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6.2.3Magnetism and Electromagnetism

Transformers

5 min read 5 parts Lesson 15 of 16 Unit quiz

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Section 1 of 5Structure

6.2.3 Transformers

A transformer has two coils wound on a common iron core.

Primary coil
  • The input coil, connected to the a.c. supply, which creates a changing magnetic field in the iron core.
Secondary coil
  • The output coil, in which the changing magnetic field induces an alternating potential difference.
Iron core
  • Carries the magnetic field from the primary to the secondary, so that little of the field is lost as it would be in air.
Turns ratio
  • The ratio of the numbers of turns on the two coils sets whether the output potential difference is larger or smaller than the input.

Section 2 of 5How a Transformer Works

Induction through the shared core transfers energy from the primary circuit to the secondary circuit.

1
An alternating current in the primary coil produces a constantly changing magnetic field in the iron core.
2
The changing magnetic field passes through the secondary coil and induces an alternating potential difference across it.
3
A transformer needs a changing input, because a steady d.c. field induces no potential difference in the secondary.
4
In a step-up transformer the secondary has more turns than the primary, so the output potential difference is larger than the input.
5
In a step-down transformer the secondary has fewer turns than the primary, so the output potential difference is smaller than the input.

Section 3 of 5Why Transformers Matter

Transformers allow electricity to be sent at high voltage and used at safe voltage.

Why Transformers Matter
Step-up use
  • Power stations use step-up transformers to raise the potential difference, which reduces the current and the energy lost in cables.
Step-down use
  • Step-down transformers near homes and factories lower the potential difference to a safe value for appliances.
Energy losses
  • Power lost in cables is given by I²R, so a smaller current from a higher voltage gives much lower losses.
Efficiency
  • Real transformers lose some energy as heat, so the output power is slightly less than the input power.

Section 4 of 5Worked Example

Question Explain why a transformer does not work when connected to a d.c. supply.

  1. A d.c. supply produces a steady current, which produces a steady magnetic field in the core.
  2. A steady magnetic field does not change, so no field lines cut the secondary coil.
  3. No potential difference is induced in the secondary coil, so the transformer produces no output.

Answer A steady d.c. field does not change, so it induces no potential difference in the secondary coil.

Section 5 of 5Exam Checklist & Tips

6.2.3 – Key Points for the Exam

  • Transformer A device that changes the potential difference of an alternating supply using two coils linked by an iron core.
  • Primary coil The input coil, which is connected to the alternating supply.
  • Secondary coil The output coil, in which a potential difference is induced.
  • Step-up transformer A transformer with more turns on the secondary than the primary, giving a larger output potential difference.
  • Step-down transformer A transformer with fewer turns on the secondary than the primary, giving a smaller output potential difference.

Common Mistakes

  • Saying a transformer changes the frequency; the frequency of the output is the same as the input.
  • Connecting a transformer to a d.c. supply, which induces no potential difference in the secondary.
  • Confusing step-up and step-down by counting the turns on the wrong coil.
  • Stating that a transformer creates energy; it only transfers energy between two circuits.
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.