Chap25&26-MagFie

ΦB closed B dA 0
surf ace
(Gauss’ law of magnetic field)
In the absence of magnetic monopoles the magnetic flux threading a closed surface is zero.
B
B
i
N=nA dl dF Nf ( nAdl )qv B idl B ( where i qnAv)
F dF idl B (force on a current)
Note: where B is external magnetic field.
I
B
moving charges
magnetic field B
moving charges
The magnetic force is: F qv B —— Lorentz’s force
B
v
B
F

+q
In a similar way the definition of electric field, as we could define B by the acting force of magnetic filed on the moving charges as:
i
25-2. Magnetic Field Lines and Flux (P581) 1. The features of magnetic field lines No origin, no termination and no cross point(无头无尾不相交,闭合曲线); Wrap around with current与电流套连; Right-hand rule with current (与电流成右手螺旋关系) 2. Magnetic flux & Gauss’ law
25-3. Magnetic Force on Moving Charges 1. Lorentz’s force (P586) The charged particles moving in magnetic field acted by magnetic force:
moving with velocity v through a magnetic F field B is always perpendicular to v and B . v q 0, F qv B F qv B q 0, F qv B q B F=qvsin (when = 0,F=0),
半导体类型判别
Hall effect can be used to find B, I, density, positive or negative and drift speed of charged carriers…
25-4. F B on a Current-carrying Wire (P605)
in the presence of both magnetic field B and the electric field E, it will feel a force
F q( E v B )
It is often called the Lorentz equation and is considered one of the basic equation in physics. If no other external forces, then qE qv B m a From it one can determine the motion of particles.
3. Samples of practical application (1) Discovery and properties of electron (P592)
Both B and E can produce a force on a charged particle: e v eE evB e v v2 evB m m B 2r v E/B m Br r
5. Ampere’s Law 6. Magnetic Materials
25-1.The Definition of B (P580)
Magnetic phenomena:
Magnets used in: compass; refrigerator-door; microphone; computer disk; …
Like poles repel; unlike poles attract
Ferromagnetic: nickel, cobalt, gadolinium, alloys
An electric charge set up an electric field that can affect other electric charges. Might you expect that a magnetic charge set up a magnetic field that can then affect other magnetic charges? Such magnetic charges are called magnetic monopoles, which are predicted by certain theory, although their existence has not been confirmed.
Let dΦB —–number of magnetic field line threading area dA,
dΦB B dA
dΦm BndA B dA Φm dΦm B dA
S S
dA
n
The flux of magnetic field intersects a closed surface:
It can be used to measure the ratio m/q of the particles moving through Thomson’s apparatus.
电子比荷
Fig.25-29 in P593
(2) The Hall effect (霍尔效应)(P594)
载流导体放在磁场中， 磁场垂直于电流方向， 则在磁场与电流垂直方 向出现横向电势差。
SI unit: tesla (特斯拉) (T), gauss or N/(A· m).
1T 104 G (gauss )
地磁场Bearth ≈
510-5 T
Dimension: I-1MT–2.
The principle of superposition of B: B B1 B2 Bi Bi
Magnetic field (line)
The interacting force between moving charges is magnetic force which is transferred by magnetic field (运动电荷之间的相互作用力---磁力,由磁场传递).
2πmv cos Pitch(螺距): h Tvcos qB
Look at example 25-6 of P588 to learn about the phenomenon of aurora, please.
If a particle of charge q moves with velocity v
1. Magnetic force on a current Freely choose a short length dl, cross area A, mass m, charge q, then the Lorentz’ force on each particle:
f qvB sin
B
F v
(3) Particle moves in an angle with B (P587): h v//= vcos, v⊥=vsin
Particle follows a spiral path
(螺旋线运动).
Radius: Period:
mv sin R qB
T 2 πR 2πm v sin qB
B
B
R
—— Circular motion with constant q>0: speed in the plane of perpendicular to B
mv Radius of track: R qB
B
q<0: R q v -
Period:
2 πR 2 πm T v qB
Chapter 25 & 26
Magnetic Field
1. Magnetic Flux and Gauss’ Law 2. The Magnetic Force on a Particle 3. Magnetic Force on a CurrentCarrying Wire
4. Magnetic Field Due to Current
Example: A straight, horizontal length of copper wire has a current I=28A through it. What are magnitude and direction of the minimum magnetic field needed to suspend the wire (to balance Fg ) on it? The linear density of wire is 46.6g/m. Solution: