# Electromagnetic induction question

1. May 16, 2012

### aznking1

1. The problem statement, all variables and given/known data

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This is the picture. Sorry i cant add a link cause i do not have 10 posts.

The part that i dont understand is why the aluminium ring takes a longer time to fall when an alternating current is used

3. The attempt at a solution

Take this scenario. Lets say the alternating current is flowing in the coil, and a North pole is induced at the top of the coil and South pole induced at the bottom of the coil.

Since it is alternating, when the current in the coil decreases, the magnetic field and hence magnetic flux through the aluminium ring decreases since it is linked with a soft iron rod.

By lenz's law, the aluminium ring would want to oppose the change, thus a north pole is induced at the top and south pole induced at the bottom of the ring to increase the flux through the ring.

Since unlike poles attract, why does the ring take a longer time to fall?? I know when the current increases the aluminium ring will be repelled but since it is attracted when the current decreases why does it still take a longer time to fall?

Your help is greatly appreciated. Thanks!

Last edited: May 16, 2012