A coil is placed in magnetic field such that plane of coil is perpendicular to the direction of magnetic field. The magnetic flux through a coil can be changed :
A. By changing the magnitude of the magnetic field within the coil.
B. By changing the area of coil within the magnetic field.
C. By changing the angle between the direction of magnetic field and the plane of the coil.
D. By reversing the magnetic field direction abruptly without changing its magnitude.
Choose the most appropriate answer from the options given below :

Solution

Physics sir · 3 steps

Flux is BA cos θ: only a change in B, A or θ changes how much flux the coil links.

1. Flux formula. For a flat coil in a uniform field, the flux depends on three things: the field strength, the coil's area inside the field, and the angle θ between the field and the coil's normal.

   Φ = BA cos θ

   Statement	What it changes	Amount of flux linked
   A	B changes	changes
   B	A changes	changes
   C	θ changes, so cos θ changes	changes
   D	only the sense of B⃗; B, A and θ are the same	stays BA

2. Reversing the field. After the reversal the field still has magnitude B, the area is still A, and the field is still perpendicular to the plane. The sign of Φ flips from +BA to -BA, but the amount of flux through the coil is still BA. None of the three factors has changed, so D is not a way of changing the flux.
3. So. Only A, B and C change a factor in BA cos θ, so the answer is (b).

The idea to keep. Before you call something a flux change, check it against Φ = BA cos θ. If B, A and θ all keep their values, the amount of flux linked stays the same.

Answer from the NTA final answer key. Solution by PrepWiser.