16.2.2—The first law of thermodynamics ∆U = q + W expressed

The first law is ∆U=q+W when q is energy supplied as heat and W is work done on the system; equivalent conventions may write ∆U=Q−W_by.

Declare the sign convention before substituting. Internal energy rises when net energy enters and falls when net energy leaves.

If 300 J of heat enters and the system does 100 J of work, ∆U=200 J using the q−W_by convention.

The first law is energy conservation, not a statement that heat and work are stored properties of the system.