and the vastness of the universe. Interactive displays allow students to manipulate variables and see the effects firsthand, such as adjusting the angle of a ramp to observe changes in the trajectory of a rolling ball.Science museums also often feature live demonstrations and workshops. These can include chemistry experiments that produce spectacular reactions, or physics demonstrations that showcase the principles of motion and energy. Such events are not only entertaining but also serve to illustrate complexscientific concepts in a way that is accessible and memorable.Another significant aspect of science museums is theirrole in inspiring future scientists and engineers. By presenting scientific principles in a fun and engaging way, these museums can spark a lifelong interest in science. For middle school students, who are at a crucial stage in their education, such exposure can be pivotal in shaping their academic and career paths.Moreover, science museums are not just about the past and present; they also serve as a glimpse into the future.Exhibits on emerging technologies, such as robotics and artificial intelligence, can open students' eyes to the possibilities of what they might one day contribute to the world of science and technology.In conclusion, science museums play a crucial role in educating and inspiring the next generation of scientists. They provide a hands-on, interactive environment wherestudents can learn about the world around them in a way thatis both enjoyable and informative. For middle school students, a visit to a science museum can be a stepping stone to a greater understanding and appreciation of the sciences.。

介绍滁州传统文化的英语作文

介绍滁州传统文化的英语作文Chuzhou, a city in the eastern Chinese province of Anhui, is a land rich in cultural heritage and traditions that have been passed down through generations. From its stunning natural landscapes to its vibrant customs and crafts, Chuzhou offers a captivating glimpse into the heart of Chinese cultural identity. In this essay, we will explore the diverse and fascinating aspects of Chuzhou's traditional culture.One of the most prominent features of Chuzhou's cultural heritage is its stunning natural landscape. The city is situated in the picturesque Huaihe River basin, surrounded by rolling hills, serene lakes, and lush, verdant forests. The region's natural beauty has long been a source of inspiration for artists, poets, and scholars, who have captured the essence of Chuzhou's natural splendor in their works.The Baihua Mountains, located just outside of Chuzhou, are a prime example of the city's natural wonders. These ancient peaks, with their striking rock formations and cascading waterfalls, have been revered for centuries as a sacred site for Taoist and Buddhist practitioners. Visitors to the Baihua Mountains can explore a network of hikingtrails that wind through the rugged terrain, offering breathtaking views of the surrounding landscape.Another integral aspect of Chuzhou's traditional culture is its vibrant culinary heritage. The city is renowned for its delectable and diverse array of local dishes, which draw inspiration from the region's abundant natural resources and centuries-old cooking techniques. From the fragrant and flavorful Hui cuisine, which blends the influences of nearby regions, to the iconic Chuzhou-style mooncakes, each dish offers a unique and satisfying taste of the city's cultural tapestry.One of the most renowned culinary specialties of Chuzhou is its famed Huaiyang cuisine, which is celebrated for its delicate flavors, intricate preparation methods, and use of high-quality local ingredients. Dishes such as the tender and succulent Chuzhou-style braised pork belly, the fragrant and aromatic Chuzhou-style fried rice, and the delicate and delectable Chuzhou-style dumplings have become beloved staples of the city's culinary landscape.Beyond its natural beauty and culinary delights, Chuzhou is also renowned for its rich and vibrant cultural traditions. The city is home to a diverse array of annual festivals and celebrations that reflect the deeply rooted customs and beliefs of the local people. The Lantern Festival, celebrated each year on the fifteenth day of the first lunarmonth, is a particularly significant event in Chuzhou's cultural calendar.During the Lantern Festival, the streets of Chuzhou come alive with a dazzling display of colorful lanterns, traditional music and dance performances, and lively cultural exhibitions. Visitors can marvel at the intricate craftsmanship of the lanterns, which are often shaped like animals, mythical creatures, and auspicious symbols, and participate in activities such as lantern riddles and lantern-lighting ceremonies.Another important cultural tradition in Chuzhou is the art of traditional Chinese calligraphy. The city is home to a thriving community of calligraphers, who have honed their skills over many years of dedicated practice and study. These talented artists create breathtaking works of calligraphic art, often using historical techniques and materials to capture the essence of the Chinese written language.Visitors to Chuzhou can explore the city's rich calligraphic heritage by visiting the many calligraphy studios and exhibitions that dot the cityscape. They can also participate in calligraphy workshops, where they can learn the basic techniques and principles of this ancient art form.In addition to its culinary and artistic traditions, Chuzhou is also renowned for its vibrant and diverse array of traditional handicrafts. The city is home to a thriving community of skilled artisans who create a wide range of exquisite and intricate handmade products, from delicate silk embroidery to intricate wood carvings.One of the most iconic traditional handicrafts of Chuzhou is the Xiuning lacquerware, a centuries-old technique of creating beautiful and durable lacquered objects. These intricate and beautifully crafted pieces, which range from decorative vases and boxes to everyday utensils and furniture, are prized for their stunning visual appeal and exceptional durability.Throughout the city, visitors can explore the workshops and galleries of Chuzhou's talented artisans, who take great pride in preserving and passing down the time-honored traditions of their craft. Whether it's admiring the delicate embroidery of a silk scroll or the intricate wood carvings of a decorative screen, visitors to Chuzhou can immerse themselves in the rich and vibrant world of traditional Chinese craftsmanship.In conclusion, the traditional culture of Chuzhou is a captivating and multifaceted tapestry that reflects the deep-rooted heritage and enduring spirit of the Chinese people. From its stunning natural landscapes and delectable culinary delights to its vibrant artistictraditions and skilled artisanal crafts, Chuzhou offers a truly unique and enriching cultural experience for all who visit. Whether you are drawn to the city's natural beauty, its rich cultural heritage, or its warm and welcoming people, Chuzhou is a destination that is sure to leave a lasting impression on all who encounter it.。

