# Homework Help: Power Loss Of An EM Wave Hitting A Cu Wire

1. Sep 6, 2010

### H_man

Hi all, :uhh:

I am trying to calculate the loss in power of a wave which is partially

reflected and partially transmitted from a wire.

The skin depth of the wire is many times the thickness of the wire.

My first attempt to solve this problem was to consider seperately the

energy lost due to currents induced by the electric field and then the

magnetic field.

I started by calculating the skin depth of the Cu wire in order to

calculate the effective resistance at the frequency of the wave.

I then thought to calculate the current I, by first taking J0=sigma.E,

using the electric field derived from the Poynting vector and the

conductivity of Cu. Then integrating with respect to area bearing in

mind that J falls off exponentially from the surface.

Finally to use P=I2R to determine power loss from the electric field.

The answer I got was non-sensical.

When then trying to calculate the losses from the changing magnetic

field I found the helpful expression:
I/l = H (where I is the current, l the length of the field in the

conductor and H the field strength).

The book didn't show any calculation relating to the electric field

which makes me think I've been barking up the wrong tree and in fact the

only losses which occur are entirely due to the magnetic field. However,

I find this very very confusing, why should one field do work and not

the other????

I think I am possibly missing some basic physical knowledge :surprised
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution