NCERT Solutions
Class 12 Physics
Magnetism and Matter

Q.7
A bar magnet of magnetic moment 1.5 J T–1 lies aligned with the direction of a uniform magnetic field of 0.22 T.
(a) What is the amount of work required by an external torque to turn the magnet so as to align its magnetic moment:
(i) normal to the field direction,
(ii) opposite to the field direction?
(b) What is the torque on the magnet in cases (i) and (ii)?
Given:
Magnetic moment, M = 1.5 J T−1
Magnetic field strength, B = 0.22 T
(i) Initial angle between the axis and the magnetic field, θ1 = 0°
Final angle between the axis and the magnetic field, θ2 = 90°
The work required to make the magnetic moment normal to the direction of magnetic field is given as:
W = -MB (cos θ2 – cos θ1)
= - 1.5 x 0.22 (cos 900 – cos 00)
=-0.33(0 -1)
=0.33J
(ii) Initial angle between the axis and the magnetic field, θ1 = 0°
Final angle between the axis and the magnetic field, θ2 = 180°
The work required to make the magnetic moment opposite to the direction of magnetic field is given as:
W = -MB (cos θ2 – cos θ1)
= - 1.5 x 0.22 (cos 1800 – cos 00)
=-0.33(-1 -1)
=0.66J
(I) For case (i) : θ2 = θ1 = 900
Therefore torque τ = MB sin θ
= 1.5 x 0.22 sin 900
=0.33 J
(II) For case (ii) : θ2 = θ1 = 1800
Therefore torque τ = MB sin θ
= MB sin 1800
=0J