1. The problem statement, all variables and given/known data (This isn't for class, but still worthy of discussion!) If you heat a room with say a fireplace, why does the total energy not increase? 2. Relevant equations PV=NkT 3. The attempt at a solution This is a qualitative answer (mostly). The total energy of the molecules in the room can not be changed, but I'm not sure why this is? The total energy of an ideal gas is U = (f/2)NkT The average energy of each molecule does increase, but again why does the total energy stay the same? The answer probably has to do with PV = constant. Again, I'm not sure, so if people have insight that would be appreciated!