A rectangular coil $PQ$ has $2n$ turns, an area $2a$ and carries a current $2I,$ (refer figure). The plane of the coil is at $60^o$ to a horizontal uniform magnetic field of flux density $B.$ The torque on the coil due to magnetic force is
A$BnaI ,sin60^o$
B$8BnaI \,cos60^o$
C$4naI\, Bsin60^o$
D
none
Diffcult
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B$8BnaI \,cos60^o$
b $\vec{\tau}=\vec{M} \times \vec{B}$
Here $\mathrm{M}=2 \mathrm{n}(2 \mathrm{I})(2 \mathrm{a})$
$M=8$ $nI \mathrm{a}$
$\therefore \quad \tau=M B \sin \left(90^{\circ}-60^{\circ}\right)$
$\tau=M B \cos 60^{\circ}$
$\tau=8$ nIa $\cos 60^{\circ}$
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