a first course on probability pan -回复

a first course on probability pan -回复“A First Course on Probability Pan”Introduction:Probability theory is a fundamental branch of mathematics that deals with the study of uncertainty and randomness. It provides a framework for analyzing and predicting the likelihood of events occurring. In this article, we will take a comprehensive journey through the basic concepts of probability, starting from the fundamental principles and gradually building up to more advanced topics. So, let's embark on our first course on probability!1. Foundations of Probability:To begin our study, we need to establish the foundations of probability theory. At its core, probability theory is based on three fundamental principles: the sample space, events, and probability axioms. The sample space is the set of all possible outcomes of an experiment. From this sample space, we define events, which are subsets representing specific outcomes or collections of outcomes. Finally, probability axioms provide a set of rules that assign probabilities to events.2. Probability Calculations:In order to calculate probabilities, we need to understand the concepts of probability measures and probability distributions. Theprobability measure assigns a number between 0 and 1 to each event, representing the likelihood of its occurrence. Probability distributions describe the probabilities of all possible outcomes of a random variable. We explore various types of probability distributions, such as discrete and continuous distributions, and learn how to calculate probabilities using them.3. Combinatorics and Counting:Counting plays a crucial role in probability calculations, especially when dealing with multiple events or outcomes. Combinatorics is the branch of mathematics that studies counting principles. We delve into the fundamental principles of combinatorics, including permutations, combinations, and the inclusion-exclusion principle. These principles help us calculate the number of possible outcomes, which is vital for determining probabilities.4. Conditional Probability:The concept of conditional probability allows us to calculate probabilities based on additional information about an event. We discuss conditional probability in detail, exploring concepts such as independence, Bayes' theorem, and the law of total probability. These concepts enable us to make more precise predictions by taking into account relevant conditions or prior knowledge.5. Random Variables and Probability Distributions:Random variables are variables whose values depend on the outcome of a random experiment. They play a crucial role in probability theory and statistics. We examine the properties and characteristics of random variables, including expected values, variance, and probability generating functions. We also study common probability distributions, such as the binomial, Poisson, and normal distributions, which are essential in modelingreal-world phenomena.6. Limit Theorems and Statistical Inference:The study of probability is closely linked to statistical inference, which involves making inferences or predictions about a population based on sample data. We explore limit theorems, such as the law of large numbers and the central limit theorem, which establish the connection between probability and statistics. These theorems allow us to draw conclusions about population characteristics and make confident statistical inferences.Conclusion:In this first course on probability, we have journeyed through the fundamental principles, calculations, combinatorics, conditional probability, random variables, and statistical inference.Probability theory provides a powerful framework for understanding and predicting uncertain events and phenomena. By mastering these concepts, we can make informed decisions, solve real-world problems, and engage with complex statistical analyses. So, let this be just the beginning of your understanding of probability, as it opens doors to countless applications across various fields of science, business, and everyday life.。

太空授课英文作文

太空授课英文作文英文：Recently, I had the opportunity to give a lecture on space exploration to a group of students. It was a unique experience, as I had to communicate complex scientific concepts in a way that was easy for them to understand. I found that using analogies and real-life examples was the best way to make the information accessible to them.For instance, when discussing the concept of gravity, I used the example of a ball rolling down a hill. I explained that just as the ball is pulled down by gravity, so too are planets and other celestial bodies pulled towards each other. This helped the students understand the basic principles of gravity and how it affects objects in space.Another example was when I explained the concept of light years. I used the analogy of a car traveling on a highway. I asked the students to imagine that the car wastraveling at the speed of light and that the distance it covered in one year was equivalent to one light year. This made it easier for them to grasp the vast distancesinvolved in space travel.Overall, I found that using relatable examples and analogies was the best way to make the information accessible to the students. It helped them understand complex concepts in a way that was easy to remember and apply.中文：最近，我有机会向一群学生授课关于太空探索的知识。

