A gas in a cylinder absorbs 500 J of heat from a burner and expands, pushing the piston out and doing 200 J of work on the surroundings. (a) Find the change in internal energy. (b) If instead the gas is compressed with 200 J of work done on it while it rejects 500 J of heat, find delta_U. Solution: (a) delta_U = Q - W = 500 - 200 = +300 J; the gas's internal energy (temperature) rises by 300 J worth. (b) Now Q = -500 J (rejected), W = -200 J (work done on the system is negative in physics convention): delta_U = (-500) - (-200) = -300 J. The gas cools, losing 300 J of internal energy.