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> However, the relevant thing is what happens immediately after the
> connection is made. What happens is that the voltage of the wire decreases
> very rapidly until it approaches the voltage of the ground. Thus the
> voltage difference between the two will be 40 kilovolts only for a really,
> really small amount of time.
Exactly - and if you were to plot the resistance (between you and the wire)
over time as you made the connection, you'll probably find the
transistor-based current limiting circuit can reduce the voltage faster than
you can touch the wire...
> Thus I think I wouldn't be really wrong when I say "it's not the current
> that kills you, it's the electrical power transfer (over time)".
All the material I've read states a level of current that is dangerous, not
power. The "power" would vary depending on how good your connection to
ground was.
As for very short pulses of dangerous current, I don't know what the outcome
would be, and don't want to try it!
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Aydan <hes### [at] hendrik-sachse net> wrote:
> As for simplicity of describing something: For some things you just need a basic
> understanding of the matter discussed.
I think it would be simple enough for anybody to understand if one would
say "yes, the voltage is really high before you touch the fence, but once
you do, the voltage drops very rapidly to almost zero, so the total amount
of current which goes through your body during this process is not enough
to kill you; the problem with the wall socket is that it maintains the
voltage, and hence the current, for as long as you touch it, which will
cause serious damage".
The classical "high voltage but low current" answer doesn't make sense
and all in itself violates the U=RI formula.
--
- Warp
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On Mon, 17 May 2010 13:37:15 +0200, gregjohn <pte### [at] yahoocom> wrote:
>
> I had an argument with someone who was apparently an insider in the
> industry about passive solar cooling. He couldn't describe it to me
> in a way that didn't seem to be a gross violation of physics. It
> sounded like a heat pipe, where you heat up one end, but then because
> some heat is naturally flowing, extra heat decides to come along for
> the ride. That just didn't seem to float. I did some googling and
> the only thing I could find was the use of electric fans powered by
> solar cells.
>
> Any ideas what is physically going on here?
http://en.wikipedia.org/wiki/Solar_chimney
--
FE
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scott <sco### [at] scottcom> wrote:
> All the material I've read states a level of current that is dangerous, not
> power.
Maybe they imply that the current is maintained indefinitely, rather than
it starting high and then dropping to zero very fast. Thus there's a time
factor implied, which I think would then be measured with watts.
(Btw, the definition of "watt" is another one which is very difficult
to get any clear answer to. It doesn't help that the unit is used for at
least two, if not three, completely different physical phenomena.)
--
- Warp
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gregjohn a écrit :
> Hi. I had an argument with a friend about solar cooling.
>
> I fully understand the physics of solar photovoltaics-- photon comes in,
> electron gets pushed somewhere. I fully understand the physics of heat pipes--
> you can end up cooling something by having a phase-change object that gets to
> move around inside a pipe.
>
> I had an argument with someone who was apparently an insider in the industry
> about passive solar cooling. He couldn't describe it to me in a way that didn't
> seem to be a gross violation of physics. It sounded like a heat pipe, where you
> heat up one end, but then because some heat is naturally flowing, extra heat
> decides to come along for the ride. That just didn't seem to float. I did
> some googling and the only thing I could find was the use of electric fans
> powered by solar cells.
>
> Any ideas what is physically going on here?
>
>
Ever heard of Peltier cooling ?
You put a current through a peltier "system", which means the Joule
effect will heat the system (as it should), and one side of the peltier
is getting colder.
In fact, IIRC, the heat transfer is about 1/4 of the Joule heat, meaning
you can extract 20 W of heat with a 100 W dissipating hot side (and 80 W
of current). It's a heat-pumping apparatus based on the traversal of
connections made of different metals. From metal A to metal B, the
section is X, but from metal B to metal A (you need a lot of alternate
A/B/A/B/A/B/A... along the current path) the section is Y, so the
intensity per unit section in A to B is different from the one in B to A.
Thermodynamics (that branch of physics about heat and energies) forbid
to have a system with only two components where the cold one get colder
when the hot one get hotter (that would be against the laws, you would
need to reverse the time, something that is not possible at that scale).
But it does not stop you to have a third component which get colder
while the first cold get hotter due to the hot second one transfering
heat to the first: while the energy/heat is transfered from the hot to
the cold, it can also take away a bit from the third component.
