When the coil of an ac generator rotates in a magnetic field, which of the following happens?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The effective area of the coil normal to the magnetic field changes.
Option (1) The area of the coil changes. — Incorrect. The coil is rigid, so its own area \( A \) stays the same. Only its orientation in the field changes as it turns.
Option (2) The effective area of the coil normal to the magnetic field changes. — Correct. As the coil turns, the angle \( \theta \) between the field \( \vec{B} \) and the area vector \( \vec{A} \) changes, so the effective area \( A\cos\theta \) facing the field changes. The flux \( BA\cos\theta \) changes and an emf is induced in the coil.
Option (3) The magnetic field of the magnets decreases. — Incorrect. The coil turns in a uniform magnetic field that stays the same. The flux changes only because the orientation of the coil changes.
Option (4) The angular momentum of the coil increases. — Incorrect. The coil is rotated with a constant angular speed \( \omega \), so its angular momentum stays constant. The emf comes from the change of flux through it.
NCERT (2026-27):
AC Generator – “… As the coil rotates in a magnetic field B, the effective area of the loop (the face perpendicular to the field) is A cos θ, where θ is the angle between A and B. This method of producing a flux change is the principle of operation of a simple ac generator.”