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] 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.

  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 
contract and relax become disorganised. I’m sure Andrel can explain it 
better as it is his subject.

-- 

Best Regards,
	Stephen

Post a reply to this message

>  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.

Post a reply to this message

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.

Post a reply to this message

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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Warp wrote:

>   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.

Static electric shocks regularly run into the kilovolt range. So why 
don't they kill people? I assure you, if you hang somebody between a 
pair of 40 kilovolt overhead power lines, their body will be reduced to 
ash - possibly explosively.

The difference is that a static shock lasts for an infintesimal fraction 
of a second. That's how long it takes the two charges to neutralise each 
other. If you touch a power line, however, there's a giant generator 
replacing the charge as fast as you can neutralise it, causing a 
_continuous_ current flow.

It's current that kills you, not potential difference or anything else.

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>> 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.

Presumably they mean that the maximum current is low. If you neutralise 
the difference in charge faster than the system can replace it, the 
voltage *changes*. So U=RI is not violated, yet the system *does* have a 
maximum currentl.

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Warp <war### [at] tagpovrayorg> wrote:
>
>   The most common one being "the voltage is high, but the current is low",
> which don't make sense.
>
> --
>                                                           - Warp

It does make sense if you know that a voltage source always has a series
resistance. The series resistance of the high voltage generator is in the kilo
ohms, whereas the series resistance of a mains supply is in the single digit
ohms or below.

Post a reply to this message

Invisible <voi### [at] devnull> wrote:
> >> 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.

> Presumably they mean that the maximum current is low. If you neutralise 
> the difference in charge faster than the system can replace it, the 
> voltage *changes*. So U=RI is not violated, yet the system *does* have a 
> maximum currentl.

  This is exactly what I'm talking about. It's impossible to get a clear,
straight, understandable answer from anybody.

  I = U/R has no "maximum". If you increase 'U', 'I' will increase likewise,
when 'R' stays the same. The relationship is linear. There's no "maximum".
There's no point after which when you keep increasing 'U' but 'I' doesn't
increase because it has reached some magical "maximum".

  If 'U' is 1 volt, then 'I' will be 1V/R. If 'U' is one billion volts, then
'I' will be 1 billion volts / R. There is not magical "maximum".

-- 
                                                          - Warp

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Invisible <voi### [at] devnull> wrote:
> It's current that kills you, not potential difference or anything else.

  No, it's not the current that kills you. It's the power transfer.

  You yourself said it: Even if the current is tens of kilovolts, it may
still not kill you.

-- 
                                                          - Warp

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Aydan <hes### [at] hendrik-sachsenet> wrote:
> >   The most common one being "the voltage is high, but the current is low",
> > which don't make sense.

> It does make sense if you know that a voltage source always has a series
> resistance. The series resistance of the high voltage generator is in the kilo
> ohms, whereas the series resistance of a mains supply is in the single digit
> ohms or below.

  But the voltage difference is between the wire and the ground, and
there's nothing else between them than the body of the person touching
the wire.

  The current *is* very high. But only for a really small amount of time.
It's not the current that kills you (as the saying goes), but the power
transfer (the amount of current over time). I think the unit is Watt.

-- 
                                                          - Warp

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>   I = U/R has no "maximum".

As an abstract equation, no.

In a real system, if you decrease R far enough, eventually U starts to 
decrease too, in each a way that I remains fixed. Thus I has a "maximum 
value".

Put simply, there is a limit to how fast the system can replace the 
charge you're neutralising. It's no different than the fact that a water 
pump has a maximum flow rate, no matter how slight the resistence of the 
water pipes becomes.

Post a reply to this message

Invisible <voi### [at] devnull> wrote:
> Put simply, there is a limit to how fast the system can replace the 
> charge you're neutralising.

  You are not talking about U=RI here (not solely). You are talking about
how the voltage/current changes over time. There's a different unit for
that. And that's the key for the answer.

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
> Aydan <hes### [at] hendrik-sachsenet> wrote:
> > >   The most common one being "the voltage is high, but the current is low",
> > > which don't make sense.
>
> > It does make sense if you know that a voltage source always has a series
> > resistance. The series resistance of the high voltage generator is in the kilo
> > ohms, whereas the series resistance of a mains supply is in the single digit
> > ohms or below.
>
>   But the voltage difference is between the wire and the ground, and
> there's nothing else between them than the body of the person touching
> the wire.
>
>   The current *is* very high. But only for a really small amount of time.
> It's not the current that kills you (as the saying goes), but the power
> transfer (the amount of current over time). I think the unit is Watt.
>
> --
>                                                           - Warp

