The First Law of Thermodynamics

The Law Any thermodynamic system in an equilibrium state possesses a state variable called the internal energy. Between andy two equilibrium states, the change in internal energy is equal to the difference in the heat transfer into the system and the work done by the system. DU= Q - W

Sign Convention

Quantity	Sign
Heat added to the system	+Q
Heat leaving the system	-Q
Work done by the system	+W
Work done on the system	-W

Examples

If heat is added to the system and work is done on the system, then we have   DU= Q - W  ð DU = Q -(-W) = Q + W

If heat is lost by the system and work is done by the system, then we have DU = Q - W  ð DU = -Q + W