Formulation of first law of thermodynamics

A system exchanges energy with its surroundings either by transfer of heat or by doing work. An energy supplied to the system increases its internal energy; on the other hand, removal of heat or work from the system decreases its internal energy.

Suppose $Q$ is the heat supplied to the system and $W$ the work done on the system by the surroundings. The internal energy of the system would increase: the increase in internal energy of the system is equal to the sum of the quantity of heat supplied to the system and the amount of work done on the system, or

$\Delta U = Q + W \qquad \text{...(4.12)}$ …