Abstract
The thermodynamic force on a dislocation is defined as the negative gradient of the Gibbs free energy of the crystal with respect to dislocation motion. This is shown to lead to a consistent force expression which contains two terms: the Peach-Koehler force due to the stress, and the Bardeen-Herring force due to the vacancy concentration. The precise definition of these two terms is arbitrary, though their sum is unique. Various forms of the force expression are derived in terms of different reference vacancy concentrations, including the original forms of Weertman and Lothe and Hirth.