Thermal equilibrium at 5 Kelvin with an electric diode

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Offline McKay

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It is not supposed to be possible to create a energy/ temperature differential from an energy/ thermal equilibrium without adding some energy from outside the system, right? We cant use the ambient thermal energy/ radiation in our surrounding air because we are at the same temperature, but we need some differential for energy to be usable, right?
Well, what about a system at a temperature of just a few Kelvin, at equilibrium - it would pretty much radiate microwaves, instead of IR radiation, right? Microwaves are rather easy to convert to current flow/ electricity - just have an antenna and a fast enough diode ( a rectifier it is called, i think (?) ).
What would happen to a super cold chamber with such a rectifier in it, that is also cold (and the diode works at such low temperatures)? Will the rectifier have a constant current flowing trough it, heating it a bit, just to have it radiate back in to the system with a bit higher energy level to be cooled down again.. and radiate microwaves to be absorbed by the rectifier... sort of keeping a constant differential ?

This bothers me.

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Offline Bored chemist

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Re: Thermal equilibrium at 5 Kelvin with an electric diode
« Reply #1 on: 31/05/2014 11:04:40 »
The answer is complicated but essentially, if you want to get the electricity to do anything, you need to connect it to a load of some sort- for the sake of simplicity, lets say a resistor.
However, that resistor has a voltage across it due to the random wanderings of the electrons.
http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise
that voltage will oppose the voltage produced by the diode.
You can get a current to flow (and do work) if you have different temperatures, but in doing so you just make a rather inefficient heat engine- a bit like using a thermocouple to produce power.
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