In an ac generator, how is the coil (armature) rotated in the magnetic field?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → By some external means
An ac generator converts mechanical energy into electrical energy, so the energy that turns its coil must be supplied from outside the machine.
Option (1) Chemically — Incorrect. No chemical reaction takes place in a generator. The coil is turned mechanically, and the emf comes from the change of magnetic flux through it.
Option (2) By the current it produces — Incorrect. The current is the output of the generator. By Lenz’s law, the induced current opposes the motion that produces it. So it can never keep the coil turning. Work has to be done against it.
Option (3) By the magnetic field — Incorrect. The uniform magnetic field only supplies the flux through the coil. The field does no work to turn the coil. The force it exerts on the induced current opposes the rotation.
Option (4) By some external means — Correct. The coil (armature) is mechanically rotated by an outside source. For example, water falling from a dam turns it in hydro-electric generators, and high-pressure steam turns it in thermal generators.
NCERT (2026-27):
AC Generator – “… The coil (called armature) is mechanically rotated in the uniform magnetic field by some external means. The rotation of the coil causes the magnetic flux through it to change, so an emf is induced in the coil.”