In a certain process, 600 J of work is done on the system which gives off 250 J of heat. What is the change in internal energy for the process?

The absorption of heat by the system tends to raise the energy of the system. The performance of work by the system, on the other hand, tends to lower the energy of the system because performance of work requires expenditure of energy. Therefore the change in internal energy ΔU, of a system is equal to the head added to the system minus the work done by the system:

ΔU = UB – UA = q + w
Here q = – 250 j and W = 600 j

Therefore, change in internal energy ΔU = – 250 J + 600 J = 350 J
Hence, the change in internal energy for the system is equal to 350 J