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Author Topic: Metal which cools when current is applied?  (Read 9899 times)

Offline Chopper

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Metal which cools when current is applied?
« on: 09/05/2005 01:09:40 »
Just wondering if there was a metal which cools when an electric current is applied?


 

Offline neilep

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Re: Metal which cools when current is applied?
« Reply #1 on: 09/05/2005 03:49:31 »
Wocha Chopper !!.............I'm not an expert on this, but I can't see how passing an electric current through a metal would cool it down. Doesn't the very nature of passing electricity through metal generate heat ?

Lets hope a passing ' electricity through metal' expert can answer this.

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« Last Edit: 09/05/2005 11:40:08 by neilep »
 

Offline daveshorts

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Re: Metal which cools when current is applied?
« Reply #2 on: 09/05/2005 10:43:10 »
Due to various bits of thermodynamics, you can't make heat dissappear, just move it around with a heat pump - this is what a fridge does.

You cannot do this with a single metal, but if you have two types of metal or more effiently two types of semiconductor you can make a heat pump. If you have two metals where the electrons are at different potential energies normally.

 If you then pass a current from the low energy metal to the high energy one, only the electrons with lots of energy (the hot ones) have enough energy to get into the high energy metal. So if you keep taking away the hot electrons from the low energy metal it will get cold - this is how a peltier cooler works.

In order to make a circuit you then have to connect the high potential metal back to the cold one - here the electrons gain energy by going down the slope and release this as heat. So the system pumps heat from  one junction to the other - a heat pump
« Last Edit: 09/05/2005 18:51:39 by daveshorts »
 

Offline chris

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Re: Metal which cools when current is applied?
« Reply #3 on: 09/05/2005 18:22:26 »
Dave - wouldn't you get an electric current then, if you put together these two metals that have different electron potentials ?

Chris

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Offline Tronix

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Re: Metal which cools when current is applied?
« Reply #4 on: 09/05/2005 18:38:04 »
i think so, and thats the "pump" i think. it would generate some heat, but it woudl be liek the heat generated by wind friction in an air conditioner,  minimal. it would probably not gety ice cold, but it would certainly be cooler, right dave-o?

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Offline daveshorts

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Re: Metal which cools when current is applied?
« Reply #5 on: 09/05/2005 18:58:32 »
If you just put one junction (between high and low electron potential metals) somewhere hot and one junction somewhere cold, you will get lots more electrons getting up the slope in the hot junction because they have lots of thermal energy. this means that you will get an overall current flowing up the slope at the hot junction and down the slope at the cold junction (releasing heat there). It is a heat engine like a steam engine (or more directly comparable a stirling engine) but using electrons rather than water as the working fluid. This system will produce a small but significant voltage (10-20mV for metals) and can be used for generating electricity

This is how thermocouples work and the Radio-thermal generators on the voyager and cassini probes (they use nuclear decay to make the heat)
 

Offline gsmollin

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Re: Metal which cools when current is applied?
« Reply #6 on: 09/05/2005 20:08:47 »
If you Google on "radioisotope thermoelectric generator" you will get a lot of hits. Here is one of them:

http://www.ne.doe.gov/space/space-desc.html

I have other information on thermoelectric generators, if you are interested in designing power conversion equipment for them. TGs can be powered from almost any low grade heat source, and you can buy them inexpensively as Peltier coolers. For instance, in an arctic winter, with no sunshine so no solar power, you could generate electricity by putting a Peltier cooler in a cabin wall. The freezing arctic winter would chill the cold side, and just the heat inside a cabin would be enough to power it. You wouldn't need a fire burning to run the TG, just have the cabin be warmer than outside.
 

Offline neilep

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Re: Metal which cools when current is applied?
« Reply #7 on: 09/05/2005 20:10:13 »
Hi Dave.......Are you actually saying that one end of the pump will be hotter than the other  yes?...so, does that mean the other end has been cooled to less than it's ambient temperature ?

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Offline daveshorts

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Re: Metal which cools when current is applied?
« Reply #8 on: 09/05/2005 20:21:31 »
If you are running as a heat pump it will move heat from one side to the other, so yes it will cool one side down and heat up the other - just like a fridge pumps heat from inside to the pipes on the back.
 

Offline moth

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Re: Metal which cools when current is applied?
« Reply #9 on: 09/05/2005 21:46:16 »
People who mess around trying to run their computers faster than specified sometimes use Peltier heat pumps to stop the cpu from burning out. They are often able to cool the cpu well below freezing point. There are two disadvantages which have stopped this from becoming a standard technology: the Peltiers consume a great deal of electricity, and temperature on the cold side can cause water to condense from the air, which then shorts out and destroys the computer.