The thing with the series resistance still holds true. The series resistance is
inside the device. If you measure the voltage with a voltmeter, the voltmeter's
resistance is usually several gigaohms, so there will not be a noticeable
voltage drop.
In the special case of the cattle fence there's the question of how the high
voltage is generated. Usually it's done with sort of a transformer. The ratio of
the transformer then is something like 1 : 5000 and the primary is driven by a
12V battery. There's a timer in the device that switches the primary on and off.
The current through the primary will increase "slowly" (kinda) because of the
inductance of the coil. But when you switch the supply off, the current wants to
keep going (magnetic energy stored in the coil) and be discharged over a sparc
gap. The rapid drop in current in the primary will induce a much higher current
in the secondary. The energy is very limited as well as the source impedance
(read resistance) because the secondary coil will have to have a much thinner
and longer wire (higher resistance) because of the many more windings it needs.
Disassemble an ignition coil of a car or motorcycle if you want to know how this
looks like.

As for simplicity of describing something: For some things you just need a basic
understanding of the matter discussed.

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Invisible <voi### [at] devnull> wrote:
> >   I = U/R has no "maximum".
>
> As an abstract equation, no.
>
> In a real system, if you decrease R far enough, eventually U starts to
> decrease too, in each a way that I remains fixed. Thus I has a "maximum
> value".
>
> Put simply, there is a limit to how fast the system can replace the
> charge you're neutralising. It's no different than the fact that a water
> pump has a maximum flow rate, no matter how slight the resistence of the
> water pipes becomes.

Your explanation is abstract as well. ;o)
The reason for the drop in voltage is a series resistance inside the supply.
This series resistance is not necessarily constant. It might increase with
current drawn.
There's no such thing as an ideal voltage source (read unlimited current supply)
in reality.
If you short a battery for exampe it will source a maximum current. Measure that
and measure the voltage without drawing current and you get the source
impedance. If you connect an load with this resistance to the battery, you'll
get half the voltage out of it.

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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!

Post a reply to this message

Aydan <hes### [at] hendrik-sachsenet> 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

Post a reply to this message

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

Post a reply to this message

>> 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.

Post a reply to this message

> 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.

Post a reply to this message

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.

Post a reply to this message

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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Am 17.05.2010 15:19, schrieb Warp:
> Aydan<hes### [at] hendrik-sachsenet>  wrote:
>>>    The most common one being "the voltage is high, but the current is low",
>>> which don't make sense.
>
>> It does make sense if you know that a voltage source always has a series
>> resistance. The series resistance of the high voltage generator is in the kilo
>> ohms, whereas the series resistance of a mains supply is in the single digit
>> ohms or below.
>
>    But the voltage difference is between the wire and the ground, and
> there's nothing else between them than the body of the person touching
> the wire.
>
>    The current *is* very high. But only for a really small amount of time.
> It's not the current that kills you (as the saying goes), but the power
> transfer (the amount of current over time). I think the unit is Watt.

Note that the probability that you're making contact with the cattle 
wire at the very moment that a pulse has just started is /very/ low.

In most cases, the person is already connected when the generator 
"fires", so the voltage will not even build up to the maximum of 
/static/ electricity the generator could provide.

Post a reply to this message

Am 17.05.2010 14:44, schrieb Warp:

>    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)".

No, normally not.

Electrical power transfer /per se/ can kill by either electrolysis or 
excessive heat.

Most people killed by electricity die of cardiac arrest though, which is 
caused neither by current nor by voltage, but primarily by a "suitable" 
frequency (though of course a certain V=I*R threshold must be exceeded 
as well).

Mains does not provide enough power to do much harm via /power 
transfer/, even with prolonged exposure: While it typically does cause 
burns at the point of contact, it takes high-voltage lines to inflict 
really serious burns - and even then they're not necessarily fatal.

Post a reply to this message

"Fredrik Eriksson" <fe79}--at--{yahoo}--dot--{com> wrote:
> > Any ideas what is physically going on here?
>
> http://en.wikipedia.org/wiki/Solar_chimney
>

Thanks. This has to be what is happening. What I thought I heard described to me
didn't involve open air flow but a closed pipe.  Either the guy didn't explain
it well or I misheard.

I do have two patents on thermoelectric (Peltier) cooling of electronic devices,
FWIW.

Post a reply to this message

Warp wrote:
> because it will be I = 40kV/R, which is much higher than with a wall socket,
> where it's I = 230V/R.

You're assuming the same R in both cases here, also. However, with a wall 
socket, R is the resistance of the human, while with the fence, R is the 
resistance down to the ground, which includes shoes, grass, perhaps gloves, 
etc. Also, if you grab the fence with both hands, you're not going to get 
current across your heart; at least, not like you do with a socket.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

clipka <ano### [at] anonymousorg> wrote:
> Note that the probability that you're making contact with the cattle 
> wire at the very moment that a pulse has just started is /very/ low.