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Fredrik Eriksson <fe79}--at--{yahoo}--dot--{com> wrote:
> http://en.wikipedia.org/wiki/Solar_chimney
If I understand the picture correctly, it's not because heat would somehow
grab some "extra heat" along the way. It's simply due to airflow: The sun's
heat is used to produce outwards airflow, which sucks air from inside the
house out.
--
- Warp
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>> All the material I've read states a level of current that is dangerous,
>> not
>> power.
>
> Maybe they imply that the current is maintained indefinitely, rather than
> it starting high and then dropping to zero very fast. Thus there's a time
> factor implied, which I think would then be measured with watts.
Current integrated over time gives an electrical charge, which is measured
in coulombs, not Watts. Anyway, you can't just state a maximum allowed
value of coulombs, because it also depends how quickly you deliver that
charge (ie the current). Of course the exact criteria of what will or will
not kill you is probably quite complex and can't be described by a single
number, but for most cases it's a good enough approximation to just state a
maximum current. For sufficiently short periods of time, you might find
that the number of coulombs is a good indicator, I don't know.
> (Btw, the definition of "watt" is another one which is very difficult
> to get any clear answer to. It doesn't help that the unit is used for at
> least two, if not three, completely different physical phenomena.)
Watts is used to measure power, or the rate at which work is done or energy
transferred. There are obviously different types of work and energy
(electrical, mechanical, thermal, chemical), so there are various
definitions of "power", all with the same units. It's very handy that they
all use the same units, as you are able to calculate the efficiency of
devices that convert energy with a simple calculation.
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> I had an argument with someone who was apparently an insider in the
> industry
> about passive solar cooling. He couldn't describe it to me in a way that
> didn't
> seem to be a gross violation of physics. It sounded like a heat pipe,
> where you
> heat up one end, but then because some heat is naturally flowing, extra
> heat
> decides to come along for the ride. That just didn't seem to float. I
> did
> some googling and the only thing I could find was the use of electric fans
> powered by solar cells.
Yes, car makers are very interested recently in using solar panels to do
cooling. The idea being that either they power fans to cool the interior of
the car while the engine is off, or more recently (with bigger and more
efficient cells) some are even trying to power some air-con system just from
solar cells.
The only other effect I could think of was similar to the "solar chimney"
already posted, it could possibly also be used in some fluid circulation
system like heat pipes. Anyway, the method seems to be getting the solar
energy to power convection, which draws in fluid from/over a cool sink.
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Am 17.05.2010 13:37, schrieb gregjohn:
> Hi. I had an argument with a friend about solar cooling.
>
> I fully understand the physics of solar photovoltaics-- photon comes in,
> electron gets pushed somewhere. I fully understand the physics of heat pipes--
> you can end up cooling something by having a phase-change object that gets to
> move around inside a pipe.
>
> I had an argument with someone who was apparently an insider in the industry
> about passive solar cooling. He couldn't describe it to me in a way that didn't
> seem to be a gross violation of physics. It sounded like a heat pipe, where you
> heat up one end, but then because some heat is naturally flowing, extra heat
> decides to come along for the ride. That just didn't seem to float. I did
> some googling and the only thing I could find was the use of electric fans
> powered by solar cells.
>
> Any ideas what is physically going on here?
Probably something broadly similar to how the water in a steam boiler
can be replenished by a steam injector (you get yourself some steam from
the boiler, shoot it through some suitably designed nozzle arrangement
back into the boiler (duh!), and have it drag some cold water with it
along the way (duh again!)): It's one of those things where us
non-enlightened folks just have to trust the scholars of the arcane art
of thermodynamics that their black magic works for real.
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Am 17.05.2010 14:36, schrieb Warp:
> 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*
That's a wrong assumption already: Cattle fence "generators" and the
mains have much different internal resistance; in the case of mains, the
human body resistance is probably dominating, but with a cattle fence,
the internal resistance is.
Also note that the cattle fence does /not/ actually produce DC, but
rather a pulsed AC; with the pulses being very short and sharp spikes,
each pulse is dominated by very high frequencies, at which currents tend
to travel close to the surface of a conductor. If that conductor happens
to be a human, this means that the pulse will travel close to the skin,
staying clear of vital organs. (Unless one of these organs happens to be
an artificial pacemaker or the like, which are usually implanted close
to the skin.)
All in all, I'm pretty sure it's a combination of various effects that
makes cattle fences much safer than mains current, so /any/ answer
mentioning only a subset of these can be considered wrong when running
in nitpicking mode.
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