plasma

30th September 2006 - 06:00 PM

Please help me with this problem.

1)When a bar magnet with its magnetic moment along the axis of a circular coil and directed towards the coil is withdrawn away from the coil, parallel to itself, the current in the coil as seen by the withdrawing magnet is:

a)Zero

b)Clockwise

c)Anti-clockwise

d)Independent of the resistance of the coil

e)Independent of the number of turns in the coil

I analyzed it in the following way:

As the magnetic moment is directed towards north pole and given that magnetic moment is directed towards the coil, the north pole of the magnet faces the coil. As the bar magnet is moved away from the coil, the magnetic flux associated with the coil changes and an e.m.f is induced and hence a current. According to Faradays law, the direction of current is such as to oppose the cause that induces the current. It opposes it, by the flow of a clockwise current. We know that a circular current coil acts as a magnetic dipole. When viewing a coil, if current flows in the clockwise direction, that particular face represents the south pole. So only if the current flows in the clockwise direction, it can oppose the change in magnetic flux. So the answer is (b)Clockwise. But the answer given in the book is ŠAnti-clockwise. Is there a mistake in my understanding?

mr_homm

1st October 2006 - 01:16 AM

Your answer is correct. You have correctly used Faraday's Law, and this is a good way to do the problem. Sometimes there is an easier way, too, and in this case, you can use it to check your answer.

The idea is that you use the two facts that (1) B cannot do work on a charge, so it transfers no energy, and (2) it takes energy to run the current in a wire loop. Because the current dissipates energy as heat, it must have a power source to continue running. But the energy cannot come from the B field, because B does no work. The only other source of energy in the system is the kinetic energy of the moving magnet. Therefore, this energy must be used up in driving the current, and therefore, the magnet must slow down (unless some outside force keeps it moving). To slow it down requires a force, and this force must pull opposite to the direction the magnet is moving. Conclusion: the current always produces a field that tends to slow whatever object is moving., because it must steal its kinetic energy to run the current.

In this case, the magnet is moving away from the coil, so the coil must attract it. Since the magnet has a N pole facing the coil, the coil must present a S pole to the magnet. Therefore, the current is CW as seen by someone looking back at the coil from the magnet.

Hope this helps!

--Stuart Anderson

plasma

2nd October 2006 - 03:51 PM

Thanks.