An expression for magnetic potential energy can also be obtained on
lines similar to  electrostatic potential energy The magnetic potential energy Um is given by
U
m = ∫τ θd
θ
( )
      =
= −
∫mB
d
mB
sin
cos
θ
θ
θ
      = –m B
(5 3)
We have emphasised in Chapter 2 that the zero of potential energy
can be fixed at one’s convenience