# NCERT Physics Class 12 Exemplar Ch 5 Magnetism and Matter Part 5

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Q.19 A bar magnet of magnetic moment and moment of inertia (about center, perpendicular to length) is cut into two equal pieces, perpendicular to length. Let be the period of oscillations of the original magnet about an axis through the midpoint, perpendicular to length, in a magnetic field. What would be the similar period for each piece?

Solution:

and

Q.20 Use

(i) The Ampere’s law for and

(ii) Continuity of lines of to conclude that inside a bar magnet,

(A) Lines of run from the pole to pole, while

(B) Lines of must run from the pole to pole.

Solution:

Consider a line of through the bar magnet. It must be closed. Let be the amperian loop.

is inside the bar.

Hence is making an obtuse angle with .