dQ:

Say the change in the configuration takes place adiabatically, which results in

dU:

Change in intrinsic energy of the body consists of two components: change associated with its temperature (dU_h ) and that associated with the change in configuration (dU_c ).

If we consider that the action takes place in the standard state, change in intrinsic energy associated with D_T is zero.

Putting together all these in equation 3.1 and considering the change in energy over a time dt results in the following expression for dU_c :

(3.8)

Note that the right hand side of equation 3.8 is an exact differential so that there exists a function which has the following properties,

(3.9)

is the potential energy, per unit volume stored up in the body by the strain and is called the strain energy function .