Question

A square loop of conducting wire is placed near a long straight current carrying wire as shown. Match the statements in column-I with the corresponding results in column-II.

$\begin{array}{cc}\text{Column-I}& \text{Column-II}\\ \text{(A) If the magnitude of current I is increased}& \text{(P) Induced current in the loop will be clockwise}\\ \text{(B) If the magnitude of current I is decreased}& \text{(Q) Induced current in the loop will be anticlockwise}\\ \text{(C) If the loop is moved away from the wire}& \text{(R) wire will attract the loop}\\ \text{(D) If the loop is moved towards the wire}& \text{(S) wire will repel the loop}\\ & \text{(T) Torque about centre of mass of loop is zero due to megnetic force}\end{array}$

• A. A – (q,r,t), B – (p,r,t), C – (p,r,t), D – (p,s,t)
• B. A – (p,r,t), B – (p,r,t), C – (q,s,t), D – (q,r)
• C. A – (q,r,t), B – (p,r,t), C – (q,s,t), D – ( p,s,t)
• D. A – (q,s,t), B – (p,r,t), C – (p,r,t), D – (q,s,t)

Answer 1

The correct option is D A – (q,s,t), B – (p,r,t), C – (p,r,t), D – (q,s,t)


A) Magnetic field ($\text{B}$) due to wire on the loop is into the plane of the paper. If $\text{I}$ is increased, flux through the coil increases. From Lenz’ law, the direction of current in the loop should be such that flux due to is out of the plane so as to oppose the change in flux due to the wire. i.e current in the loop has to be counter-clockwise.

The current in AD will be from A to D and that in BC will be from C to B. The straight wire will repel AD with greater force (as magnetic field acting at AD is more) and attract BC with lesser force. Hence net force will be repulsion.


(B) If $\text{I}$ is decreased, flux through the coil decreases. From Lenz’ law, the direction of current in the loop should be such that flux due to is into the plane so as to oppose the change in flux due to the wire. i.e current in the loop has to be clockwise.

The current in AD will be from D to A and that in BC will be from B to C. The straight wire will attract AD with greater force (as magnetic field acting at AD is more) and repel BC with lesser force. Hence net force will be attraction.

(C) When the wire is moved away from the loop, the flux through the loop will decrease. Using Lenz’s law, the direction of current is clockwise.

(D) When the wire is towards the loop, the flux through the loop will increase. Using Lenz’s law, the direction of current is counter clockwise.

In all cases, the torque due to magnetic force about the COM is zero as it passes through the center of mass.