基本原理英文

基本原理英文Basic Principles。

The basic principles are the fundamental concepts or ideas that form the foundation of a subject. In this document, we will explore the basic principles of various fields, including science, mathematics, and engineering.In science, the basic principles are the laws and theories that explain the natural world. These principles are based on empirical evidence and are used to make predictions about the behavior of the physical universe. For example, the basic principles of physics include Newton's laws of motion and the laws of thermodynamics. These principles form the basis of our understanding of how the universe works and are essential for the development of new technologies and innovations.In mathematics, the basic principles are the fundamental concepts that underlie the study of numbers,quantities, and shapes. These principles include the basic operations of addition, subtraction, multiplication, and division, as well as the principles of geometry and algebra. These principles are used to solve problems and make calculations in a wide range of fields, from engineering to finance to computer science.In engineering, the basic principles are the fundamental concepts and laws that govern the design and construction of machines, structures, and systems. These principles include the laws of mechanics, the principles of materials science, and the principles of thermodynamics. Engineers use these principles to design and build everything from bridges and buildings to airplanes and automobiles.In conclusion, the basic principles are the foundationof our understanding of the natural world, mathematics, and engineering. These principles are essential for making predictions, solving problems, and designing new technologies. By understanding and applying the basicprinciples, we can continue to advance our knowledge and make new discoveries in these fields.。

冰山理论-中英

Iceberg PrincipleThe iceberg principle is a writing theory stated by a famous American writer, Ernest Hemingway, as follows: If a writer of prose knows enough about what he is writing about he may omit things that will have a feeling of those things as strongly as though the writer had stated them. The dignity of movement of an iceberg is due to only one-eighth of it being above water. A good writer does not need to reveal every detail of a character or action. 〔冰山原则是一个写作理论由一位著名的美国作家,海明威,如下:如果一个作家的散文足够了解他写他可能忽略的事情,会有一种感觉的事情虽然作者说他们一样强烈.冰山运动的尊严在于只有八分之一露出水面.一个好的作家不需要显示一个字符或动作的每个细节〕Now this principle has been applied in various fields. We could regard the small part which can be easily noticed as the 1/8 of an iceberg, and the part can’t be easily noticed as the part under the surface. What can’t be seen is the most important, because it’s the base of an iceberg, what it to the part above water is what the root of a tree to the trunk. We may admire a famous person very much because of his wealth and fame, but how many people know the struggle and sufferings behind it? Are they dispensable? We can use the model of "iceberg principle". His wealth and fame is the small part we can see, and his sufferings and struggle is the part under water, without it, the person wouldn’t have acqu ired the fame and wealth he owns today.〔现在这一原则已经应用于各个领域.我们可以把这一部分很容易注意到冰山的1/8,和部分不容易注意到的部分表面.什么不可以被认为是最重要的,因为它是一座冰山的基础,它上面的部分水是什么根树的主干.我们可以欣赏一个非常著名的人因为他的财富和名声,但是有多少人知道它背后的挣扎和痛苦吗?他们是可有可无的吗?我们可以用"冰山原则"的模型.他的财富和名声是我们可以看到的一小部分,和他的苦难和斗争是在水下的部分,没有它,就不会获得名誉和财富的人他今天拥有.〕This principle also could be used in teaching. The amount of material that teachers could present to their students is a small portion of the sum total of knowledge and skills that they possess. Like an iceberg, the level of expertise that is visible to the public is dependent upon a larger body of knowledge that is submerged out of sight. So, it is particularly important for instructors to continually increase their knowledge and skill level because only a portion of the total body of knowledge will appear to be available; the same goes to learning. What the teacher could teach us is very limited, so if we want enrich our knowledge; we have to learn to use what we have learned to explore the unknown things.<这一原则也可用于教学.的材料数量,教师可以给学生是一小部分的总和为他们所拥有的知识和技能.像一座冰山,专业知识水平,可见公众依赖于一个更大的知识,被淹没在看不见的地方.是特别重要的,教师不断提高他们的知识和技能水平,因为只有部分知识的全身会出现可用;同样的去学习.老师可以教会我们什么是非常有限的,所以如果我们想要丰富我们的知识,我们必须学会使用我们已经学会探索未知的事物>Ernest Hemingway’s "iceberg theory" suggests that the writer include in the text only a small portion of what he knows, leaving about ninety percent of the content a mystery that grows beneath the surface of the writing.This type of writing lends itself naturally to a version of dream-interpretation If a writer of prose knows enough about what he is writing about he may omit things that will have a feeling of those things as strongly as though the writer had stated them. The dignity of movement of an iceberg is due to onlyone-eighth of it being above water. A good writer does not need to reveal every detail of a character or action.〔海明威的"冰山理论"表明,作者包括文本中只有一小部分他知道什么,离开大约百分之九十的内容一个谜,生长在表面之下的写作.这种类型的写作有助于自然版的释梦法如果一个作家的散文足够了解他写他可能忽略的事情,会有一种感觉的事情虽然作者说他们一样强烈.冰山运动的尊严在于只有八分之一露出水面.一个好的作家不需要显示一个字符或动作的每个细节.〕The Iceberg Theory is a term used to describe the writing style of American writer Ernest Hemingway. The theory is this: The meaning of a piece is not immediately evident, because the crux of the story lies below the surface, just as most of the mass of a real iceberg similarly lies beneath the surface. For example, The Old Man and the Sea is a meditation upon youth and age, even though the protagonist spends little or no time thinking on those terms."冰山理论"是一个用来形容美国作家欧内斯特?海明威的写作风格的术语.具体是指：一篇作品的的意义不是显而易见的,因为故事的要义往往隐藏在表面之下.比如,《老人与海》是是关于对年龄的思考,但作品中的主要人物并没有在这个问题上花费什么时间.In Death in the AfternoonIf, Hemingway wrote: a writer of prose knows enough of what he is writing about he may omit things that he knows and the reader, if the writer is writing truly enough, will have a feeling of those things as stronglyas though the writer had stated them. The dignity of movement of an ice-berg is due to only one-eighth of it being above water.在《午后之死》中,海明威写道：如果一个作家真正知道自己在写什么的话,即使他省略掉这些东西,读者还是能够理解他想要表达的内容.冰山运动之雄伟壮观,是因为他只有八分之一在水面上.]。

