# Magnets and Eddy Currents

Q:
When a neodymium magnet is dropped down a copper tube, what happens to gravity's constant and how do I find it from there? I want to know the change, if any, that eddy currents place on acceleration.
- Daniel (age 16)
vienna,VA, USA
A:
As the magnet starts to fall, it does induce eddy currents in the copper. These create a form of friction, since current flowing through a conductor heats up the conductor. That friction builds up as the falling magnet speeds up. It gets close to the value where the frictional force balances the gravitational force, so the acceleration almost ceases. If you give the magnet a little shove so that it travels at just the speed where the eddy current friction balances gravity, the acceleration will be zero. That's the upper limit to the speed the magnet reaches after you drop it.

I'm not sure how you could go about calibrating these effects to determine the local strength of gravity. That seems easier to do by just timing free fall or pendulums, since there are fewer complications and unknowns. Maybe I'm missing something.

Mike W.

(published on 01/07/2013)

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