I think internal energy and heat work are the variables that is represented by the formula for the first law of thermodynamics. According to the first law of thermodynamics the change in internal energy of a system is equal to the net heat transfer into the systems minus the net work done by the system. The equation is; ΔU = Q - W
where ΔU is the change in internal energy of the system, Q is the net heat transfer into the system while W is the work done on or by the system.
