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Q & A: Onsager reciprocity relations

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Most recent answer: 04/20/2013
Q:
Does the Onsager reciprocity relations apply when local thermodynamical equilibrium hypothesis is not valid?
- Luis Costa (age 68)
Lisbon-Portugal
A:
Generally speaking, no. They're derived using the assumption of thermal equilibrium. In many cases, however, they do apply to quasi-equilibrium systems, in which some very slowly varying quantities are out of equilibrium, but they're static on the time scale of the dynamical processes described by the Onsager relations.

Here's an example that became familiar in my line of work. Say that the electrical properties of some material slowly fluctuate as defects move around. Probably the defect configuration is far from equilibrium. The material might even be a non-equilibrium glass. You measure things like electrical resistance and thermoelectric power, etc. At any time, those properties all obey Onsager relations, since on the relevant time scale for electrical and thermal transport the defect configuration is nearly fixed.

In cases where there's some significant relevant flow, the relations need not hold.

Mike W.


(published on 04/20/2013)

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