Eddy currents in action

[Joe Gwinn]

The reason the plate takes so long to fall in the MRI machine's magnetic field is that the motion causes eddy currents whose magnetic fields oppose the original magnetic field (Lenz's Law).

http://www.youtube.com/watch?v=fxC-AEC0ROk

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[NightWinder]

so, resistance.........the plate was repelling against the magnet?

[Joe Gwinn]

so, resistance.........

Eh??

the plate was repelling against the magnet?

Yes. Well, the motion of the plate in a magnetic field caused eddy currents to be generated in the plate, and by Lenz's Law, these currents flow in the direction to generate a magnetic field opposing the original magnetic field. So the plate is in effect pushing against the oiginal field. Once the plate stops moving, no more force.

You can also see this by letting a strong magnet slide down a copper or aluminum plate set at an angle. Or drop a rare earth magnet down a piece of copper waterpipe. It will fall very slowly.

[Sweetfinger]

I thought I was too old...

..to learn new stupid bar tricks! I've just been dropping magnets down everything in the shop. Freakin' COOL!

[David Schwab]

This is also how the breaks work on roller coasters.

[erikbojerik]

I got your eddy currents right here.