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Jano L.
Jano L. is offline
#7
Feb17-13, 10:31 PM
P: 1,027
If there is no motion, the temperature is zero. What's confusing about that?
Well, I do not necessarily want to use the idea that temperature is direct measure of motion. It is conceivable that one has a system that has temperature and no motion - take model of two state spins for example, which can have energy and entropy but no motion. Or take ideal fluid defined by the equation of state

$$
pV = nRT
$$

and total energy

$$
U = ncT.
$$

with some constant ##c##. According to the second equation, temperature is a measure of total energy ##U##. But energy is an abstract concept, which can be negative in principle. (It will be other contributions to energy are negative and great enough). Then according to the second equation, the temperature will be negative too.