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Q & A: induction and eddy currents

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Most recent answer: 07/18/2018
Q:
In reading about traffic detectors, most sources state that "the induction of the buried wire loops decreases due to eddy currents induced in the car above them". But I thought induction was a property of the geometry of the loops. How do the external eddy current in the car body change the induction of the loops?
- Bob Davis
46989
A:

You can think of the coil's induction L as a measure of how much magnetic field energy a current I through the loop makes, LI2/2. The magnetic field energy density is proportional to the square of the local field. At very low frequency those metal plates don't affect L because very small eddy currents are induced in them, so the magnetic field is nearly the same as it would be if they weren't there. At higher frequency, the eddy currents make their own fields that largely cancel the fields directly from the coil. So they reduce the total magnetic field energy, and reduce the effective L at those frequencies.

I once encountered this effect rather dramatically in the lab. We were trying to apply an ac field to a sample, but didn't see the effects we expected. The sample was mounted on a cold piece of pure copper, a very good conductor. The field from its eddy currents was canceling the field we were applying.  Replacing the copper with a worse conductor fixed the problem.

Mike W.


(published on 07/18/2018)

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