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Brief explanation of
Unattainability of absolute zero
by quantum physicist
Christopher Foot of the University of Oxford
(This is essentially the 3rd law of thermodynamics)
To paraphrase a standard textbook example, consider how you
cool things to low temperature. Clearly it is no good saying that we put the
object in contact with something colder since otherwise we can never get below
the temperature of the fridge, so we need to consider how refrigeration works.
One method is to allow a gas to
expand while keeping it insulated from its surroundings so that no heat can flow
into the object. Generally the gas gets colder as it expands. This process can
be repeated.
If we assume the expanded gas has temperature T1 then we can
use this to cool a sample of compressed gas to temperature close to T1. We then
let that sample of compressed gas expand to reach a lower temperature T2, and so
on to lower and lower temperatures.
The catch is that the particles of the gas are almost
stationary at low temperatures and the expansion produces less and less cooling
as the temperature approaches absolute zero. This series of smaller and smaller
steps never reaches zero. For example, if the temperature is halved each time
then we have a sequence like 1, ½, ¼, 1/8 …….. this is just an example of what
might happen.
I hope that this helps. A rigorous explanation requires the
concept of entropy and that entropy tends to zero (or some constant value) as
temperature goes to zero. With this one can prove in general that no sequence of
processes can reach absolute zero, eg. a sequence of alternating isothermal
compressions and adiabatic expansions.
(Answered April 13, 2008)
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