Thermodynamic force conjugate to an internal variable
= Thermodynamic force conjugate to an internal variable
{title2=$f_r=-\rho_0\,\partial U/\partial\xi_r$}
The derivative of <internal energy> at fixed <deformation gradient> and <entropy>, or of specific <Helmholtz free energy> at fixed <temperature>, defines this force. Its product with the internal-variable rate contributes mechanical dissipation to the <Clausius-Duhem inequality>.