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



We are still missing the time factor. 100mA will not kill a person if the exposure is short enough as many survivors have demonstrated.
Sent from my MetroPCS 4G LTE Android device
------ Original message------From: Richard TararaDate: Tue, Dec 18, 2018 1:29 PMTo: phys-l@mail.phys-l.org;Cc: Subject:Re: [Phys-L] Current or Voltage is the thing that kills?
On 12/18/2018 10:43 AM, Paul Nord wrote:

Just to put some numbers with this--these are hand to foot, 1 second
exposures.  As JD has posted, there are other complications (frequency
for example), but these will give a rough idea.  Not sure where I got
these values (some textbook decades ago), but this is one part of the
Electricity lessons that even my Gen-Ed students paid attention to and
could answer questions on.

CurrentEffect

0-0.5 mAnone

0.5-2 mAThreshold of feeling

2-10 mAPain; muscular contractions

10-20 mAIncreased muscular effect, someinjury; above 16 mA is the ‘let go’

current above which a person cannot release held objects.

20-100 mARespiratory paralysis

100 mA –3 AVentricular fibrillation; fatal unless resuscitation occurs
immediately.

above 3 ACardiac arrest; heart can be restartedif shock is very brief;
severe burns


rwt

Working at the national lab in Los Alamos there were some signs which said,
"Danger High Current." This was on a 5 volt bus that supplied power to a
few thousand detectors. Sure, you could touch the terminals with your
hand, but drop a wrench across those terminals and it would blow up in your
face.

This leads to another point on the hazards of electricity... there are
multiple ways that it can kill you.
1) There is the obvious cooking way where lots of power is dissipated in
your flesh, raises body temperature and chars important organs. This seems
to be the way that most of you are thinking about.
2) Arc Flash - a very brief flash from a breaking contact could simply set
you on fire. Then again it may produce a hot plasma cloud in your
vicinity. The human reflex to such a shock is to inhale. Needless to say,
hot plasma is bad for the lungs. (Always inhale before you flip a circuit
breaker.)
3) Wires or stuff simply exploding.
4) Enough current to cause your muscles to tense and freeze in place. This
might not burn you, but if one of the muscles is your diaphragm, your won't
be able to breathe.
5) Stopping your heart. I'm no expert on the physiology of cardiac shock,
but I am certain that a typical AED does not provide sustained high
current. Whatever the voltage and pulse duration, it has a serious impact
on the cardiac rhythm. What other sources might cause this, I wouldn't
venture to guess.

That's probably not an exhaustive list, but it's enough to give you a
healthy respect for electrical power.

Paul

On Tue, Dec 18, 2018 at 9:12 AM Bill Norwood via Phys-l <
phys-l@mail.phys-l.org> wrote:

As in most other potentially hazardous exposures time is a factor.

If I remember correctly one can conduct 10 amperes for 1 microsecond from a
van de graff generator and be unharmed. Done it myself hundreds of times.

Bill Norwood
U of Md at College Park

On Tue, Dec 18, 2018, 8:47 AM Paul Fedoroff via Phys-l <
phys-l@mail.phys-l.org> wrote:

How about just saying Power?

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

Hello,

I have always used the phrase "current kills" when discussing
electrical
safety for my lab safety lectures. One of my references is:
https://www.physics.ohio-state.edu/~p616/safety/fatal_current.html

Now, one of my students has found:
http://www.electroboom.com/?p=450

that contradicts it. In response to this video, a thoughtful student
wrote
me the following:
"...here 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?

Thanks,
Peter Schoch
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--
Richard Tarara
Professor Emeritus
Saint Mary's College

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