The maximum theoretical work obtainable, when a system interacts to equilibrium with a reference environment, is called

A source at temperature of $$500K$$ provides $$1000$$ $$kJ$$ of heat. The temperature of environment is $${27^ \circ }C.$$ The maximum useful work (in $$kJ$$) that can be obtained from the heat source is __________

Availability of a system at any given state is