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Class 12
Physics
Magnetism and Matter
a bar magnet of magnetic moment 1 5 j t 1 nbsp lie...

Question:
A bar magnet of magnetic moment 1.5 J T^-1lies 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)?

Answer:

(a)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 90° - cos 0°)
= -0.33 (0 - 1)
= 0.33 J

(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 180° - cos 0°)
= -0.33 (-1 - 1)
= 0.66 J

(b)For case (i): θ = θ₂ = 90°

∴Torque, T = MB sinθ
= 1.5 x 0.22 sin 90°
=0.33 J

Forcase (ii): θ = θ₂ = 180°

∴Torque, T = MB sin θ
=MB sin 180° = 0

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