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Re: math woes



Original poster: "Jan Florian Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

> I want to find the skin depth penetration of certain diameters of human body
> parts in centimeters(thumb, chest) using a resistivity of 40 ohm-meters for
> skin covered flesh and then 70 for open flesh, and using 10,000hz.

You should always use SI units, not inches or other stuff... ;o)

Skin depth actually comes from a formula that states field strength at a
certain depth in a lossy material. Skin depth is defined to be
the depth where the field drops to 1/e=1/2.7182... so there IS current
flowing deeper than this!!

Anyway lets forget about this horrible EM-theory stuff and come to the
formula:

  skindepth = SQRT(2*resistivity) / SQRT(2*pi*freq*my)
  
Resistivity you got, and frequency too. 
Just plug them in into the formula. 

Now "my" (mangetic permeability) is something I'm not so sure of - for
free space it would be defined as 4*pi*10^-7, but I don't know if this is
100% true for a human body. Probably not... anyway it wouldn't be too far
off from the true value so why not use it anyway?

So you write 4*pi*10^-7 (i.e. [pi][*][4][E/EXP][7][+-] on the calculator)
for my. (10^1=10, 10^2=100, 10^3=1000, 10-1=0.1, 10^-2=0.01, you get the
trend? ;o)

For 100kHz you write 100 000 Hz

The resulting distance, as always with SI units, is in metres, so you
multiply the value by 100 to get centimetres ("cent"=1/100 so 1m=100cm).

cheers,
  - Jan

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