Physics · Alternating Current · NEET
A transformer works only when the magnetic flux through its coils keeps changing, because it runs on electromagnetic induction (emf appears only when flux changes). Steady DC gives a constant flux, so no emf is induced in the secondary and the voltage cannot be stepped up or down. Without stepping up the voltage, DC would have to travel at low voltage and high current, which causes huge I²R heat loss in the wires. That is why the whole grid uses AC.
The power delivered is P = V·I. To send the same power P, if you make V large, then I becomes small (because I = P/V). The heat lost in the transmission wire is P(loss) = I²R, which depends on the current squared. So halving the current cuts the loss to one-quarter. By stepping the voltage up thousands of times, the current becomes tiny and the loss becomes very small. This is the key idea NEET tests.
For the loss in the transmission line, always use P(loss) = I²R. Here R is the fixed resistance of the wire and I is the current actually flowing through that wire. Do NOT use V²/R with the supply voltage — that V is the voltage across the load/line ends, not the small voltage drop across the wire itself. The current I is the same everywhere in a series line, so I²R is the safe, correct formula for line loss.
No — this is the classic confusion. The loss is I²R, and it depends on current, not on the transmission voltage. Raising the voltage lowers the current for the same power, so the loss actually goes DOWN, not up. High voltage is dangerous to touch, but it is exactly what makes transmission efficient. At the city, a step-down transformer brings it back to a safe 220 V.
Power is generated at a plant (around 11 kV), then a step-up transformer raises it to very high voltage (like 220 kV or more) for the long journey through transmission towers. Near your city, step-down transformers lower it in stages back to 220 V for your home. This step-up-then-step-down chain is only possible with AC, which is the whole reason AC is chosen.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because AC voltage can be stepped up by a transformer to a very high value, which makes the current very low, so the I²R loss in the long wires is minimal. DC voltage cannot be changed easily by a transformer.
A transformer works on electromagnetic induction, which needs a changing magnetic flux. DC gives a steady, unchanging flux, so no emf is induced in the secondary coil and no voltage transformation happens. Only the changing flux of AC induces the secondary emf.
Power loss = I²R, where I is the current in the line and R is the resistance of the wire. It depends on the square of the current, so lowering the current strongly lowers the loss.
By stepping the voltage up. Since P = V·I for a fixed power P, a high V forces a low I. A step-up transformer at the generating station raises the voltage before the power enters the long transmission lines.
High voltage is dangerous to touch, so transmission lines are placed high on towers with insulation. Before reaching homes, step-down transformers reduce it to a safe 220 V. The high voltage is used only for the long-distance efficient transfer.
