Stephen <mca### [at] aoldotcom> 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 
> contract and relax become disorganised. I???m sure Andrel can explain it 
> better as it is his subject.

  Well, that's precisely what I'm talking about. You are somehow assigning
some magical properties to the basic "U=RI" formula by making odd statements
like "cattle fences have a limited amount of current".

  If we solve how much current a certain voltage difference produces, it's
I = U/R. Here the resistance 'R' is the resistance of the human body. If we
assume that this resistance is the same in both situations, then the *only*
thing which affects current 'I' is the voltage difference 'U'. Clearly if
'U' is 230 volts, the current 'I' will be much lower than if 'U' is 40000
volts, in which case 'I' will be almost 200 times larger.

  There's no "limited current". If the voltage difference is 40 kilovolts,
the current will be almost 200 times larger than of the voltage difference
is 230 volts.

  The correct answer has to do with what happens once the human body
connects the fence to the ground. At that instant the current will indeed
be 40kV/R, but what happens then?

  (If your answer would have been "the fence has a limited power transferring
capacity", that would have probably been closer to the truth.)

-- 
                                                          - Warp

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