Einstein spent a period of his life desinging fridges (he had shares in the company) and came up with many novel designs. One used the Peltier effect in metals. I can't google details of that one but this one is even crazier: newbielink:http://www.cam.net.uk/home/StKilda/electrolux.html [nonactive]
 

Offline chimera

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Re: Metal which cools when current is applied?
« Reply #10 on: 09/05/2005 22:35:11 »
Doesn't the ole' Sterling engine work similarly? Gotta look it up, but I remember it had one cold and one hot end...

also found this (scroll down to Cooling Efficiently:

http://www.eneco-usa.com/technology/content.htm#

seems there's still some surprises there, too...



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Offline gsmollin

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Re: Metal which cools when current is applied?
« Reply #11 on: 10/05/2005 15:27:21 »
All heat engines have one cold end and one hot end. The thermoelectric generator is a heat engine. The Peltier cooler is a heat pump, and the two are just each other running in reverse of the other. The only fly in this ointment is that the efficiency of the TG is about 6%, and the Peltier cooler is similar.  Peltier coolers have a nasty habit of going into thermal runaway if overloaded.
 

Offline Chopper

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Re: Metal which cools when current is applied?
« Reply #12 on: 11/05/2005 03:07:46 »
Thanks for these replies everyone.  I have one other question - not really science based though:  Why aren't radar detectors illegal?  Do they serve some other function other than helping people break the law?
 

Offline gsmollin

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Re: Metal which cools when current is applied?
« Reply #13 on: 13/05/2005 13:46:49 »
quote:
Originally posted by Chopper

Thanks for these replies everyone.  I have one other question - not really science based though:  Why aren't radar detectors illegal?  Do they serve some other function other than helping people break the law?



In some states of the U. S. they are illegal. In other states, they are not illegal. I don't know of any use for them besides evading police speed radar.

So are they illegal in New Zealand? I also wonder about other countries.
 

Offline Bass

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Re: Metal which cools when current is applied?
« Reply #14 on: 13/05/2005 17:56:57 »
They're good for at least a couple of other functions:

Emergency vehicles (fire, ambulance, etc) emit a special signal that most detectors pick up and sound a special alarm as the vehicles approach, which allows you more time to move to the side of the road.

The detectors are great at finding airports- at least mine goes bonkers whenever I get within a few miles of any airport.

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Offline davids

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Re: Metal which cools when current is applied?
« Reply #15 on: 16/05/2005 10:15:00 »
Radar detectors are illegal because it is a criminal offence to intercept a police radio transmission. Not everyone is aware of that and it is infact illegal to listen in on emergency services tranmissions and people do get convicted of it !!

A radar type ' speed trap' or speed camera etc is also an official police transmission and intercepting the signal;  not even jamming it ,just checking if its there, is illegal.

So thats why radar detectors are banned.
 

Offline gsmollin

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Re: Metal which cools when current is applied?
« Reply #16 on: 16/05/2005 13:40:04 »
quote:
Originally posted by davids

Radar detectors are illegal because it is a criminal offence to intercept a police radio transmission. Not everyone is aware of that and it is infact illegal to listen in on emergency services tranmissions and people do get convicted of it !!

A radar type ' speed trap' or speed camera etc is also an official police transmission and intercepting the signal;  not even jamming it ,just checking if its there, is illegal.

So thats why radar detectors are banned.



Once again, the (il)legality of this depends upon your jurisdiction. Some states of the US make it illegal, and some do not. So where are you reporting from?
 

Offline Sparky

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Re: Metal which cools when current is applied?
« Reply #17 on: 23/05/2005 18:38:17 »
quote:
Originally posted by gsmollin

quote:
Originally posted by davids

Radar detectors are illegal because it is a criminal offence to intercept a police radio transmission. Not everyone is aware of that and it is infact illegal to listen in on emergency services tranmissions and people do get convicted of it !!

A radar type ' speed trap' or speed camera etc is also an official police transmission and intercepting the signal;  not even jamming it ,just checking if its there, is illegal.

So thats why radar detectors are banned.



Once again, the (il)legality of this depends upon your jurisdiction. Some states of the US make it illegal, and some do not. So where are you reporting from?

 

Offline Sparky

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Re: Metal which cools when current is applied?
« Reply #18 on: 23/05/2005 18:49:04 »
quote:
Originally posted by Sparky

quote:
Originally posted by gsmollin

quote:
Originally posted by davids

Radar detectors are illegal because it is a criminal offence to intercept a police radio transmission. Not everyone is aware of that and it is infact illegal to listen in on emergency services tranmissions and people do get convicted of it !!

A radar type ' speed trap' or speed camera etc is also an official police transmission and intercepting the signal;  not even jamming it ,just checking if its there, is illegal.

So thats why radar detectors are banned.



Once again, the (il)legality of this depends upon your jurisdiction. Some states of the US make it illegal, and some do not. So where are you reporting from?



 

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Re: Metal which cools when current is applied?
« Reply #18 on: 23/05/2005 18:49:04 »

 

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