Which of the following can produce alternating current?

The correct answer is Option (4) → A coil rotated in a uniform magnetic field

Option (1) Transformer — Incorrect. A transformer only steps an alternating voltage up or down. It works by mutual induction and needs an alternating current already fed to its primary coil. So it changes ac but does not produce it.

Option (2) Electric motor — Incorrect. A motor uses an electric current to produce rotation. It changes electrical energy into mechanical energy, so it does the reverse of a generator.

Option (3) Cell — Incorrect. A cell keeps one terminal positive and the other negative, so its emf has a fixed polarity and it drives a direct current.

Option (4) A coil rotated in a uniform magnetic field — Correct. As the coil turns, the magnetic flux through it changes, \( \Phi_B=BA\cos\omega t \). The induced emf \( \varepsilon=NBA\omega\sin\omega t \) reverses its polarity periodically. So the current it drives also changes direction periodically. This is the principle of the ac generator.

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.”

AC Generator – “… Since the value of the sine fuction varies between +1 and –1, the sign, or polarity of the emf changes with time. … The direction of the current changes periodically and therefore the current is called alternating current (ac).”