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How does a transformer work?

Class 12 PhysicsNCERT chapter: Alternating Currentइसे हिंदी में पढ़ें

Exam-ready answer

A transformer changes an alternating voltage using mutual induction. It has a primary and a secondary coil wound on a laminated soft iron core. Alternating current in the primary produces a changing magnetic flux in the core, which links the secondary and induces an emf in it. For an ideal transformer, Vs/Vp = Ns/Np = Ip/Is.

Explanation

When an alternating voltage vp is applied to the primary coil, the alternating current produces a magnetic flux φ that keeps changing with time. The soft iron core guides almost all of this flux through the secondary coil. Assuming no flux leakage, the same φ links each turn of both coils, so the emf induced in the secondary is εs = −Ns dφ/dt and the back emf in the primary is εp = −Np dφ/dt.

If the primary has negligible resistance, vp = εp, and if the secondary current is small, vs = εs. Dividing gives vs/vp = Ns/Np. In a step-up transformer Ns > Np, so the output voltage is higher; in a step-down transformer Ns < Np, so it is lower. In an ideal transformer, input power equals output power, ip vp = is vs, so the current changes in the inverse ratio.

A transformer works only with AC, because steady DC produces constant flux and no induced emf. Real transformers lose some energy through flux leakage, resistance of the windings, eddy currents in the core (reduced by a laminated core) and hysteresis (reduced by soft iron). Power is sent over long distances at high voltage and low current to cut I²R heating loss in the lines; step-down transformers lower the voltage near homes.

Points to remember

  1. Principle: mutual induction between two coils linked by a common changing magnetic flux.
  2. Voltage ratio: Vs/Vp = Ns/Np (the turns ratio).
  3. Ideal transformer: VpIp = VsIs, so Is/Ip = Np/Ns.
  4. Step-up: Ns > Np (voltage increases, current decreases); step-down: Ns < Np (voltage decreases).
  5. Energy losses: flux leakage, resistance of windings, eddy currents and hysteresis; a laminated soft iron core reduces core losses.

Exam tip

Draw a labelled diagram showing the primary coil, secondary coil and laminated core, then derive Vs/Vp = Ns/Np step by step from εs = −Ns dφ/dt and εp = −Np dφ/dt. List at least two energy losses with their remedies.

Questions students also ask

Why does a transformer not work on DC?
A steady direct current produces a constant magnetic flux in the core. Induced emf depends on the rate of change of flux, and here the flux does not change, so no emf is induced in the secondary. A transformer needs alternating current to create continuously changing flux.
Does a step-up transformer increase power?
No. A transformer cannot create energy. In an ideal transformer, input power equals output power. When the voltage is stepped up, the current is stepped down in the same ratio, so the product VI stays the same. Real transformers give slightly less output because of losses.
Why is the core of a transformer laminated?
A solid iron core would carry large eddy currents induced by the changing flux, wasting energy as heat. Thin insulated sheets (laminations) break up the paths of these currents, making them much smaller and reducing the energy lost in the core.

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