1. Example e- Example 2: Motion of an electron in a magnetic field.
An electron enters a magnetic field as shown below. e-
Draw a diagram to show the subsequent path of the electron.

2. Example
The direction of current is opposite to the motion of the electron. By the motor rule, the magnetic force acts downwards, making the electron change its direction as shown. e-

3. Example
Example 3: A metal rod AB is placed on two fixed smooth conducting rails, as shown below.
conducting rail A B S1 S2 E1 E2
metal rod

4. Example
The magnetic field is pointing into the paper. The e.m.f. in E1 equals that in E2.
What is the direction of motion of the rod AB if
a) only switch S1 is closed
Current flows from B to A through the rod. By the motor rule, the rod moves to the left.

5. Example b) only switch S2 is closed
Current flows from A to B through the rod. By the motor rule, the rod moves to the right.
(c) both switches are closed?
No current flows through the rod so it does not move.

6. Example
Example 4: A current-carrying wire is put between the arms of a U-shaped magnet. The resultant magnetic field is shown below.
wire

7. Example a) What are the polarities of X and Y?
X is the north pole and Y is the south pole.
b) In what direction does the current flow in the wire?
The current flows out of the paper.

8. Example
c) Draw an arrow to show the magnetic force acting on the wire.
wire F

9. Example
d) Why is the magnetic field under the wire stronger than that above the wire?
Above the wire the magnetic field from the magnet and that from the current-carrying wire cancel each other out.
Below the wire the magnetic fields from the magnet and the wire reinforce each other.

10. Example End e) What will happen to the force
(i) if the direction of the current is reversed
The direction of the force is reversed.
(ii) if both the current and the field are reversed?
The force does not change.
End