交通运输科技英语

第一讲1. contract system tested in all railway operationsThe contract system will be practiced in all operations of the Ministry of Railways-transport, capital construction and technological upgrading, ministry officials told Xinhua recently.The Guangzhou Railway Administration has been chosen to experiment with ways of implementing with ways of implementing the system throughout, the officials said.The contract system is necessary because since April 1986, the State has ceased to allocate funds to the ministry. Instead, it collects taxes from the ministry, which can use its own profits to develop its own operations, the officials said.“Th at means we have to depend exclusively on ourselves and that´s why we use the contract system to stimulate workers initiative for higher turnovers.〞 One official said.Although the contract system was practiced last year only in its transport sector, the ministry was able to handle 3.7 per cent more cargo than in 1985.In the first half of this year, the total volume of cargo and passenger transport was up 2.4 and 6.5 per cent over the same 1985 period, respectively, the officials reported.Meanwhile, a feeder railway funded by famers in a village in Zhanjiang, Guangdong Province, is expected to open to traffic in October, according to the overseas edition of People´s Daily.Farmers in Xinhua village raised 2 million yuan(about $540,000) to build a 600-metre feeder railway line to speed up transport between urban and rural areas. (Xinhua)Words and ExpressionsContract /´kɑn‚t rækt/ n. 承包；契约，合同Contract system 承包制，大包干Administration /æd‚mini´streʃǝ n/ ǝ n.局；管理，经营Experiment /ik´spɛrǝ mǝ nt/ n.&v.i.试验To……w i th……试验……To……upon……用……作实验Implement /´implǝ mǝ nt/ v.t.实现，落实Thoughout prep.在整个……期间；全……，……到处´ adv.彻底地，全部地Cease /sis/ v.t.停止Allocate /´ælǝ‚ket/ v.t.分配，拨给Fund /fΛnd/ n.经费，资金Tax /tæks/ n.税；税款Profit /´prɑfit/ n.利润，赢利；利益Exclusively /ik´sklusivli/ adv.仅仅，只有；独占地Stimulate /´stimjǝ‚let/ v.t.鼓励，刺激Initiative /i´niʃǝ tiv/ n.首创精神，主动性，积极性Turnover / /´tǝn‚ovǝ/ n.营业额，产值Sector /´sɛktǝ, -‚tↄr/ n.扇形；地段，区域；方面Feeder /´fidǝ/ n.铁路支线；加料漏斗2. East railways to be upgradedRailway lines in East China will be improved with an investment of 7 billion yuan (about $1.89 billion), the overseas edition of People´s Daily reported on Thursday.The project involves 1.660 kilometers of railways, with 470 kilometers of new lines to be built, 510 kilometers to be electrified and 680 kilometers double-tracked.According to Zhou Congqing, the official in charge of the project, more than 3,300hectares of land will be requisitioned and many dwellings relocated.Technicians are working on an engineering plan, and the whole project is expected to be completed in1990.Words and ExpressionsUpgrade /´Λp‚ɡred/ Λ v.t.加强；使升级，改进In charge of 负责……；主管……Hectare /´hɛk‚tɛr/ n.公顷Requisition /‚rɛkwi´ziʃǝ n/ n.征用Dwelling /´dwɛliŋ/ n.住房Relocate /ri´loket/ v.t.搬迁3. Rail capacity doublesEach kilometers of railway in China carried 596.9 billion tons of freight in the first six months of this year on average, double that in1978. Officials of the Railways Ministry said that the present structure can hardly meet the needs of the developing economy.The ministry is now concentrating on expanding the handling capacity of the existing railroads, while constructing or upgrading new lines. Among the 38 marshalling yards in China,28 have increased the number of trains they send out every day by 10 to 30 per cent. The yards sent out 38,000 trains every day last year, 15 per cent more than in theprevious year.Since April, the number of passenger and freight trains has increasedby 22 per cent and 20 per cent, respectively, compared to 1978. Nowpassenger trains are capable of transporting three million people a day.The project to add a double-track along the line between Hengyang inHunan Province and Guangzhou is nearly complete. And the project ofexpanding the railway network in eastern China is going on smoothly. (Xinhua)Words and ExpressionsFreight /fret/ n.货物Average /´ævǝ riʤ, ´ævriʤ/ v n.平均，均值On average 按均值计算，平均Marshalling/´mɑ:ʃǝ liŋ/ 排列，整理，编组Marshalling yard 编组场To send……out发出，寄出，放出ŋImproving railway operationsNew trendsBy Dr. techn., Prof. E.A. SOTNIKOV,All-Union Railway Scientific Research Institute, Ministry of Railways of the USSRSUMMARYThe Soviet Railways handle 52 per cent of the world´s railway freight turn-over although they account only of about 11 per cent of their total mileage. This determines an extremely high traffic density of the lines exceeding 6 to 10 times and even more, similar values of the railways of technically advanced countries.In order to meet the requirements for handling constantly growing volumes of traffic, apart from improving railway equipment, reinforcing the tracks and raising the rolling stock carrying capacities, the operational procedure has to perfected as well, it has to attaining this overall goal is the intensification of the utilization of the existing technical means. In this report new trends improving railway operations are discussed.