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4.4.8Electricity and Magnetism

Transformers — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 50 of 65 1 quiz

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Lesson summary

4.4.8 Transformers — IGCSE PHYSICS notes: Describe the structure and action of a transformer and explain why transformers are used with a.c. for efficient power transmission.Purpose — a transformer changes the size of an alternating p.d.

  • Primary coil
  • Secondary coil
  • Soft iron core
  • Laminated core
  • Power stations
  • Low current

Part 1 of 8Structure of a transformer

Structure of a transformer
Purpose
  • Purpose a transformer changes the size of an alternating p.d. or current.
Primary coil
  • Primary coil the input coil, connected to the supply.
Secondary coil
  • Secondary coil the output coil, connected to the load.
Soft iron core
  • Soft iron core it is easily magnetised and links the two coils, which are not electrically connected.
Laminated core
  • Laminated core thin insulated sheets reduce eddy currents and heating.

Part 2 of 8How a transformer works (Extended)

1
An alternating current is supplied to the primary coil.
2
This produces a changing magnetic field around the primary coil.
3
The iron core carries the changing field through the secondary coil.
4
The changing field induces an alternating e.m.f. in the secondary coil.
5
If the secondary is part of a complete circuit, an alternating current flows.
Frequency
  • Frequency the induced e.m.f. has the same frequency as the supply.
d.c.
  • d.c. a transformer does not work on d.c. because a steady current gives a steady field that induces nothing.

Part 3 of 8Step-up and step-down

TypeTurnsVoltage
Step-upMore turns on the secondary (Ns > Np)Vs > Vp
Step-downFewer turns on the secondary (Ns < Np)Vs < Vp
  • Power
    • a transformer that raises the voltage does not give more power, because the current falls.

Part 4 of 8High-voltage transmission

Power stations
  • Power stations produce electricity at around 25 kV, which is stepped up to hundreds of kilovolts for long distances.
Low current
  • Low current since P = I V, a high voltage allows the same power to be sent with a small current.
Less heating
  • Less heating the power lost in the cables is I2R, so a small current gives a much smaller loss.
Thinner cables
  • Thinner cables a lower current allows thinner, cheaper cables.
Near users
  • Near users the voltage is stepped down in stages to a safe level, such as 230 V for homes.

Part 5 of 8Reasons for the soft-iron core

Easily magnetised
  • Easily magnetised soft iron follows the alternating current with little energy loss.
Lamination
  • Lamination thin insulated sheets reduce circulating eddy currents in the core.

Part 6 of 8Energy losses in real transformers

Winding resistance
  • Winding resistance the copper wires have resistance, so there is I2R heating.
Eddy currents
  • Eddy currents currents flow in the core, though lamination reduces them.
Reversing magnetisation
  • Reversing magnetisation each reversal of the iron warms it.
Leakage
  • Leakage not all of the flux from the primary links with the secondary.
Efficiency
  • Efficiency large units are often above 98% efficient.

Part 7 of 8Everyday transformers

Phone charger
  • Phone charger a step-down transformer with a rectifier turns 230 V a.c. into a few volts.
Microwave oven
  • Microwave oven a step-up transformer raises the mains voltage to about 2000 V.
Hum
  • Hum the core is magnetised and demagnetised 100 times a second at 50 Hz, causing a 100 Hz hum.

Part 8 of 8More from the specification

More from the specification
Electromagnetic Induction in a Conductor
  • Moving a conductor wire across magnetic field lines induces an electromotive force detected by a sensitive voltmeter;
    • reversing the direction of motion induces a voltage in the opposite direction.
Magnetic Field of a Solenoid
  • Passing an electric current through a coiled solenoid produces a uniform magnetic field inside;
  • increasing the current or number of coil turns increases magnetic field strength.
Motor Effect and Electromagnetic Forces
  • A current-carrying conductor in a magnetic field experiences a mechanical force perpendicular to field lines; this interaction drives the continuous rotation of an electric motor.
    • (Fleming's left-hand rule)
Electromagnetic Induction in a Conductor
  • Moving a conductor wire across magnetic field lines induces an electromotive force detected by a sensitive voltmeter;
    • reversing the direction of motion induces a voltage in the opposite direction.
Magnetic Field of a Solenoid
  • Passing an electric current through a coiled solenoid produces a uniform magnetic field inside;
  • increasing the current or number of coil turns increases magnetic field strength.
Motor Effect and Electromagnetic Forces
  • A current-carrying conductor in a magnetic field experiences a mechanical force perpendicular to field lines; this interaction drives the continuous rotation of an electric motor.
    • (Fleming's left-hand rule)
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.
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.
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
Vp × Ip
Vs × Is [1 mark]
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil

Key terms

TermMeaning
TransformerDevice that changes the size of an alternating p.d.
Primary coilInput coil of a transformer.
Secondary coilOutput coil of a transformer.
Step-up transformerTransformer with more turns on the secondary, raising the voltage.
Step-down transformerTransformer with fewer turns on the secondary, lowering the voltage.
Laminated coreCore made of thin insulated sheets of soft iron.

Avoid these mistakes

1
Saying a transformer works on d.c. — it needs a changing field.
2
Claiming a step-up transformer increases the power — the current falls as the voltage rises.
3
Saying the coils are connected by wire — the link is the magnetic field in the core.
4
Using a solid iron core — it should be laminated.

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.
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