The first law of thermodynamics states that ΔE= Q− W. Is this also a statement of the principle of conservation of energy? No, the heat that is added to the system is only used to do work. No, the change in internal energy is the energy lost in the system. Yes, the heat added and the change in internal energy of the gas equal the work done by the piston. Yes, the heat that flows into the system is used to change the internal energy of the gas and becomes work done by the piston.