The frequencies of a.c. generator depends on:

The correct answer is Option (3) → speed of the rotation of the coil

A coil of N turns and area A rotating at angular speed \( \omega \) in a field B generates the emf \( e = NBA\omega\sin\omega t \). This alternates with the same frequency as the rotation, \( \nu = \omega/2\pi \).

The amplitude \( e_0 = NBA\omega \) depends on the size of the coil, the number of turns and the field, but the frequency depends only on the speed of rotation of the coil. A.c. has no wavelength that the generator controls.

NCERT (2026-27):
AC Generator – “… The direction of the current changes periodically and therefore the current is called alternating current (ac). Since \( \omega = 2\pi\nu \), Eq (6.20) can be written as

    \( \displaystyle e = e_0\sin 2\pi\nu t \qquad (6.21) \)

where \( \nu \) is the frequency of revolution of the generator’s coil.”