Which equation best expresses the first law of thermodynamics, assuming Q is heat, U is internal energy, and W is work?

A.ΔW = ΔQ + ΔU

B.ΔW = ΔU - ΔQ

C.ΔQ = ΔU+W

D.ΔU=Q - W

The first law of thermodynamics is expresses by

D. ΔU=Q-W

which means change in internal energy of system = Heat added to system minus work done by the system

All are expressed in Joules.

This law is based on principle of conservation of energy.