NCERT Solutions for Class 12 Physics Chapter 5: Magnetism and Matter – Free PDF Download

Complete, verified solutions to the NCERT Class 12 Physics Chapter 5 (Magnetism and Matter) exercise questions. These Class 12 Physics Chapter 5 solutions are also useful as quick revision notes before exams.

NCERT Exercise Solutions

5.1 A short bar magnet of moment 0.32 J/T placed at 30° with an external field of 0.15T. (a) Torque on the magnet. (b) Work done in turning it to be normal to the field. (c) Work done for 180°.
Ans: (a) τ=mB sinθ=0.32×0.15×sin30°=0.024 J. (b) W=mB(cosθ₁−cosθ₂)=mB(cos30°−cos90°)=0.32×0.15×0.866≈0.0416 J. (c) For 180°: W=mB(cos30°−cos180°)=0.32×0.15×(0.866+1)=0.32×0.15×1.866≈0.0896 J.

5.2 A short bar magnet of magnetic moment 0.48 J/T is placed in a horizontal plane. Find the magnitude and direction of the magnetic field at a point 10cm from its centre on (a) the axis, (b) the equatorial line.
Ans: (a) Axial: B=(μ₀/4π)×2m/r³=10⁻⁷×2×0.48/(0.1)³=0.96×10⁻⁷/10⁻³=9.6×10⁻⁵ T, along the direction of the magnetic moment. (b) Equatorial: B=(μ₀/4π)×m/r³=10⁻⁷×0.48/10⁻³=4.8×10⁻⁵ T, opposite to the direction of the magnetic moment (i.e., anti-parallel).

5.3 What are the direction and magnitude of the magnetic moment of a current loop, and what determines it?
Ans: Magnitude m=NIA (number of turns × current × area); direction given by the right-hand rule (curl fingers along current direction, thumb points along m), perpendicular to the plane of the loop.

5.4 A short bar magnet placed with its axis at 30° with a uniform external magnetic field of 0.25T experiences a torque of 4.5×10⁻² J. (a) Magnetic moment. (b) Work done in moving it from the most stable to most unstable position.
Ans: (a) τ=mBsinθ ⇒ m=τ/(Bsinθ)=0.045/(0.25×0.5)=0.36 J/T. (b) Most stable is θ=0°, most unstable is θ=180°: W=mB(cos0°−cos180°)=0.36×0.25×2=0.18 J.

Frequently Asked Questions

What is the formula for the axial field of a bar magnet?
B=(μ₀/4π)(2m/r³) for short bar magnet, along the magnet’s axis.

At what angle is the torque on a magnetic dipole maximum?
At θ=90° (dipole perpendicular to the field), where τ=mB.

Written by Satish

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