Question

The electromagnetic force developed in any physical system acts in such a direction as to tend to _____________

(a) decrease the co-energy at constant mmf

(b) increase the co-energy at constant flux

(c) decrease the co-energy at constant flux

(d) increase the co-energy at constant mmf

I have been asked this question at a job interview.

This key question is from Singly Excited Magnetic Systems in portion Magnetic Circuits of Electrical Machines

Answer 1

The correct choice is (d) increase the co-energy at constant mmf

The best explanation: fe = (∂Wfld^1 (i,x))/∂x = (∂Wfld^1 (F,x))/∂x, the positive sign before ∂Wfld^1 indicates that force fe acts in a direction as to tend to increase the co-energy at constant mmf.