A hollow copper pipe is held vertically, and a magnet is dropped down the pipe. The magnet is oriented with its north pole down. What direction do the eddy currents flow as viewed from above the pipe? If copper bar of approximately the same size and weight as the magnet is dropped down the tube instead, will it fall faster or slower than the magnet?
A) CCW above the magnet and CW below the magnet; copper bar will fall slower
B) CCW above the magnet and CW below the magnet; copper bar will fall faster
C) CW above the magnet and CCW below the magnet; copper bar will fall slower
D) CW above the magnet and CCW below the magnet; copper bar will fall faster

Question

Quizplus · Accepted Answer

When a magnet is dropped down a copper pipe, it induces eddy currents in the pipe which creates a magnetic field that opposes the motion of the magnet. The direction of the eddy currents can be determined by Lenz's Law which states that an induced current will flow in a direction to oppose the change that caused it. In this case, the magnet is moving downwards, so the eddy currents will flow in a direction to try and make the magnet fall slower. This means that there will be a clockwise (CW) current above the magnet and a counterclockwise (CCW) current below the magnet.

When a copper bar of approximately the same size and weight as the magnet is dropped down the tube, it will also induce eddy currents that will create a magnetic field that opposes the motion of the bar. However, since the bar is not a magnet, it will not create as strong of an induced current as the magnet did. Therefore, the copper bar will fall slower than the magnet due to the weaker eddy currents it induces.

A Hollow Copper Pipe Is Held Vertically, and a Magnet