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The rate of Joule heat dissipation per unit volume in a conductor (with usual symbols: J = current density, E = electric field, sigma = conductivity) equals:

1. J E
2. sigma E
3. sigma J
4. None of these

Correct answer: J E

Solution

The power dissipated per unit volume is the dot product of current density and electric field, J E (equivalently sigma E² or J²/sigma).

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