  You make it sound like touching the fence is about the same as touching
a 1.5-volt battery poles. Wouldn't that kind of defeat the whole purpose?

-- 
                                                          - Warp

Post a reply to this message

Warp wrote:
> clipka <ano### [at] anonymousorg> wrote:
>> Note that the probability that you're making contact with the cattle 
>> wire at the very moment that a pulse has just started is /very/ low.
> 
>   You make it sound like touching the fence is about the same as touching
> a 1.5-volt battery poles. Wouldn't that kind of defeat the whole purpose?

The farmer doesn't want to kill the cow. He wants to tingle the cow enough 
that the cow learns not to lean against the fence.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

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 
> contract and relax become disorganised. I’m sure Andrel can explain it 
> 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?

Post a reply to this message

Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > clipka <ano### [at] anonymousorg> wrote:
> >> Note that the probability that you're making contact with the cattle 
> >> wire at the very moment that a pulse has just started is /very/ low.
> > 
> >   You make it sound like touching the fence is about the same as touching
> > a 1.5-volt battery poles. Wouldn't that kind of defeat the whole purpose?

> The farmer doesn't want to kill the cow. He wants to tingle the cow enough 
> that the cow learns not to lean against the fence.

  Hence 1.5 volts isn't going to do it.

-- 
                                                          - Warp

Post a reply to this message

On 5/17/2010 5:07 AM, 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
> contract and relax become disorganised. I’m sure Andrel can explain it
> better as it is his subject.
>
Mind, a 9 volt battery will do the same thing, but you have to have 
direct contact to your insides, since the skin has too much resistance. 
Apparently, there is a standing rule on ships that you *do not* stab 
yourself with leads, connected to a 9 volt battery, precisely due to 
some moron trying this. Then again, I may just be propagating a rumor of 
something that happened. But, someone who was in the military told me 
about it.

-- 
void main () {
   If Schrödingers_cat is alive or version > 98 {
     if version = "Vista" {
       call slow_by_half();
       call DRM_everything();
     }
     call functional_code();
   }
   else
     call crash_windows();
}

<A HREF='http://www.daz3d.com/index.php?refid=16130551'>Get 3D Models, 
3D Content, and 3D Software at DAZ3D!</A>

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On 17/05/2010 8:21 PM, andrel wrote:
> 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 note I was taught that it was important to remove any metallic 
body jewellery from around the chest area before using a defibrillator. 
Ouch!
Another point about electrocution is that mains voltage is not always 
fatal. I’ve had more 240V shocks than I can remember several 415V shocks 
and one 800V dc that really woke me up. Insulating footwear is a 
lifesaver. :-D


-- 

Best Regards,
	Stephen

Post a reply to this message

On 17-5-2010 22:23, Patrick Elliott wrote:
> On 5/17/2010 5:07 AM, 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
>> contract and relax become disorganised. I’m sure Andrel can explain it
>> better as it is his subject.
>>
> Mind, a 9 volt battery will do the same thing, but you have to have 
> direct contact to your insides, since the skin has too much resistance. 
> Apparently, there is a standing rule on ships that you *do not* stab 
> yourself with leads, connected to a 9 volt battery, precisely due to 
> some moron trying this. Then again, I may just be propagating a rumor of 
> something that happened. But, someone who was in the military told me 
> about it.
> 
Applying a 9 volt battery directly on the heart is the standard way to 
induce fibrillation. I wasn't aware that somebody tried this on himself 
without the backup of a heart-lung machine.

Post a reply to this message

Stephen wrote:
> On 17/05/2010 8:21 PM, andrel wrote:
>> 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 note I was taught that it was important to remove any metallic
> body jewellery from around the chest area before using a defibrillator.
> Ouch!
> Another point about electrocution is that mains voltage is not always
> fatal. I’ve had more 240V shocks than I can remember several 415V shocks
> and one 800V dc that really woke me up. Insulating footwear is a
> lifesaver. :-D

I was told it was all metal jewelery, but that could also be caused by
the use of electric scalpels and the possibility of ground issues.

That is really not what you want to hear from a surgical nurse or
anesthesiologist, that the hospital might have grounding issues.

Post a reply to this message

On Mon, 17 May 2010 17:52:32 -0400, Sabrina Kilian wrote:

> I was told it was all metal jewelery, but that could also be caused by
> the use of electric scalpels and the possibility of ground issues.

