apply first law of thermodynamics to a resistor carrying a current i identify which of the quantities dQ , dU , dW are zero which are positive and which are negative
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2 Answers
62Well this is a wonderful question for testing the basics of thermoD...
SYSTEM : The Conductor (Electrons enter and leave the system, so you could say that this is an open system)
dQ will be zero. (No external heating/cooling)
dU will be +ve bcos heat generation (i2RT) is positive.
dW will be -ve bcos wire provides resistance to current flow. In that sense, the system is trying to do -ve work (opposing work)
This also justifies the first law :)
62This above explanation comes under the assumption that there is no loss of heat from the surface of the wire.
If there is heat dissipation, then the dQ would be -ve and magnitude of other things involved would differ by some amount!