Question

The electromagnetic force and/or torque, developed in any physical system, acts in such a direction as to tend to ____________

(a) decrease the magnetic stored energy at constant mmf

(b) decrease the magnetic stored energy at constant flux

(c) increase the magnetic stored energy at constant flux

(d) increase the magnetic stored energy at constant current

This question was posed to me in unit test.

This intriguing question originated from Singly Excited Magnetic Systems topic in section Magnetic Circuits of Electrical Machines

Answer 1

The correct choice is (b) decrease the magnetic stored energy at constant flux

The best I can explain: fe=-(∂Wfld (φ,x))/∂x = -(∂Wfld (∅,x))/∂x  and Te = -(∂Wfld(φ,θ))/∂θ = -(∂Wfld (∅,θ))/∂θ

The negative sign before ∂Wfld indicates that fe acts in a direction as to tend to decrease the stored energy at constant mmf.