Question

A circular loop of flexible conducting material is kept in a magnetic field direction perpendicularly into its plane. By holding the loop at diametrically opposite points its is suddenly stretched outwards, then
A) No current is induced in the loop
B) Anti-clockwise current is induced
C) Clockwise current is induced
D) Only emf is induced.

Answer 1

Hint:This question is easy to answer as it is based on Lenz's law which states that the magnetic field opposes the change in magnetic flux through the loop in order to ensure that original flux is maintained through the loop when current flows in it. As we stretch, the area gets reduced and hence the flux gets reduced and oppose in flux will induce current in clockwise direction.

Complete step by step answer:
Step 1:
Before solving the question let us see about the Lenz’s law as it plays an important role in answering the question:
Lenz’s law states that induced electromotive force with different polarities induces a current whose magnetic field opposes the change in magnetic flux through the loop in order to ensure that original flux is maintained through the loop when current flows in it.
Lenz's Law has done corrections in the formula of Faraday’s law. Here the negative sign is contributed from Lenz’s law. The expression is –$Emf = - N\left( {\dfrac{{\Delta \phi }}{{\Delta t}}} \right)$
Where, Emf is the induced voltage (also known as electromotive force); N is the number of loops, $\Delta \phi $ Change in magnetic flux, $\Delta t$ Change in time.

Step 2:
We are given:
A circular loop of flexible conducting material is kept in a magnetic field direction perpendicularly into its plane.
By holding the loop at diametrically opposite points it is suddenly stretched outwards.
As the field is perpendicular to the plane so the coil has certain flux.
As we stretch it outward the area (A) of the coil gets decreased so flux=BA gets reduced.
According to Lenz’s law it must be opposing the decrease in the flux so that the induced current should produce the inward field to support the original field.
This is the reason that induced current will be in the clockwise direction.

Option C is correct.

Additional information:
Applications of Lenz’s law:
-Eddy current balances
-Metal detectors
-Eddy current dynamometers
-Braking systems on train
-AC generators
-Card readers
-Microphones

Note:
Lenz's law explains the direction of many effects in electromagnetism, such as the direction of voltage induced in an inductor or wire loop by a changing current, or the drag force of eddy currents exerted on moving objects in a magnetic field.