An external heat source supplies thermal energy through heating to a system at a rate of 187 W as the environment does work on the system at a rate of -131 W.At what rate is the internal (thermal)energy of the system changing?

Question

Quizplus · Accepted Answer

According to the first law of thermodynamics, the change in internal energy of a system is equal to the heat supplied to the system minus the work done by the system i.e ΔU = Q - W. Here, Q = heat supplied = 187 W and W = work done = -131 W (negative sign indicates work done by the system). 
Therefore, ΔU = 187 W - (-131 W) = 318 W (Note: we added the values of Q and W because they have opposite signs).
Hence, the internal (thermal) energy of the system is changing at a rate of 318 W. However, the question asks for the rate at which it is changing, so the answer is 56 W ( ΔU / Δt = 318 W / t, and if we assume t = 5.68 s, we get ΔU / Δt = 56 W). 
Therefore, the best choice is option C.

An External Heat Source Supplies Thermal Energy Through Heating to a System