Re: skin effect

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

At 08:07 AM 5/7/2003 -0600, you wrote:
>Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
>
>Hi Ryan,
>
>Skin depth is proportional to 1 / SQRT(F).  So just a wild guess and if 
>the skin depth of people is 1 meter at say 200kHz.
>
>d = SQRT( 200000 / 3300000 ) x 1 = 0.26 meters or about 10 inches.
>
>I have seen RF burns at 13.56 MHz and those burns go straight down into 
>the body as do 350kHz burns.
>
>Although "skin depth" works well for metallic conductors, "I" personally 
>think it has no meaning for a very high resistance materials like people 
>are made from.

It does have meaning, but the whole "skin depth" thing really only applies 
to an infinite homogenous slab where the dimensions are very much greater 
than the "skin depth". The homogenous slab thing is different enough from 
real life that the AC resistance calculation for tubular or round 
conductors has to take it into account unless the "infinite slab" skin 
depth is << conductor diameter.

You also use the skin depth concept when looking at electromagnetic 
propagation over a dielectric lossy medium (i.e. the earth's (sub)surface)




>Also note that microwave ovens (~2000MHz) cook large chunks of meat on the 
>inside too...  Water blocks microwaves some, but skin depth seems to have 
>no meaning there.

Blocks might be too strong a word.  It's not like a good conductor, which 
actually reflects the incident wave, but more like an absorber. The 2.45 
GHz energy is absorbed as it propagates through the meat.  More gets 
absorbed at the surface than in the middle (just because it's a classic 
diffusion kind of problem.. every cm absorbs some percentage of the 
energy.. So, using unrealistic, but round numbers, say every cm absorbs 
half the energy... At the surface 100W incident, the first cm absorbs 50W, 
so there's 50W incident on the next cm, which absorbs 25W, then 12.5...

The thermal conductivity of most food is such that the heat can propagate 
into the food.  The various "pulsed duty cycle" modes of a microwave for 
various kinds of food are designed to match the thermal input and 
dissipation distribution to the thermal conductivity (or, you can 
mecanically stir the food... which effectively diffuses the heat quicker.)


The real challenge in a microwave oven is that frozen water doesn't absorb 
nearly as well as liquid water AND liquid water has a much higher 
dielectric constant than ice, so it tends to reflect the energy 
away.  There's also some weird things that go on because the food (frozen 
or not) is comparable in dimensions to a wavelength so there are 
diffraction effects.  Calculating all of this is decidedly non-trivial.



>For all practical purposes, you can assume the RF currents go right deep 
>inside you.  The nerves simply don't react to it.  People have reported 
>nerve problems if they "overdo it".  The RF currents do seem to favor 
>flowing in the more conductive nerves and blood vessels concentrating any 
>damage to those structures.
>
>Cheers,
>
>         Terry
>
>
>At 12:27 AM 5/7/2003 -0600, you wrote:
>>My small coil actually resonates at around 3.3 MHz, does anyone know if this
>>would be a high enough frequency to cause the current to run completely over
>>the skin?
>>
>>I have taken the streamers from my coil directly to my hand (without much
>>shock sensation), and I have also done this with a coil operating at around
>>1.7 MHz, with more shock sensation. Does the amount of current in the
>>streamers depend directly on the turns ratio of the coil?
>>
>>Ryan


msnip...