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Re: Watts or Amperes





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Hi al,
I thing there was a discussion some time ago
on that topic but I cannot find any file refering
to this:

Current kills! What is critical in determining the effect of electrical
contact, is the amount of current that flows through the person. Damage
occurs at about the 20mA level and currents above 100mA are usually fatal
-- disrupting normal muscle control for both the heart and lungs. The
amount of current that flows can be estimated reasonably well with Ohm's
law although the situation can become more complicated (such as people
getting hit by lightning but the current taking a surface path rather than
going through the body).

The 'normal' hand to foot resistance for a dry person is about 100 kohms
while a wet person can have a resistance on the order of only 1 kohm. Of
course for shorter paths (say hand to hand) the resistances would be lower.

So, first the voltage source must be able to produce a current (via Ohm's
law) large enough to be dangerous, but second, the source must be able to
sustain that current long enough (especially at lower currents) for the
situation to be lethal. (If a short burst of current sends the heart into
fribrillation or into cardiac arrest, then there is a _serious_ problem
even if the current is stopped). Considering currents under 100 mA though,
the ultimate damage also depends on the time current flows and this will
be partly determined by the current capacity of the voltage source. Hence
a car battery is much more dangerous that a flashlight battery even if both
have the same voltage rating. With enough current and a long enough
exposure, then body-cells will be burned--but this is serious only at high
currents > 1A where cardiac arrest usually happens (but CPR can be
effective) if burn damage is excessive . This is a question of energy
absorbed. How quickly that happens then involves power, but the MAJOR
concern is alway the amplitude of current through the body.


Why does it kills you when touching for instance
the electrode of a car battery? (12V & 30-400A)
Is it the high current the can supply or the
power? may be both thing depending on the quantity
of watts or amperes the can supply us when we touch
the battery?

My believe was that our body-cells get burned hence
power was responsible.

Regards
msantos@etse.urv.es

*****************************************************
Richard W. Tarara
Department of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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