An electric heater supplies heat to a system at a rate of 100 W. If the system performs work at a rate of 75 J/s, then the rate at which internal energy increases will be: (a) 75 W (b) 100 W (c) 125 W (d) 25 W

Answer: (d). First law per second: dU/dt = dQ/dt - dW/dt = 100 - 75 = 25 W So the answer is (d) 25 W.

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Heat, Work & First Law of Thermodynamics

An electric heater supplies heat to a system at a rate of 100 W. If the system performs work at a rate of $75\text{ J/s}$ , then the rate at which internal energy increases will be:

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Answer: (d)

First law per second: $\dfrac{dU}{dt} = \dfrac{dQ}{dt} - \dfrac{dW}{dt}$
$= 100 - 75 = 25$ W
So the answer is (d) 25 W.

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