Physics · Alternating Current · NEET
A transformer needs a CHANGING magnetic flux to induce an emf in the secondary (Faraday's law). AC current is always changing, so it makes a changing flux and the secondary gets a voltage. DC is steady, so the flux is constant, its rate of change is zero, and the induced emf is zero. This is exactly why power is distributed as ac and not dc.
The principle is MUTUAL INDUCTION between two coils. When ac flows in the primary, it sets up an alternating magnetic flux in the soft-iron core. This flux passes through (links) the secondary coil, and because it keeps changing, it induces an alternating emf in the secondary. So one coil's changing current creates a voltage in the other coil without any electrical connection between them.
Soft iron is used for two reasons. First, it has high magnetic permeability, so it concentrates and guides almost all the flux from the primary into the secondary (very little flux leaks out). Second, soft iron is easily magnetised and demagnetised each cycle, which reduces hysteresis energy loss. A strong, well-linked flux means an efficient transformer.
No. Power is conserved. For an ideal transformer, input power = output power, so V_p x I_p = V_s x I_s. When voltage is stepped up, the current in the secondary drops by the same factor, so power stays the same. A transformer changes the voltage-current combination, it never makes free energy.
The changing flux induces circulating eddy currents inside the iron core, which waste energy as heat. Laminating the core into thin sheets, each insulated with a coat of varnish, breaks up these current loops and greatly reduces eddy-current loss, making the transformer more efficient.
In an ideal transformer, the turns ratio is N_p/N_s = 1/2. The ratio V_s : V_p is equal to (the symbols carry their usual meaning):
A step down transformer connected to an ac mains supply of 220 V is made to operate at 11 V, 44 W lamp. Ignoring power losses in the transformer, what is the current in the primary circuit?
A 12 V, 60 W lamp is connected to secondary of step down transformer, whose primary is connected to ac mains of 220 V. Assuming the transformer to be ideal, what is the current in the primary winding?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A transformer is a device that raises or lowers ac voltage using two coils on a common iron core, linked by a changing magnetic flux (mutual induction).
On the principle of mutual induction: a changing current in the primary coil induces an emf in the secondary coil through a shared changing magnetic flux.
No. DC gives a constant flux with zero rate of change, so no emf is induced in the secondary. Transformers work only with ac.
A laminated soft-iron core, a primary (input) coil of N_p turns, and a secondary (output) coil of N_s turns, with the two coils insulated from each other.
V_s / V_p = N_s / N_p, and for current I_p / I_s = N_s / N_p, because input power equals output power (V_p I_p = V_s I_s).
