91Ó°ÊÓ

A magnetized wire of moment \(M\) is bent into an arc of a circle subtending an angle of \(60^{\circ}\) at the centre, the new magnetic moment is (A) \(\frac{2 M}{\pi}\) (B) \(\frac{M}{\pi}\) (C) \(\frac{3 \sqrt{3} M}{\pi}\) (D) \(\frac{3 M}{\pi}\)

An iron rod of cross-sectional area \(4 \mathrm{sq} \mathrm{cm}\) is placed with its length parallel to a magnetic field of intensity \(1600 \mathrm{amp} / \mathrm{m}\). The flux through the rod is \(4 \times 10^{-4}\) weber. The permeability of the material of the rod is (In weber/amp-m). (A) \(0.625\) (B) \(6.25\) (C) \(0.625 \times 10^{-3}\) (D) None of these

Assertion: When a test charge moves through a magnetic field, its momentum changes but kinetic energy remains constant. Reason: The magnetic force acts as a centripetal force, which is perpendicular to the instantaneous velocity and so does no work. (A) A (B) \(\mathrm{B}\) (C) \(\mathrm{C}\) (D) D

Assertion: A charged particle moves perpendicular to a uniform magnetic field then its momentum remains constant. Reason: Magnetic force acts perpendicular to the velocity of the particle. (A) \(\mathrm{A}\) (B) \(\mathrm{B}\) (C) \(\mathrm{C}\) (D) D

Figure \(16.66\) shows four rods having \(\lambda=0.5 \Omega / \mathrm{m}\) resistance per unit length. The arrangement is kept in a magnetic field of constant magnitude \(B=2 \mathrm{~T}\) and directed perpendicular to the plane of the figure and directed inwards. Initially, the rods form a square of side length \(\ell=15 \mathrm{~m}\) as shown. Now each wire starts moving with constant velocity \(v=5 \mathrm{~m} / \mathrm{s}\) towards opposite wire. Find the force required in newton on each wire to keep its velocity constant at \(t=1 \mathrm{~s}\).