# Copper rod vs. bundle of wires in a magnetic field

1. Apr 9, 2012

### tigrathi

I was wondering what the difference in current is if there is one when you have a bundle of copper wire passing through alternating magnetic fields verses a solid copper rod of the same diameter. Will the rod have more current because there is more copper? Would it have less because more copper means more residual magnetic field when the polarity switches? Or would it make no difference at all?

2. Apr 9, 2012

### Bob S

Eddy currents will be generated in the rod.

3. Apr 9, 2012

### tigrathi

Wouldn't the same eddy current be produced in each wire of the bundle just in smaller amounts, and when all of those small amounts are added up won’t they equal the current in the rod?

4. Apr 10, 2012

### Bob S

Actually not.
Consider a small differential current loop in the copper of radius r and width dr. The induced voltage (Faraday induction) is proportional to πr2 and the length of the loop is proportional to 2πr, so the induced current i is proportional to ρr/2 (where ρ is the resistivity of copper), and the dissipated power to i2ρ=ρr2. So larger eddy current loops lead to more power dissipated in the copper. This is why transformer iron is lamiated (see http://wiki.answers.com/Q/What_is_the_purpose_of_laminating_an_iron_core_in_transformers), and why Litz wire (see http://en.wikipedia.org/wiki/Litz_wire) is used in ac coils. To minimize losses in the Litz wire, each strand is covered with a thin insulating coating.

Last edited: Apr 10, 2012
5. Apr 10, 2012

### tigrathi

Perhaps an explanation of what made me ask this question could help. I was looking at all the different designs of axial flux generators, and nearly all of them use an "O" shaped coil of wire on one side of a magnet. I was thinking why would they not use a "C" shape coil and use both sides of the magnet. Then I got thinking that it would be kind of hard to route all of that wire. I was thinking one could use a "C" shaped piece of iron and put the coil on that, but it would cause too much feedback when the next opposite polarity magnet passes by. So I was thinking to use copper instead of iron & just connect the ends and use that as the coil to get around the feedback.