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Question
The relaxation time τ is nearly independent of applied E field whereas it changes significantly with temperature T. First fact is (in part) responsible for Ohm’s law whereas the second fact leads to variation of ρ with temperature. Elaborate why?
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Solution
Relaxation time is bound to depend on velocities of electrons and ions. Applied electric field affects the velocities of electrons by speeds at the order of 1 mm/s, an insignificant effect. Change in T, on the other hand, affects velocities at the order of 102 m/s. This can affect τ significantly.
[ρ = ρ(E, T) in which E dependence is ignorable for ordinary applied voltages.]
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