第二讲1、Train SchedulingThe train schedule is the basis for managing train traffic. It integrates the activities of all railway services. Observing exactly the technological procedures laid down for yards and stations, for locomotive and wagon shops, for track, signaling and communication facilities as well as for other railway bodies is a vital necessity for trains being operated strictly to the schedule.In order to keep the dispatchers staff informed on the train situation data provided through the automatic traffic control systems of the various railway regions is used. Data on the availability of trains and locomotives at each of the stations and at the classification yards of the given route is submitted; to the dispatchers staff of the upper control echelon in 3 hours intervals, to the dispatchers staff of the railway region headquarters and of the various sections on a continuous basis as the status at the stations and yards changes.This data is also taken into consideration at the railway region border stations where the wagons are being transferred from one railway region to the other. When necessary, the data is used at other stations as well.This makes it possible to monitor train traffic all along the route, to make timely calculations with regard to assigning locomotives to trainsto reduce to the minimum delays of trains waiting for motive power and to raise the level of running trains strictly to the schedule.The improvement of train operations requires further development of the dispatcher control system through further automating and centralizing the management of train traffic. With regard to improving train operation the following major principles have to be observed.1. The activity of the train dispatchers which is at the moment primarily concentrated at keeping records of the train operations with a certain delay with regard to the actual situation should pass over to a strategy of advanced management using train operation forecast and of applying optimized algorithms for wording out operational plans covering foreseeable periods of future activities.2. A continuous automated monitoring of possible handicaps has to be introduced. This will enable the train dispatchers staff by using the man-machine dialog mode of operation to consider within several minutes a number of managerial alternatives to choose the most effectives of them to prevent minor handicaps form developing into major traffic problems.3. The supply of information to the dispatchers staff has to be automated using specific facilities developed for the purpose, as well as data provided by the existing automated operations management system. Presently the train dispatchers spend 30 to 40% of their working time to acquire this information. This will also make it possible to automate accounting activity of train operations.4. Based on using automated information systems and on introducing up-to-date control equipment the routes covered by the dispatchers at the sectional, regional and ministerial levels should be extended, and traffic control over long routes should be integrated.5. Centralizing train dispatching control, setting up of big dispatching centers equipped with modern means of communication, computers and with automated consoles for the dispatchers is of major importance. Such a control console, for a sectional train dispatcher, for instance, should consist of:a multi-sectional panel with a colored presentation of the section track lay-out or with the presentation of the entire route indicating train numbers as well as other data supplied by the computer;a graphical display to keep the planned schedule;an alphabetical display for the train dispatcher to keep a dialog with the computer;a printer for an automated record keeping of the operation;of a graph-plotting display for drawing a graph of the actual train operation.The introduction of new methods aimed at running the trains strictlyto the schedule will result in an overall improvement of train operations. Words and ExpressionsSummary /´sΛmǝ ri/ n.摘要Freight turn-over 货物周转量To account of 〔数量上〕占……Mileage /´mailiʤ/ n.里程Traffic density 运输密度Reinforce /‚riin´fↄrs, -´fors/ v.t.加强Rolling stock 机车车辆〔有时专指车辆〕Rationalize /´ræʃǝ nǝ‚laiz/ a v.t.使合理化Utilization /‚jutilai´zeʃǝ n/ n.利用Means /mi:nz/ n.方法，手段；工具，设备Train scheduling 铺画列车运行图Integrate /´inti‚ɡret/ v.t.把……连接成一个整体Railway service 铁路业务；铁路部门Observe /ǝ b´zǝv/ v.t.观察，遵守To lay down 放下，使……躺下，铺设，规定，制定Vital /´vaitl/ adj.非常的，至关紧要的Dispatcher /dis´pætʃǝ/ n.列车调度员To keep informed on 随时向……报告To submit to 向……提出……；把……提请……批准Echelon /´ɛʃǝ‚lɑn/ 梯队，级别Dispatches staff ofUpper control echelon 上级调度人员Border station 分界站To keep a record of 把……记载下来Algorithm /´ælɡǝ‚riðǝ m/ n.算法Handicap /´hændi‚kæp/ n.不利条件，障碍，困难Man-machine dialog modle 人机对话方式Accounting activcity 统计工作Up-to-date adj.现代化的，最新式的Console /kǝ n´sol/ n.控制台Multi-sectional panel 多个区间的显示盘Presentation/‚prɛzǝ n´teʃǝ n,prizǝ n-/ n.