What is the purpose of rotating the armature in the field of two electromagnets in an ac generator?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → To generate an emf
Option (1) To keep the magnetic flux through the coil constant — Incorrect. Rotation changes the angle between the field and the coil, so the flux \( NBA\cos\omega t \) keeps changing. A constant flux would induce no emf at all.
Option (2) To magnetise the electromagnets — Incorrect. The current in their own windings magnetises the electromagnets. The rotating armature only uses the field they set up.
Option (3) To generate an emf — Correct. As the armature rotates, the magnetic flux through it changes continuously, so by electromagnetic induction an emf \( \varepsilon=NBA\omega\sin\omega t \) is induced in it. In this way the generator converts mechanical energy into electrical energy.
Option (4) To convert dc into ac — Incorrect. An ac generator takes no dc input. It produces an alternating emf from the mechanical energy of rotation.
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.”