When a bar magnet of dipole moment m is placed in a uniform magnetic
field B,
Rationalised 2023-24
Physics
150
     (a)
the force on it is zero,
     (b)
the torque on it is m × B,
     (c)
its potential energy is –m B, where we choose the zero of energy at
the orientation when m is perpendicular to B 3 Consider a bar magnet of size l and magnetic moment m, at a distance
r from its mid-point, where r >>l, the magnetic field B due to this bar
is,
0
3
2
r
=µ
π
m
B
    (along axis)
   =
0
3
– 4
r
µ
π
m     (along equator)
4