PYQ NEET- Moving Charges And Magnetism L-10

A square current carrying loop is suspended in a uniform magnetic field acting in the plane of the loop. If the force on one arm of the loop is F, the net force on the remaining three arms of the loop is#

A) $3 \vec{F}$

B) $-\vec{F}$

C) $-3 \vec{F}$

D) $\vec{F}$

Answer: (B) $-\vec{F}$#

Sol:#

When a current carrying loop is placed in a magnetic field, the coil experiences a torque given by $\tau=N B i A \sin \theta$. Torque is maximum when $\theta=$ $90^{\circ}$,i.e., the plane of the coil is parallel to the field $\tau_{\max }=N B i A$

Forces $\overrightarrow{F_1}$ and $\overrightarrow{F_2}$ acting on the coil are equal in magnitude and opposite in direction. As the forces $\overrightarrow{F_1}$ and $\overrightarrow{F_2}$ have the same line of action their resultant effect on the coil is zero. The two forces $\overrightarrow{F_3}$ and $\vec{F}_4$ are equal in magnitude and opposite in direction. As the two forces have different lines of action, they constitute a torque. Thus, if the force on one arc of the loop is $\vec{F}$, the net force on the remaining three arms of the loop is - $\vec{F}$.