NCERT · Class 12 · Physics · Alternating CurrentExplain the working principle of a transformer. Why cannot a transformer be used to step up or step down Direct Current (DC) voltage?

Principle of a Transformer:\nA transformer works on the principle of mutual induction. It consists of two coils—primary and secondary—wound on a soft iron core. When an alternating voltage is applied across the primary coil, a continuously changing current flows through it. This creates a continuously changing magnetic flux in the core. Since the secondary coil is linked with the same iron core, this changing magnetic flux links with the secondary coil as well, inducing an alternating electromotive force (emf) in it according to Faraday's law of electromagnetic induction.

Why Transformer cannot work on Direct Current (DC):

1. Direct current (DC) has a constant magnitude and direction, so the current flowing through the primary coil remains constant over time.
2. As a result, the magnetic flux generated in the iron core remains steady and constant ($d\Phi/dt = 0$).
3. According to Faraday's law of electromagnetic induction, induced emf in the secondary coil is proportional to the rate of change of magnetic flux ($e = -N \frac{d\Phi}{dt}$).
4. Since there is no change in magnetic flux for DC, no emf is induced in the secondary winding. Thus, a transformer does not operate on DC and cannot step up or step down DC voltages.