Consider a set of interacting current circuits in a linear magnetic medium. Al: circuits with the exception of circuit 1 are held stationary, but circuit 1 is allowed to move rigidly. The curents are all held constant by means of batteries. Show from the combination of Eqs. (12-4), (12-6), and (12-18) that the mechanical work done by the moving circuit is dW = 1, dP,, where dP, is the change in flux through circuit 1. %3D