A magnet is hung by a string and then placed near a wire as shown. When the swit

Question

A magnet is hung by a string and then placed near a wire as shown. When the switch is closed, the magnet rotates such that the ends of the magnet move as indicated by the arrows.

At the instant the switch is closed determine...

1. The direction of the current through the wire segment nearest the magnet. Explain.

2. The direction of the net force exterted by the magnet on the wire segment at the instant that the magnet is in the position shown. Explain.

***Note, saying "because of the RHR" is NOT an explanation.

Explanation / Answer

First you analyze the situation based on how the magnet rotates initially. In the perspective view it moves towards us.

Next we will use that information to conclude which side of the loop is "N" and "s" pole, there will be a magnetic field moving through the middle of the loop.

Since the south end of the magnet moves towards us, we can conclude that the "N" end of the loop is also towards us, since "n" and "s" attract each other.

You should have concluded in class, another right hand rule regarding loops. In this case we want the N pole of the magnetic field through the loop to be towards us. The rule is take your right hand and move your fingers in the direction of the current around the loop, and your thumb will point to the N pole of that field. So in this case your fingers will move counterclockwise in order to have a "N" pole pointing towards us to attract the "s" end of the magnet.

answer: Current move counterclockwise

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