《物理双语教学课件》Chapter 19 Magnetic Field 磁场

Chapter 19 Magnetic Field

We have discussed how a charged plastic rod produces a vector field-the electric field E -at all points in the space around it. Similarly, a magnet produces a vector field-the magnetic field B -at all points in the space around it.

19.1 The Definition of B

1. We determined the electric field

E at a point by putting a test particle of charge q at rest at that point and measuring the electric force

E F acting on the particle. We then defined E as q F E E =.

2. To define the magnetic field

B , we can fire a charged particle through the point where

B is to be defined, using various directions and speeds for the particle and determining the force B F that acts on the particle at that point.

(1) After many such trials we would find that when the

particle ’s velocity v is along a particular axis through the

point, force B F is zero .

(2) For all other directions of

v , the magnitude of B F is always proportional to φsin v , where φ is the angle between

the zero-force axis and the direction of v .

(3) Furthermore, the direction B F is always perpendicular to

the direction of v (These results suggest that a cross product

is involved).

3. We can then define a magnetic field

B to be a vector quantity that is directed along the zero-force axis . We can next measure the magnitude of B F when v is directed perpendicular to

that axis and then define the magnitude of

B in terms of that force magnitude: v

q F B B =, where q is the charge of the particle. 4. We can summarize all these results with the following vector equation: B v q F B ⨯=.

5. Finding the magnetic force on a particle: The force

B F acting on a charged particle moving with velocity

v through a magnetic field B is always perpendicular to

v and B . 6. The SI unit for B is the netown per coulomb-meter per

second. For convenience, that is called the tesla (T). An earlier (non-SI) unit for B , still in common use, is the gauss (G), and 1 tesla = 104 gauss.

7. Right table lists the

magnetic fields that

occur in a few

situations.

8. We can represent magnetic fields with field lines , just as we

did for electric field. Similar rules apply. That is, (1) the direction of the tangent to a magnetic field line at any point gives the direction B at that point, and (2) the spacing of the lines represents the magnitude of B -the magnetic field is stronger where the lines are closer together, and conversely. 9.Figures show the magnetic field lines for magnet with different shapes. We find that the lines all pass through the magnet, and they form closed loops(even those that are not

shown closed in the figures). The closed field lines enter one end of a magnet and exit the other end. The end of a magnet from which the field lines emerge is called the north pole of the magnet; the other end, where field lines enter the magnet, is called the south pole.

10.W hen two magnets are moved to each other, we find that opposite magnetic pole attract each other, and like magnetic pole repel each other.

11.E arth has a magnetic field that is produced in its core by still