Choose the incorrect statement from the following:

The correct answer is Option (1) → A transformer can produce ac power

A transformer works by mutual induction: the alternating current in the primary coil produces a changing flux in the core, and this changing flux induces an emf in the secondary coil.

Option (1) A transformer can produce ac power — The incorrect statement. A transformer has no source of energy of its own. It only transfers the power given to the primary, and the output power is never more than the input (equal for an ideal transformer).

Option (2) A transformer cannot work on dc — A correct statement. A steady direct current gives a constant flux, which induces no emf in the secondary.

Option (3) In a transformer when ac voltage is stepped up x times, the alternating current reduces to \( \frac{1}{x} \) times — A correct statement. For an ideal transformer the power is unchanged, \( V_p I_p = V_s I_s \), so if \( V_s = x V_p \) then \( I_s = I_p/x \).

Option (4) A transformer cannot change the frequency of alternating current — A correct statement. The secondary emf is induced by the same changing flux, so it has the same frequency as the primary supply.

NCERT (2026-27):
Points to Ponder – “… 11. A transformer (step-up) changes a low-voltage into a high-voltage. This does not violate the law of conservation of energy. The current is reduced by the same proportion.”