What is the power transferred per unit volume into joule heat in a resistor ?

Explanation

Power, `P = I^(2)R = I^(2)rho l//A ," "( :' R = rho l//A)`
Volume of resistor, `V = Al`
`:.` Power transferred per unit volume
`P/V = (I^(2) rho l//A)/(Al) = (I/A)^(2) rho = J^(2) rho`
Where, `I//A = J` = current density.