A rectangular loop carrying a current $i$ is placed in a uniform magnetic field $B$. The area enclosed by the loop is $A$. If there are n turns in the loop, the torque acting on the loop is given by
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(a) $\vec{\tau}=\vec{M} \times \vec{B}$
$=n i(\vec{A} \times \vec{B})(\because \vec{M}=n i \vec{A})$
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