Physics · Alternating Current · NEET
NCERT lists exactly four: (1) Flux leakage — not all primary flux reaches the secondary. (2) Resistance of the windings — the wire has resistance, so heat is lost as I^2 R (called copper loss). (3) Eddy currents — the changing flux induces circulating currents in the iron core that heat it. (4) Hysteresis — the core magnetisation keeps reversing with the AC, and this repeated re-magnetising wastes energy as heat. Losses 3 and 4 happen in the iron core, so together they are called iron loss (or core loss).
The alternating magnetic flux induces eddy currents in the iron core, and these circulating currents heat the core (energy wasted). A solid block would allow large eddy currents. By building the core from thin sheets (laminations) that are insulated from each other, the path for eddy currents is broken into thin layers, so the induced currents are much smaller and the heating loss drops sharply. This is why every transformer core looks like a stack of thin plates, not one solid iron block.
Copper loss is the I^2 R heating in the copper windings, caused by the resistance of the wire. It depends on the load current, so it rises when the transformer supplies more current. Iron loss (core loss) happens inside the iron core and has two parts: eddy current loss and hysteresis loss. Iron loss depends mainly on the flux and frequency, so it stays roughly constant whether the transformer is lightly or heavily loaded. For NEET, remember: copper loss = windings, iron loss = core.
During each AC cycle the core's magnetisation is reversed twice, and reversing the tiny magnetic domains each time wastes energy that appears as heat. This waste per cycle equals the area of the material's B-H hysteresis loop. To reduce it, the core is made of a soft magnetic material with a low hysteresis loss (a thin, narrow B-H loop), such as soft iron or silicon steel, instead of hard steel which has a fat loop.
Flux leakage means some of the magnetic flux produced by the primary does not pass through the secondary — it escapes through air gaps or a poorly designed core. That leaked flux does no useful work, so it counts as a loss. It is reduced by winding the primary and secondary coils one over the other (concentric windings) on the same core limb, so almost all the flux links both coils.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Some energy is always lost as heat through flux leakage, copper loss, eddy currents and hysteresis. But NCERT notes a well-designed transformer can be more than 95% efficient, so treating it as ideal (power in = power out) is a good approximation for NEET numericals.
Eddy current loss and hysteresis loss together form the iron loss, because both occur inside the iron core. Copper loss (I^2 R in the windings) is separate.
No. A transformer needs a changing (alternating) flux to induce a voltage in the secondary. Steady DC gives constant flux, so no EMF is induced in the secondary. This is why transformers are used only with AC — a key reason AC is used for power transmission.
Stepping up the voltage lowers the current for the same power (P = VI). Lower current means much smaller I^2 R heating loss in the long transmission wires, so more of the generated energy reaches homes. Near the consumer, step-down transformers bring the voltage back to a safe ~240 V.
Copper loss changes with load because it depends on the current (I^2 R) the transformer is supplying. Iron loss stays almost constant because it depends on the flux and frequency, not on how much current the load draws.
