The first law of thermodynamics is a formulation of the law of conservation of energy, adapted for thermodynamic processes. It distinguishes in principle two forms of energy transfer, heat and thermodynamic work for a system of a constant amount of matter. The law also defines the internal energy of a system. The law of conservation of energy states that the total energy of any isolated system, which cannot exchange energy or matter, is constant. Energy can be transformed from one form to another, but can be neither created nor destroyed. The first law for a thermodynamic process is often formulated as ΔU=Q-W where ΔU denotes the change in the internal energy of a closed system (for which heat or work through the system boundary are possible, but matter transfer is not possible), Q denotes the quantity of energy supplied to the system as heat, W denotes the amount of thermodynamic work done by the system on its surroundings.