# The copper plate riddle

1. May 8, 2007

### ragav

Two days ago in my physics class, one of my friends asked me a question:

"There are 2 identical circular coils with radius 'r', length 'l' and 'n' no. of turns placed with their axes lying on a straight line, and current is let to pass through them so that mutual induction is observed.What happens to the magnitude of mutual induction when a copper plate is introduced between these two coils?"

I said "The coefficient of mutual induction increases and thereby the mutual induction increases."

He said "Eddy currents are produced in the Cu plate and thereby mutual induction gets reduced".

And i still don't know which one is true.

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rAgAv

2. May 8, 2007

### lpfr

Sorry, but it is your friend who is right. The eddy currents induced in the conducting plate opposes to the penetration of time varying magnetic fields. If the copper plate is thick (and big) enough it will shield the two coils. The mutual inductance will be zero (or near zero).

3. May 8, 2007

### ragav

thanks. That helped

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