Question

Two similar circular loops carry equal currents in the same direction. On moving coils further apart, the electric current will:

• A. Increase in both
• B. Decrease in both
• C. Remains unaltered
• D. Increase in one, decrease in the second

Answer 1

The correct option is A

Increase in both

The explanation of the correct option

Option( A) -Increase in both

1. Magnetic Flux $\left({\varphi }_B\right)$: It is the measure of the total magnetic field that passes through an area.
2. Mathematically it is expressed as, ${\varphi }_B=BA\cos \theta$, where $B$ is the magnetic field, $A$ is the area and $\theta$ is the angle between a vector perpendicular to the area and magnetic field.
3. Lenz's Law: According to Lenz's law, “An induced electric current flows in a direction such it resists the change which induced it ”.
4. Mathematically it is expressed as, $\epsilon =-N\frac{\partial {\varphi }_B}{\partial t}$, where $\epsilon$ is induced emf, $N$ is the number of turns in the coil, $\partial {\varphi }_B$ is the change in magnetic flux, and $\partial t$is change in time.
5. As the coil moves farther from each other, their magnetic field will weaken and thus, the magnetic flux will reduce.
6. Therefore, by Lenz's law, the current will increase in both the loops in order to increase the magnetic flux that decreases due to moving them apart.

The explanation of incorrect options-

Option (B) - Decrease in both

1. There cannot be a decrease in current since magnetic flux reduces due to the increasing distance between the coils.
2. To bring back the magnetic flux to the same amount, the current has to increase

Option (C) - Remains unaltered

1. In cases where there is a change in magnetic flux, there is going to be a change in current according to Lenz's law
2. Therefore current cannot remain unaltered

Option (D) - Increase in one, decrease in the second

1. Since loops are similar and equal current flows through them in the same direction, the current flowing in both the loops will have a similar effect.

Thus, the current in both the loops increases.

Hence, option A is the correct answer.