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From: neufeld@aurora.physics.utoronto.ca (Christopher Neufeld)
Subject: Re: Surviving Electrocution
Message-ID: <1991Apr15.141917.28570@helios.physics.utoronto.ca>
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References: <18678@csli.Stanford.EDU>
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Date: Mon, 15 Apr 1991 14:19:17 GMT

In article <18678@csli.Stanford.EDU> cphoenix@csli.Stanford.EDU (Chris Phoenix) writes:
>
>A sufficiently high voltage will stay on the outside of any conductor.  
>I think several tens or hundreds of kilovolts would do it.  Of course,
>when you increase the voltage, you dissipate more energy... if the pulse
>can be *very* short, you could probably make a pulse that would flash-boil
>the outermost layer of skin, and then move in *very* quickly and cool the
>remaining skin off, for something like a second-degree burn.  
>
   Not quite. A sufficiently high frequency will cause the current to
travel dominantly in a region near the outer surface of the conductor.
At DC levels you can express the current as a function of the
conductivity and potential difference across the body without having to
worry about complicating time variances, and when you do you see that
the conductor is permeated by a current density which is inversely
proportional to the resistivity when that quantity doesn't change much
over the bulk. A person's skin has a high resistivity, it's most of the
300kohms hand to hand resistance most people measure the first time they
get an ohmmeter. Once under the skin the body is primarily an ionic
solution in water. That has a very low resistance. DC to megahertz
frequency applied voltages will not drive currents only along the skin.
Most of the current will cross the skin and pass through the body to the
other contact.  High voltages are fatal at DC or at low frequencies
(read lower than a few megahertz). It's dangerous to believe otherwise.

>This is stretching the rules a bit, but it might even be possible to make 
>the pulse go through the steam rather than the solid body, so that most
>of the energy dissipated would not be in direct contact with the body.  
>You could probably maintain tens or hundreds of amps for several seconds 
>this way, and keep the person alive afterwards. 
>
   Recall that pure water has a very low conductivity, and steam is pure
water. Further, it's very difficult to get ion transfer in such a dilute
medium. In general, if a gas at atmospheric pressure is conducting
electricity it's probably a plasma. You don't want to be on the inside
of a cloud of plasma. While steam is technically a vapour, the argument
still applies.

>Reader's Digest had a "Drama in Real Life" a while ago that involved a 
>person shorting out a high-tension power line for a long time (at least 
>several seconds, I don't remember.)  She got severe burns where the spark 
>hit her, but as I recall survived with no other ill effects.

   And there are stories of people surviving falls of thirty thousand
feet. Doesn't mean I'd like to base a passenger space launch on the
principle. (That was the intent of the original question).


-- 
 Christopher Neufeld....Just a graduate student  | Flash: morning star seen
 neufeld@aurora.physics.utoronto.ca    Ad astra! | in evening! Baffled
 cneufeld@{pnet91,pro-cco}.cts.com               | astronomers: "could mean
 "Don't edit reality for the sake of simplicity" | second coming of Elvis!"