A thermodynamic system, its boundary, and its surroundings must be defined with all the roles of the components fully explained before we can analyze a situation Thermal equilibrium is reached with two objects if a third object is in thermal equilibrium with the other two separately

A general equation of state for a closed system has the form f ( p , V , T ) = 0 , with an ideal gas as an illustrative example The internal energy of a thermodynamic system is a function of state and thus is unique for every equilibrium state of the system

first law of thermodynamics — the change in internal energy for any transition between two equilibrium states is Δ E int = Q - W. closed system — system that is mechanically and thermally isolated from its environment. internal energy — average of the total mechanical energy of all the molecules or entities in the system. equilibrium — thermal balance established between two objects or parts within a system.