On 17-5-2010 14:07, Stephen wrote:
> On 17/05/2010 12:51 PM, Warp wrote:
>>    For example, ask anybody, an expert or layman, why you can die from
>> touching an electrical wall socket (which has at most 230 volts), but
>> usually not from an electric cattle fence (which has tens of kilovolts)
>> and you won't get a straight, rational answer. I have yet to get a
>> proper answer from anybody (which wouldn't seem to violate the basic
>> "U=RI" formula).
>>
> 
> It is all to do with current and the path it takes. Cattle fences have a 
> limited amount of current and it is dc. Domestic supplies are ac and 
> although the current is limited it is in the region of amps. It is the 
> current that kills you. More importantly id the current passed through 
> your heart the ac component disrupts the electrical signals to the heart 
> and in the heart so it starts to fibrillate that is the signals to 

> better as it is his subject.
> 

A major factor is the 50 (or 60) Hz. If you stimulate a muscle it will 
twitch. If you stimulate it again shortly after that, it will twitch a 
bit more. The way to let it contract the most is by stimulating it 
rapidly at a frequency of about, you guessed it, 50 Hz. So the frequency 
at which you need the least power to kill someone is 50 Hz. 60 is not as 
effective, but it comes close. Connected to that a bit of advise: if you 
want to test if some wire is life with your bare hands (in general: 
don't), use the backside of a finger. Then if it is, your muscles will 
contract and pull the finger away from the wire. Never use the inside of 
your finger and certainly never grasp a wire, then muscle contraction 
will increase your grip.

The best way to not kill a person is by using MHz frequencies, then you 
have the skin effect, all current flows only on the outside. If you have 
a generator that generates that sort of electricity you can safely touch 
the electrodes even if there are arcs of ten centimeters between the 
electrodes.

A defibrillator gives a short 'DC'-pulse that stimulates all cells in 
the heart at once, and resets them. Hopefully the heart will then reboot 
gracefully. As a side effect all other muscles will also contract, which 
explains the strange movements. Sometimes the patients are also a bit 
disoriented afterwards, as if awaken rudely from a dream. There have 
been occasions when we had to dive on the patient to keep him on the 
table, because they definitely wanted to step out of bed. Which is not a 
good idea if there are still all sorts of wires connected and catheters 
inserted into the bloodstream.
A well timed defibrillator pulse delivered to a not fibrillating heart 
might actually induce fibrillation. That is why you should not touch a 
person that is being defibrillated (apart from being painfull 
experience). Funnily the same holds for a hit on the chest. There are 
cases of people hit by a baseball in the chest at precisely the right 
moment at the right place that died from ventricular fibrillation. And 
OTOH the precordial thump was a procedure sometimes used before the 
defibrillator to stop fibrillation.

Does that somehow answers some questions?

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