1. The problem statement, all variables and given/known data A tank containing air is stirred by a paddle wheel. The work input to the paddle wheel is 9000 kj and the heat transferred to the surroundings from the tank is 3000 kj. The external work done by the system is ? 2. Relevant equations dU = Q-W 3. The attempt at a solution Work input = -9000 kj heat given to the surroundings = -3000 kj The answer given is Zero, but no explanation has been given . Is it zero because there is no movement of the boundary in this case?