# Transformer > Transformer for NEET Physics: turns ratio, step-up and step-down operation, losses in real transformers and power transmission economics. - Canonical URL: https://prepelephant.com/topics/neet-ug/physics/transformer-neet - Exam / course: NEET-UG · Subject: Physics - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Transformer", PrepElephant, https://prepelephant.com/topics/neet-ug/physics/transformer-neet ## Direct answer Mains electricity reaches an Indian home at 220 V, yet a doorbell needs a few volts and a transmission line wants several lakh — the transformer performs both conversions with no moving parts. An alternating current in the primary sets up changing flux in a laminated soft-iron core, inducing an emf in the secondary by mutual induction: E_s/E_p = N_s/N_p, with an ideal machine conserving power so I_s/I_p = N_p/N_s. Step-up raises voltage and lowers current; step-down does the reverse; and DC gives a steady flux and no emf — the machines are AC-only, which is why long-distance transmission steps the voltage up: halving the current quarters the line's I^2R loss. ## What you must remember - **Turns ratio:** E_s/E_p = N_s/N_p; the voltage ratio equals the turns ratio, and for an ideal transformer I_p V_p = I_s V_s so currents go the opposite way. - **Step-up versus step-down:** N_s > N_p raises voltage and lowers current; N_s < N_p does the reverse — a transformer changes voltage and current, never frequency and never DC power. - **Why AC only:** a steady DC flux does not change, so dΦ/dt = 0 and no emf appears in the secondary; feeding DC can also burn the primary, whose opposition to current is then only its small resistance. - **Four losses:** copper loss I^2R in the windings (reduced by thick conductors), eddy-current heating (reduced by laminations), hysteresis loss per cycle (reduced by soft silicon steel with a narrow loop), and flux leakage (reduced by a closed core winding both coils on a common limb). - **Efficiency:** real power transformers commonly run above 95 per cent, large units near 99. - **Transmission logic:** line loss = I^2R_line; stepping voltage up by a factor of n cuts current n-fold and loss n^2-fold — the entire case for high-voltage transmission. ## A doorbell transformer, accounted to the last decimal A doorbell transformer has N_p = 2200 turns on the primary connected to the 220 V mains, and N_s = 100 turns on the secondary. The secondary voltage follows the ratio: E_s = 220 × (100/2200) = 10 V. The bell draws 2 A at 10 V — 20 W — and an ideal transformer would draw exactly 20 W from the mains, I_p = 20/220 ≈ 0.09 A. Now run the machine backwards: connect 10 V AC to the 100-turn side and the 2200-turn side delivers 220 V — the same device is a step-up transformer now, because "primary" only names the coil you feed. And run the loss bookkeeping: if this real unit is 90 per cent efficient, the primary draws 22.2 W, the extra 2.2 W shared among copper heating, eddies in the laminations and hysteresis in the iron every cycle. Each loss has a named cure, and matching loss to cure is half the marks in this chapter. ## Where the turns hide the traps The most frequent error is treating the transformer as a power source: when the secondary voltage rises, the secondary current falls, and options that show both increasing violate conservation — the examiner includes them to catch formula-without-physics. The second trap is the DC question: a transformer supplied with DC delivers nothing (after a transient), and the primary may burn because only the winding's small resistance limits the current with no reactive opposition — the justification expected in assertion-reason form. Third, frequency is invariant: a 50 Hz input gives a 50 Hz output, whatever the turns ratio, since the secondary emf follows the same changing flux. Fourth, loading: an unloaded transformer draws only a small magnetising current, and the primary current grows only as the secondary delivers power. Finally, efficiency direction: efficiency = output power/input power, and the losses scale differently — copper loss with load current squared, hysteresis and eddy losses with frequency regardless of load — so "which loss remains even on no load" has a definite answer. ## Frequently asked questions ### On what principle does a transformer work? Mutual induction: an alternating primary current produces a changing core flux that links the secondary and induces an emf proportional to its number of turns. ### Why cannot a transformer work on direct current? A steady DC produces a constant flux and no induced emf; worse, the primary may overheat for lack of inductive opposition. ### How does stepping up voltage reduce transmission losses? Power at higher voltage needs lower current, and the line's heat loss I^2R falls as the square of that current reduction — the reason grids run at very high voltages. ### Which losses occur in a real transformer? Copper (I^2R) losses in the windings, eddy-current heating in the core, hysteresis loss in the iron each cycle, and flux leakage that fails to link both coils. ### Can the same transformer be used as step-up and step-down? Yes — the coil you feed is the primary; reversing the roles inverts the turns ratio, so a 220 V-to-10 V unit used backwards steps 10 V up to 220 V.