JEE Physics 2023 Question with Solution

Given: A coil is placed in a magnetic field, and magnetic flux through the coil is to be examined for different changes.

Find: Which listed changes can change the magnetic flux through the coil.

Magnetic flux through a coil is given by

$$\Phi = \vec{B} \cdot \vec{A} = BA \cos \theta$$

where $B$ is the magnetic field magnitude, $A$ is the area of the coil, and $\theta$ is the angle between $\vec{B}$ and the area vector.

• Changing $B$ changes $\Phi$ directly, so A is correct.
• Changing $A$ changes $\Phi$ directly, so B is correct.
• Changing $\theta$ changes $\cos \theta$ and hence changes $\Phi$, so C is correct.
• The provided solution notes that reversing the magnetic field changes the sign of flux, but the source solution itself concludes the most appropriate listed option is A, B and C only.

Therefore, the correct option is B.

Given: Magnetic flux depends on magnetic field, area, and orientation.

Find: The most appropriate option.

From the source working,

$$\Phi = \vec{B} \cdot \vec{A} = BA \cos \theta$$

For option A, changing the magnitude of $B$ changes flux.

For option B, changing the area $A$ changes flux.

For option C, changing the angle changes $\cos \theta$, so flux changes.

The source also writes for reversal of field direction:

$$\cos(\theta + 180^\circ) = -\cos \theta$$

so the sign of flux reverses. However, the solution explicitly states The Correct Option is B and again states The correct answer is option (B): A, B and C only with the note Most suitable ans is B [Otherwise ABCD].

Hence, following the solution as primary source, the answer is B.