For thermodynamic processes of energy transfer without transfer of matter, the first law of thermodynamics is often expressed by the algebraic sum of contributions to the internal energy, U , from all work, W , done on or by the system, and the quantity of heat, Q , supplied or withdrawn from the system. The historical sign convention for the terms has been that heat supplied to the system is positive, but work done by the system is subtracted. This was the convention of Rudolf Clausius, so that a change in the internal energy, Δ Δ U , is written