显示的图像第三讲2.Increasing the weights and speeds of trains Increasing the weights and speeds of trains is a major factor in handling the constantly growing volume of traffic .A variety of ways and means are used to raise the tonnage and the length of trains.First of all the length of the exisiting tracks must be used to the full. With this in mind comprehensive measures of the undertaken at present to raise the static weights of the wagons by increasing the net weight of the commodity being forwarded .The permissive axle load of the wagons has been raised, a good share of bulky goods are forwarded in disassembled status, the most rational way are use to accommodate the good in the wagons ,the outer dimensions set for the rolling stock at a given region are applied since at use some extra space for loading cargo ,etc. Making proper use some each and every meter o f the tracks is of substantial importance too. Applying computer techniques in handling traffic made it possible to take into account the length of each individual wagon rather than of the types of wagons which was common practice before. As a result one or two wagons were added to every train. Likewise the length of each individual stationand/or yard track is taken into consideration: that made it possible to make up trains that are longer than the set standard.In case when the weight and the length of trains are limited by the traction capacities of locomotives, alternative standards are being introduced with regard to the weights of trains. To handle trains with weights exceeding the standard double locomotive traction, or pusher locomotives are used on part of the route.Often the length of the train is limited by the length of station tracks rather than by limited tractive capacities of the locomotives. On a number of railways trains are run which are 20 to 25% longer than the station tracks can accomodate. Also superlong trains are run on special schedules that have been worked out in such a way that they do not interfere with the presently of the other trains .15 to 20% of the trains presently operated on the main lines exceed the standard length .On high traffic density lines station and/or yard tracks are being extended to provide for the operation of long trains. As additional extended station tracks are put into operation the number of long trains run on special schedules is increased respectively.The handling of jointed trains, that is doubled-up and trippled-up trains up to 16000 tons heavy ,is of special importance .Some of those trains even exceed that weight .On April 18,1984,to quote one example ,a 30220 ton train 4668 meters long was run over a distance of 1130 km from the Ekibastus station to the Tobol station .By raising the tonnage and the length of trains the throughput capacity of lines is enhanced substantially .It also makes it possible to allocate the time needed for track maintenance and to cut down the operational expenditures. Part of the system will be up-graded to ensure the operation of jointed trains.Long term measures are undertaken to raise the tonnage oftrains .Extending station tracks along certain axes, putting 6-axe wagons into operation on a wide scale –those are measures that should be mentioned first .The utilization of 8-axle wagons will raise the per-meter of load of the tra-10.5 tons. Though this will be attained only if the outer dimensions set for the rolling stock is fully used by the industry when building the wagons.Raising the per meter load of the tracks will result in increasing the weight of standard 850metres long trains up to 8000 tons and even up to 10000 tons if the station tracks are extended to 1050 meter. Passenger trains are made longer as well. For suburban traffic ,trains are put into operation consisting of 12 coaches ,and on some directions doubled-up trains consisting of 24 or 32 sleeping cars are planned to be introduced .This will make it possible to handle the growing volume of passenger train traffic.Speeds of passenger trains are growing too .A high speed train has been put into operation on the Moscow to Leningrad line .Speeds of trains are being raised on other lines as well, first of all on international lines .Since in the USSR passenger and freight trains are run on the same tracks it is important to raise the speeds of both passenger and goods trains.Introducing roller bearing axle-boxes on a larger scale will contribute a great deal to an early attaining of this objective.Words and Expressionsthe full 充分地，完全地wi th……in 考虑到comprehensive /‚kɑmpri´hɛnsiv/ adj.理解的，综合的comprehensive measures 综合措施undertake /‚Λndǝ´tek/ v.t.承当，着手，从事static /´stætik/ adj.静态的，静止的static weight 静载重net /nɛt/ adj. 纯洁的net weight 净重commodity /kǝ ´mɑditi/ n. 商品，货物forward /´fↄrwǝ d/ adj. 向前的；v.t促进；发送permissive /pǝ´misiv/ adj.允许的axle load /´æksǝ l,´æksl/ 轴重wagon /´wæɡǝ n/ n. 货车share /ʃɛr/ n. 份额，局部a good shar e of ……大局部……bulky /´bΛlki /adj. 体积大的bulky goods 笨重货物disassemble /‚disǝ ´sɛmbǝ l/ v.t. 分散，拆散accommodate /ǝ ´kɑmǝ‚det/ v.t. 供给，供给substantial sǝ b´stænʃǝ l/ adj. 实际的，重大的likewise /´laik‚waiz/ adv. 同样地double locomotive traction 双机牵引pusher locomotive 补机superlong train 超长列车doubled-up train 双列组合列车trippled-up train 三列组合列车throughput /´θru‚put/ n. 生产量；通过量thoughput capacity 通过能力enhance /ɛn´hæns/ v.t. 提高expenditure /ik´spɛndǝ tʃǝ/ n. 交出，费用NoteApplying computer techniques in handling traffic made it possible to take into account the length of each individual wagon rather than of the types of wagons which was common practive before.(运用计算机技术来管理运输，使我们可以按照每一辆货车的实际长度来计算列车总长，而不是像以前那样按列车类型进行近似计算。

