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4.5.6Electricity and Magnetism

THE Transformer — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 64 of 65 1 quiz

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Lesson summary

4.5.6 The Transformer — IGCSE PHYSICS notes: Describe the transformer, use the turns-ratio and ideal-power equations and explain high-voltage transmission.Coils — a primary coil and a secondary coil are wound on a laminated soft-iron core.Purpose — it changes the size of an alter…

  • Vp / Vs = Np / Ns
  • IpVp = IsVs
  • Efficiency
  • Step-up transformers
  • Step-down transformers
  • Low current

Part 1 of 7Construction

Coils
  • Coils a primary coil and a secondary coil are wound on a laminated soft-iron core.
Purpose
  • Purpose it changes the size of an alternating voltage.
Soft iron
  • Soft iron it is easily magnetised and demagnetised, and laminations reduce energy wasted by eddy currents.

Part 2 of 7Turns-ratio equation

1Vp / Vs
Np / Ns
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
  • V is voltage (V) and N is the number of turns.
2Step-up
  • Ns > Np, so Vs > Vp.
3Step-down
  • Ns < Np, so Vs < Vp.

Part 3 of 7How a transformer works (Extended)

1
An alternating voltage is applied to the primary coil, so an alternating current flows.
2
This current produces a changing magnetic field in the soft-iron core.
3
The core links this changing field to the secondary coil.
4
The changing field induces an alternating e.m.f. in the secondary coil.
5
The size of the secondary voltage depends on the turns ratio.
  • d.c.
    • a transformer does not work with steady d.c., because the field would not change and no e.m.f. would be induced.

Part 4 of 7Ideal transformer (Extended)

IpVp
IsVs
  • for a 100% efficient transformer, power in equals power out.
Current
  • Current stepping the voltage up steps the current down by the same factor.
Efficiency
  • Efficiency useful power output divided by total power input, multiplied by 100%.

Part 5 of 7High-voltage transmission

1Step-up transformers
  • Step-up transformers at the power station, to raise the voltage.
2Step-down transformers
  • Step-down transformers near homes, for safety.
3Low current
  • Low current high voltage means low current, so less energy is wasted as heat in the cables.
4Cables
  • Cables the lower current allows thinner, cheaper cables.

Part 6 of 7Power loss in cables (Extended)

P = I2R
What the symbols mean: P = power (W) · I = current (A) · R = resistance (Ω)
  • power loss (W) equals current squared times resistance.
Halving the current
  • Halving the current it reduces the loss to one quarter.
FeatureStep-upStep-down
VoltageIncreasesDecreases
TurnsNs > NpNs < Np
CurrentDecreasesIncreases
UsePower stationHomes and chargers

Part 7 of 7More from the specification

More from the specification
Transformer structure
  • Two coils, primary and secondary, wound on a soft iron core;
    • an alternating current in the primary creates a changing magnetic field in the core.
Electromagnetic induction
  • The changing field links the secondary coil and induces a voltage across it, as a moving conductor cutting field lines would.
Transformer equation
  • Vp/Vs = Np/Ns, so more turns on the secondary gives a step-up voltage and fewer turns gives step-down.
Power and efficiency
  • For an ideal transformer, VpIp = VsIs, so a higher voltage means a lower current.
Transmission and motor contrast
  • Step-up transformers reduce current and energy loss in cables;
    • a motor does the opposite job by using current in a field to produce a force.
Generator
  • Device that turns kinetic energy into electrical energy.
Brushes
  • Carbon contacts pressing on the slip rings.
Field lines
  • Lines showing the direction and strength of a magnetic field.
Potential difference
  • Work done by a unit charge passing through a component.
Electromagnetic induction
  • Production of an e.m.f. by relative movement of a conductor and a magnetic field.
Permanent magnet
  • Magnet whose field is always there.
Solenoid
  • Long coil of wire carrying a current
Conductor
  • Material that allows current to flow.
Voltmeter
  • Instrument that measures p.d., connected in parallel.

Key terms

TermMeaning
TransformerDevice that changes the size of an alternating voltage.
Turns ratioRatio of the number of turns on the two coils.
Ideal transformer100% efficient transformer where power in equals power out.
Eddy currentsUnwanted currents induced in the core, reduced by lamination.
EfficiencyUseful power output divided by power input.

Avoid these mistakes

1
Saying a transformer works with d.c. — it needs alternating current.
2
Claiming a step-up transformer increases the power — the power stays the same for an ideal transformer.
3
Inverting the turns ratio — the larger voltage belongs to the coil with more turns.
4
Using a solid iron core instead of a laminated one.

Last updated:

Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: Vp / Vs = Np / Ns = Is / Ip

  • Step-up reduces grid thermal loss (P = I2R).
  • Laminated core prevents eddy currents.