Re: Skin Effect & Primary Current?

Kevin wrote:

> I'm not trying to be PC, but -no women on this list?

I recall someone figureing how deep the penetration was of
the primary current into the conductor. Could someone who
knows or remembers how to figure it, please tell me.

The skin depth is defined as:

d= square root [ 1/(pi*mu*c*f)]

where (SI units)

d=depth in meters
mu=permeability of material in Henries/meter
c= conductivity in 1/(ohm-meters) (not a DVM!)
f= frequency in Hertz

For copper mu is about the same as for vacuum since
copper is not very magnetic. So use mu=4*pix10^-7 H/m

The conductivity of copper can vary considerably
depending on purity and temperature, but one reference
suggests c=1/(1.7x10^-6 ohm-cm) = 5.9x10^7 1/(ohm-meters) 

for a typical f=100kHz

I get

d=2x10^-4 meter
 =0.0082 inches

Of course, it is not a good design rule to assume
no currents flow below this depth. Nor is it true
that currents always flow uniformly along the surface of
a conductor with this given depth at the prescribed
frequency. For more information on skin and proximity
effects you might want to look at these references:

"Analysis of eddy currents" by Stoll

"Low Frequency Electromagnetic design" by ....

I'll send along the more complete references in a
post to follow this one.

-Ed Harris