ASTM D1735

Designation:D1735–08Standard Practice forTesting Water Resistance of Coatings Using Water Fog Apparatus1This standard is issued under theﬁxed designation D1735;the number immediately following the designation indicates the year of original adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.A superscript epsilon(´)indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1.Scope1.1This practice covers the basic principles and operating procedures for testing water resistance of coatings in an apparatus similar to that used for salt spray testing.1.2This practice is limited to the methods of obtaining, measuring,and controlling the conditions and procedures of water fog tests.It does not specify specimen preparation, speciﬁc test conditions,or evaluation of results.N OTE1—Alternative practices for testing the water resistance of coatings include Practices D870,D2247,and D4585.1.3The values stated in SI units are to be regarded as the standard.The values given in parentheses are for information only.1.4This standard does not purport to address all of the safety concerns,if any,associated with its use.It is the responsibility of whoever uses this standard to consult and establish appropriate safety and health practices and deter-mine the applicability of regulatory limitations prior to use.2.Referenced Documents2.1ASTM Standards:2B117Practice for Operating Salt Spray(Fog)Apparatus D609Practice for Preparation of Cold-Rolled Steel Panels for Testing Paint,Varnish,Conversion Coatings,and Related Coating ProductsD610Practice for Evaluating Degree of Rusting on Painted Steel SurfacesD714Test Method for Evaluating Degree of Blistering of PaintsD823Practices for Producing Films of Uniform Thickness of Paint,Varnish,and Related Products on Test PanelsD870Practice for Testing Water Resistance of Coatings Using Water ImmersionD1193Speciﬁcation for Reagent WaterD1654Test Method for Evaluation of Painted or Coated Specimens Subjected to Corrosive EnvironmentsD1730Practices for Preparation of Aluminum and Aluminum-Alloy Surfaces for PaintingD2247Practice for Testing Water Resistance of Coatings in 100%Relative HumidityD2616Test Method for Evaluation of Visual Color Differ-ence With a Gray ScaleD3359Test Methods for Measuring Adhesion by Tape Test D3363Test Method for Film Hardness by Pencil TestD4541Test Method for Pull-Off Strength of Coatings Using Portable Adhesion TestersD4585Practice for Testing Water Resistance of Coatings Using Controlled Condensation3.Summary of Practice3.1Coated specimens are placed in an enclosed chamber to permit free settling of fog on all specimens.The temperature of the chamber is usually maintained at38°C(100°F).The exposure condition is varied by selecting the duration of the test.Water permeates the coating at rates that are dependent upon the characteristics of the coating.Any effects such as color change,blisters,loss of adhesion,softening,or embrittle-ment are observed and reported.4.Signiﬁcance and Use4.1Water can cause the degradation of coatings,so knowl-edge of how a coating resists water is helpful in predicting its service life.Failure in water fog tests may be caused by a number of factors,including a deﬁciency in the coating itself,1This practice is under the jurisdiction of ASTM Committee D01on Paint andRelated Coatings,Materials,and Applications and is the direct responsibility ofSubcommittee D01.27on Accelerated Testing.Current edition approved Nov.1,2008.Published December2008.Originallyapproved st previous edition approved in2002as D1735–04.2For referenced ASTM standards,visit the ASTM website,,orcontact ASTM Customer Service at service@.For Annual Book of ASTMStandards volume information,refer to the standard’s Document Summary page onthe ASTM website.Copyright©ASTM International,100Barr Harbor Drive,PO Box C700,West Conshohocken,PA19428-2959,United States.--` , ` , , ` , ` ` , , ` ` , ` ` , , ` , ` , , ` , ` ` ` ` ` -` -` , , ` , , ` , ` , , ` ---contamination of the substrate,or inadequate surface prepara-tion.The test is therefore useful for evaluating coatings alone or complete coating systems.4.2Water fog tests are used for research and development of coatings and substrate treatments,speciﬁcation acceptance,and quality control in manufacturing.These tests usually result in a pass or fail determination,but the degree of failure may also be measured.A coating system is considered to pass if there is no evidence of water-related failure after a speciﬁed period of time.4.3Results obtained from the use of water fog tests in accordance with this practice should not be represented as being equivalent to a period of exposure to water in the natural environment,until the degree of quantitative correlation has been established for the coating or coating system.4.4The test apparatus is similar to that used in Practice B117,and the conversion of the apparatus from salt spray to water fog testing is feasible.Care should be taken to remove all traces of the salt from the cabinet and reservoir when convert-ing from salt spray to water fog testing.5.Apparatus5.1Descriptions for the Test Chamber,Test Specimen Sup-ports,and Fog Collecting Devices,are listed under Apparatus Section in Practice B117.5.2Unless otherwise speciﬁed,provide for continuous re-cording,such as a chart recorder or data logger,of the temperature within the chamber during the exposure period.6.Test Specimens6.1This practice does not cover the preparation of test specimens.The substrate composition and surface preparation, specimen preparation,and the number of specimens should be agreed upon prior to testing.N OTE2—Applicable methods for the preparation of test panels and substrates are given in Practices D609and D1730.Practices D823cover application techniques of the production of uniformﬁlms.6.2It’s recommended that a control specimen of a paint with known durability be included with each test.Such control specimens can provide warning of changes in test severity in a given apparatus,and can indicate variations in test severity between different apparatuses.6.3It’s recommended that at least two replicate specimens of each different coating be used,so as to compensate for variations between specimens and variations in test conditions within the apparatus.7.Procedure7.1Fill the reservoir with reagent water conforming to Type IV or better of Speciﬁcation D1193.7.2Heat the water to the desired temperature with the circulating system in operation.If no temperature is speciﬁed, heat the water to3862°C(10064°F).Maintain the temperature throughout the test.7.3Support or suspend specimens15°from the vertical, with the plane of the specimen parallel to the direction of the fogﬂow.Slotted wood supports are suitable forﬂat specimens. Each specimen shall be placed to permit unencumbered expo-sure to the fog.A minimum spacing between specimens of30 mm is recommended.Arrange the specimens so that the water from one specimen does not drip on other specimens.7.4Place two containers within the chamber to collect fog for e collectors that have approximately the same position within the chamber as the test e a glass or plastic funnel with a diameter of area80cm2(area100 mm)with the stem of the funnel extending into the collection container.Locate one container within10cm of the fog nozzle and the other as far as possible from the nozzle.Place the collectors so that they collect only the fog from the nozzle. Collection rates are to be measured every24h except on weekends.Record the collection rate in mL/h.Refer to customer speciﬁcation for acceptable collection rates.7.4.1Use at least four collectors for cabinets that have more than one fogging nozzle.Position a collector10cm from each nozzle.Place the other collectors at positions as far as possible from each nozzle.7.4.2It is recommended to map the collection rates within the exposure zone of the cabinet once a year.Place collectors throughout the exposure zone to evaluate variations within the cabinet.Areas that are out of compliance should be taped off and specimens not tested there.Refer to manufacturer’s rec-ommendations to correct the problem.N OTE3—Collection rate mapping of a chamber throughout the expo-sure zone to ensure conformance to the collection rate requirements is recommended as a means of reducing test variability.N OTE4—Larger cabinets that have more than one fogging nozzle,may need more than two collectors to obtain adequate collection information. Refer to manufacturers’recommendations and customers’requirements for speciﬁc requirements.7.5To control for variability within the apparatus,reposi-tion the specimens on a regular basis so that all specimens spend equivalent amounts of time in the various areas of the apparatus(front,back,left,right,and center).7.6Adjust the atomizing air supply so that1.0to3.0mL of water/h is collected in a collector with a diameter of100mm based on an average run of at least16h.The collection rate should be agreed upon between the customer and the supplier before starting the test.The collection rate of the water should be recorded daily.N OTE5—The fog rate speciﬁed in Practice B117is only1.0to2.0 mL/h for a collector with a diameter of100mm.7.7Operate the test continuously with the test chamber closed unless otherwise speciﬁed.Short interruptions to inspect or remove specimens or to replenish the water supply are permitted,but such interruptions should occur no more than once each day.7.8Conclude the test after a speciﬁed period of time or after effects from exposure to water fog are observed.7.9Wipe the test specimens dry.Rate specimens for changes in color,blistering,etc.Evaluate specimens no less than5min and no more than10min after removal from test, as the effects from water exposure can change within a short time.Remove only as many specimens as can be rated within the speciﬁedtime. --`,`,,`,``,,``,``,,`,`,,`,`````-`-`,,`,,`,`,,`---N OTE 6—The 0to 10scale described in the ASTM STP 5003is preferred for rating.Relevant procedures for evaluating water effects are described in Test Methods D 610,D 714,D 1654,D 2616,D 3359,D 3363,and D 4541.7.9.1If possible,rate the specimens again after they have been removed from the test for a recovery period long enough that moisture absorbed within the specimens dries out and the specimens reach moisture equilibrium with room air.A recov-ery period of 12from 24h is generally sufficient.The post-recovery rating allows evaluation of the permanent effects of the exposure as distinct from the transient effects,and is especially important for evaluation of color and gloss.8.Report8.1Report the following information:8.1.1Sample identiﬁcation.8.1.2Results of the evaluation(s).8.1.3Reference to Practice D 1735.8.1.4Hours of test duration.8.1.5Test temperature.8.1.6Rate of fog collected.8.1.7Special conditions of test or any deviations in test procedure.9.Keywords9.1adhesion;blistering;resistance-water;rust;water fogASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentioned in this ers of this standard are expressly advised that determination of the validity of any such patent rights,and the risk of infringement of such rights,are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every ﬁve years and if not revised,either reapproved or withdrawn.Your comments are invited either for revision of this standard or for additional standards and should be addressed to ASTM International Headquarters.Your comments will receive careful consideration at a meeting of the responsible technical committee,which you may attend.If you feel that your comments have not received a fair hearing you should make your views known to the ASTM Committee on Standards,at the address shown below.This standard is copyrighted by ASTM International,100Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959,United States.Individual reprints (single or multiple copies)of this standard may be obtained by contacting ASTM at the above address or at 610-832-9585(phone),610-832-9555(fax),or service@ (e-mail);or through the ASTM website ().3Paint and Coating Testing Manual,14th ed.,ASTM,1995.--`,`,,`,``,,``,``,,`,`,,`,`````-`-`,,`,,`,`,,`---。

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