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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?
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gregjohn <pte### [at] yahoo com> wrote:
> He couldn't describe it to me in a way that didn't
> seem to be a gross violation of physics.
Somehow that seems to be pretty common with certain subject matters.
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).
Or ask any physicist about the candle-in-a-glass experiment: You have
a plate with some water, and a candle in the middle. Light the candle,
put a drinking glass upside down on the plate so that the candle is inside.
When the candle goes out, the water will be sucked inside the glass. Why?
You will get approximately as many different answers as people you ask,
none of which will be correct.
--
- Warp
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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.
--
Best Regards,
Stephen
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> 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).
An electric fence has a very low value current limiter (of the order of a
few milliamps), whereas you can get 10's of amps from wall sockets.
U=RI is never violated, because a current limiting circuit will control the
voltage to ensure the current limit is not exceeded. If you connected a
voltmeter to the electric fence you would see the voltage drop significantly
when you touched it. BTW this is exactly how an LED driver circuit works,
because they need a constant current supply, rather than voltage.
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Warp <war### [at] tagpovrayorg> wrote:
> gregjohn <pte### [at] yahoocom> wrote:
> > He couldn't describe it to me in a way that didn't
> > seem to be a gross violation of physics.
>
> Somehow that seems to be pretty common with certain subject matters.
>
OK, I'll try my best ;o)
> 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).
>
The cattle fence works with a defined charge at a high voltage, which will
result in a
quick pulse (only a few milliseconds) with very low overall energy. This doesn't
screw up your nerve system too much.
Mains current (230Vac) will keep your nerve system jammed long enough (your time
of contact will usually be much longer than a few milliseconds) so your heart
stops beating.
>
> Or ask any physicist about the candle-in-a-glass experiment: You have
> a plate with some water, and a candle in the middle. Light the candle,
> put a drinking glass upside down on the plate so that the candle is inside.
> When the candle goes out, the water will be sucked inside the glass. Why?
> You will get approximately as many different answers as people you ask,
> none of which will be correct.
>
> --
> - Warp
When you put the glass over the candle, the air inside the glass will be warmer
than the environment. When the candle has used up all the oxygen, the air will
cool down and the inside pressure sinks, sucking in the water from the outside.
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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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scott <sco### [at] scottcom> wrote:
> An electric fence has a very low value current limiter (of the order of a
> few milliamps), whereas you can get 10's of amps from wall sockets.
> U=RI is never violated, because a current limiting circuit will control the
> voltage to ensure the current limit is not exceeded. If you connected a
> voltmeter to the electric fence you would see the voltage drop significantly
> when you touched it. BTW this is exactly how an LED driver circuit works,
> because they need a constant current supply, rather than voltage.
I think that what your explanation lacks is a description of how the
voltage (and hence the current, as 'R' doesn't change, as in this case
it's the resistance of the human body which is connecting the wire to
the ground) changes over time when the connection is made.
If the wire is at 40 kilovolts and the ground is at 0 volts, when a
person connects them through his body, the current 'I' will be quite high,
because it will be I = 40kV/R, which is much higher than with a wall socket,
where it's I = 230V/R.
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.
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)".
--
- Warp
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On 17/05/2010 1:36 PM, Warp wrote:
> (If your answer would have been "the fence has a limited power transferring
> capacity", that would have probably been closer to the truth
So you know the truth?
Then why ask if you know the answer?
When people ask that question the answer is generally given in
simplistic terms.
--
Best Regards,
Stephen
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Aydan <hes### [at] hendrik-sachsenet> wrote:
> When you put the glass over the candle, the air inside the glass will be warmer
> than the environment. When the candle has used up all the oxygen, the air will
> cool down and the inside pressure sinks, sucking in the water from the outside.
Yeah. You wouldn't believe how convoluted explanations I have seen.
--
- Warp
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Stephen <mca### [at] aoldotcom> wrote:
> On 17/05/2010 1:36 PM, Warp wrote:
> > (If your answer would have been "the fence has a limited power transferring
> > capacity", that would have probably been closer to the truth
> So you know the truth?
> Then why ask if you know the answer?
I didn't ask. I was telling it as an example where you often can't get
a straightforward understandable explanation even from professionals.
> When people ask that question the answer is generally given in
> simplistic terms.
The most common one being "the voltage is high, but the current is low",
which don't make sense.
--
- Warp
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