<P> The δ's before the heat and work terms are used to indicate that they describe an increment of energy which is to be interpreted somewhat differently than the d U (\ displaystyle \ mathrm (d) U) increment of internal energy (see Inexact differential). Work and heat refer to kinds of process which add or subtract energy to or from a system, while the internal energy U (\ displaystyle U) is a property of a particular state of the system when it is in unchanging thermodynamic equilibrium . Thus the term "heat energy" for δ Q (\ displaystyle \ delta Q) means "that amount of energy added as the result of heating" rather than referring to a particular form of energy . Likewise, the term "work energy" for δ W (\ displaystyle \ delta W) means "that amount of energy lost as the result of work". Thus one can state the amount of internal energy possessed by a thermodynamic system that one knows is presently in a given state, but one cannot tell, just from knowledge of the given present state, how much energy has in the past flowed into or out of the system as a result of its being heated or cooled, nor as the result of work being performed on or by the system . </P> <P> Entropy is a function of the state of a system which tells of limitations of the possibility of conversion of heat into work . </P> <P> For a simple compressible system, the work performed by the system may be written: </P> <Dl> <Dd> δ W = P d V, (\ displaystyle \ delta W = P \, \ mathrm (d) V,) </Dd> </Dl>

Who discover the law of conservation of energy