Mythbusters tested this one - their result (by no means definitive, 
though) was that a burn could happen only if the defibrillator was used 
incorrectly.

http://mythbustersresults.com/episode88

Jim

Post a reply to this message

Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Warp wrote:
>>> clipka <ano### [at] anonymousorg> wrote:
>>>> Note that the probability that you're making contact with the cattle 
>>>> wire at the very moment that a pulse has just started is /very/ low.
>>>   You make it sound like touching the fence is about the same as touching
>>> a 1.5-volt battery poles. Wouldn't that kind of defeat the whole purpose?
> 
>> The farmer doesn't want to kill the cow. He wants to tingle the cow enough 
>> that the cow learns not to lean against the fence.
> 
>   Hence 1.5 volts isn't going to do it.

Yes, but a very brief high spike, which is what you were responding to, 
*will* do it if the cow is pressing against the fence for several seconds.

He didn't say 1.5 volts. *You* said 1. volts. He said very short pulses.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

Post a reply to this message

On 05/17/10 06:13, Warp wrote:
> Invisible <voi### [at] devnull> wrote:
>> It's current that kills you, not potential difference or anything else.
> 
>   No, it's not the current that kills you. It's the power transfer.
> 
>   You yourself said it: Even if the current is tens of kilovolts, it may
> still not kill you.

	Yes, but a voltage supply in the kiloamps range just may...

-- 
A man attempting to walk around the world DROWNED today...

Post a reply to this message

On 17/05/2010 10:52 PM, Sabrina Kilian wrote:
> I was told it was all metal jewelery, but that could also be caused by
> the use of electric scalpels and the possibility of ground issues.
>

I’m thinking more “first aid” rather than hospitals where jewellery will 
be removed on admission. A burn from a necklace while painful and 
unsightly is a talking point in at the bar. But a burn on the delicate 
tissue people are piercing nowadays could be a little more painful.

> That is really not what you want to hear from a surgical nurse or
> anesthesiologist, that the hospital might have grounding issues.

No it’s not ;-)

-- 

Best Regards,
	Stephen

Post a reply to this message

On 17/05/2010 10:56 PM, Jim Henderson wrote:
> On Mon, 17 May 2010 17:52:32 -0400, Sabrina Kilian wrote:
>
>> I was told it was all metal jewelery, but that could also be caused by
>> the use of electric scalpels and the possibility of ground issues.
>
> Mythbusters tested this one - their result (by no means definitive,
> though) was that a burn could happen only if the defibrillator was used
> incorrectly.
>

It is not unknown, you know. Especially in the field.

> http://mythbustersresults.com/episode88
>
> Jim


-- 

Best Regards,
	Stephen

Post a reply to this message

And lo On Mon, 17 May 2010 13:46:29 +0100, Warp <war### [at] tagpovrayorg> did  
spake thusly:

> 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.

Just to act on the dumb side. If you use a graduated beaker with a candle  
in it, add the water, and the second beaker, and then measure the water  
height. Shouldn't the increase in temperature in the up-turned beaker  
create a high-pressure that forces the water out from under it and thus an  
increase in the water height as measured in the graduated beaker? So why  
is the level of water forced out by the high-pressure less than the amount  
'sucked' in by the low-pressure?

:-)

-- 
Phil Cook

--
I once tried to be apathetic, but I just couldn't be bothered
http://flipc.blogspot.com

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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > Darren New <dne### [at] sanrrcom> wrote:
> >> Warp wrote:
> >>> clipka <ano### [at] anonymousorg> wrote:
> >>>> Note that the probability that you're making contact with the cattle 
> >>>> wire at the very moment that a pulse has just started is /very/ low.
> >>>   You make it sound like touching the fence is about the same as touching
> >>> a 1.5-volt battery poles. Wouldn't that kind of defeat the whole purpose?
> > 
> >> The farmer doesn't want to kill the cow. He wants to tingle the cow enough 
> >> that the cow learns not to lean against the fence.
> > 
> >   Hence 1.5 volts isn't going to do it.

> Yes, but a very brief high spike, which is what you were responding to, 
> *will* do it if the cow is pressing against the fence for several seconds.

> He didn't say 1.5 volts. *You* said 1. volts. He said very short pulses.

  I didn't say he said 1.5 volts. I said that "you make it sound like it was
like 1.5 volts or something" (instead of a huge shock).

-- 
                                                          - Warp

Post a reply to this message

Phil Cook v2 <phi### [at] nospamrocainfreeservecouk> wrote:
> Just to act on the dumb side. If you use a graduated beaker with a candle  
> in it, add the water, and the second beaker, and then measure the water  
> height. Shouldn't the increase in temperature in the up-turned beaker  
> create a high-pressure that forces the water out from under it and thus an  
> increase in the water height as measured in the graduated beaker? So why  
> is the level of water forced out by the high-pressure less than the amount  
> 'sucked' in by the low-pressure?

  I didn't understand the question.

-- 
                                                          - Warp

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