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Re: [Phys-L] Current or Voltage is the thing that kills?

I always say current kills, but voltage produces current (V = IR). You an
have V without I, but not I without V. That's why we see signs that say,

On Tue, Dec 18, 2018 at 8:27 AM Peter Schoch <> wrote:


I have always used the phrase "current kills" when discussing electrical
safety for my lab safety lectures. One of my references is:

Now, one of my students has found:

that contradicts it. In response to this video, a thoughtful student wrote
me the following:
" are my calculations and conclusions: ohms law: [image: I={\frac
{V}{R}},] states that current is equal to voltage divided by resistance.
This means that current is limited by by the voltage of the circuit and the
inverse of the resistance of the circuit. That being said, humans are not
good conductors, relative to metal such as in wires. In fact, a typical
resistance for 'human' is anywhere from 500(very wet skin)- 100,000(dry
skin) Ohms. For comparison, a typical wire is usually around the .1 - .01
ohm range, meaning that there is a minimum of 3 orders of magnitude
difference between the resistance of wet skin (e.g. your tongue) and even a
bad wire. Thus, if you have a power supply producing 10 volts, even if the
current it can produce is infinite, the maximum that could run through your
tongue is given by: [image: LaTeX:
I=\frac{10}{500}=.02\:amps\:=\:20\:miliAmps]I=10500=.02amps=20miliAmps This
is enough to hurt and even cause strong muscle contractions, but it isn't
enough to kill, which is 60 miliAmps (for DC it is actually higher than
this), and this is on your tongue. In fact, in the video, this is exactly
what Mehdi Sadaghar does to himself. However, for dry or even mildly damp
skin, this value would be < 1 miliAmp, which would not even be felt. So
while it is in fact the current that kills you, saying only that "current
kills, not voltage" is similar to saying that cancer doesn't kill people,
organ failure does. While this isn't strictly wrong, it isn't completely
true either, and it can give people the wrong impression. This would also
explain why warning signs say 'High voltage' not 'High Current'."

While I agree that you can't have a current without a sufficient voltage to
move it, they don't 'discover' that in lab until the second or third week
(the first week or two being spent on just familiarizing themselves with
the various peices of equipment and what they do).

Ultimately, my question is -- Should I stay with my simple "current kills"
message to drive home the necessity for safety; or, should I ammend it in
some way?

Peter Schoch
Forum for Physics Educators



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