phy electromagnetism

1. When the wire AB is pulled out of the magnetic field, there is no induced current. Thus the pointer returns to zero.

2. By Faraday's Law, the induced emf depends on the rate of change of magnetic flux only. The change of magnetic flux depends on the no. of turns in the coil, its cross-sectional area and the magnetic field intensity. It doesn't relate to the resistance of the coil.

Notice that the induced current, however, depends on the coil resistance because of Ohm's Law.

3. After switch is closed, current in P flows in counterclockwise direction as seen by the observer. By Lenz's Law, the induced current must flow in the opposite direction, i.e. clockwise.

The force is repulsive, as the currents in the two coils are flowing in opposite direction. They have opposite magnetic polarity facing each other (try to use the Right-Hand Screw Rule to find their polarities).

4. I suppose the horizontal direction is x, and the vertical direction is y.
There is no induced emf, hence no induced current, when the coil is moving horizontally along the x-direction, as there is no change of magnetic flux linked with the coil.

The is induced current when PQ is moving vertically in the y-direction out of the magnetic field, but with RS still inside. No induced current when both PQ and RS are inside the field.

5. Net magnetic forces at positions 1 and 2 are both away from the wire

6. The aluminimum ring is being pushed up the rod because it has a magnetic polarity opposite to that of the coil.

2014-09-17 23:28:35 補充：
Correction to Q5.
The net magnetic force at position 1 is away from the wire. At position 2, it is towards the wire.

2014-09-22 08:02:02 補充：
Your suppl question:
Be aware that my x-direction is horizontal. Should the coil move horizontally, there is NO cutting of field lines, hence no induced emf.

2014-09-22 08:04:43 補充：
Induced current flows along RQPS when the coil moves VERTICALLY with the side PQ is out of the field. That is, when only RS is cutting field lines.

2014-09-24 15:33:16 補充：
See 意見 column

2014-09-24 15:34:39 補充：
Your further question on Q4
When the coil moves horizontally, it is moving along the field lines. Hence, there is no field line cutting.

2014-09-25 13:12:35 補充：
Q:that means sides PS and QR are not cutting magnetic field lines when the loop is moving along x direction?
A: The field lines are running horizontally. Should the coil move horizontally, there would not be any cutting of field lines.