# A couple conceptual Magnetism questions

## Homework Statement

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:
a) the direction of the current through the wire segment nearest the magnet.
b) the direction of the net force exerted by the magnet on the wire segment at the instant that the magnet is in the position shown.

Right hand rule

## The Attempt at a Solution

For a) I know that the B field must be pointing in the opposite direction of magnet because the south pole vector points toward the bar magnet for its self. And by the rotation it's being repelled. By pointing my thumb in the direction of current and curling my fingers around the wire so that they end up away from the bar magnet I find that the current must be flowing counterclockwise.

For b) the net force must be in the right direction because the field lines of the south pole, which is closest to the wire, point in the right direction.

Yay or nay? Thanks for any help.

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## Answers and Replies

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PeterO
Homework Helper

## Homework Statement

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:
a) the direction of the current through the wire segment nearest the magnet.
b) the direction of the net force exerted by the magnet on the wire segment at the instant that the magnet is in the position shown.

Right hand rule

## The Attempt at a Solution

For a) I know that the B field must be pointing in the opposite direction of magnet because the south pole vector points toward the bar magnet for its self. And by the rotation it's being repelled. By pointing my thumb in the direction of current and curling my fingers around the wire so that they end up away from the bar magnet I find that the current must be flowing counterclockwise.

For b) the net force must be in the right direction because the field lines of the south pole, which is closest to the wire, point in the right direction.

Yay or nay? Thanks for any help.
Applying newton's third law, I think the force on the wire should be to the left.

due to magnetic effects, the wire pushes on the magnet, so the magnet pushes on the wire. The magnet is free to move, so it does - and presumably in the direction of the force.
So the magent is being pushed to the right, meaning the wire must be pushed to the left. Of course if the wire is not free to move it won't move to the left, but it will still be pushed that way.

Now part 1: the current should be up, as the field of the magnet goes into a South pole so if I point the fingers of my right hand at the magnet, and have my thumb directe up, then my palm will be pointing left - the direction of the push.

An upward current in the wire would correspond to what you called an anticlockwise current.

Of course I could be wrong

PeterO, I agree with you again. I looked at this and imagined what would happen to the wire.... The force is in the opposite direction to the force on the magnet.
I used Fleming's left hand rule for the force on a current in a magnetic field.
With the first finger pointing into the S pole as the field direction and the thumb pointing basically to the left for the force on the wire gives the second finger (current) pointing 'up'.