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1.9.6Electricity, Energy and Waves

Transformers

5 min read 5 parts Lesson 52 of 54 Unit quiz

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Section 1 of 5Structure of a Transformer

1.9.6 Transformers

A transformer changes the size of an alternating voltage without changing its frequency.

Primary coil
  • The input coil, connected to the alternating supply, which creates a changing magnetic field.
Secondary coil
  • The output coil, in which a changing field from the core induces an alternating voltage.
Iron core
  • Soft iron links the two coils and is easily magnetised, so it carries the changing field from primary to secondary.
Laminated core
  • The core is built from thin insulated sheets, which reduces energy wasted by currents induced in the iron itself.

Section 2 of 5How a Transformer Works

1
An alternating voltage is applied to the primary coil, so an alternating current flows in it.
2
The alternating current produces a magnetic field that is constantly increasing, decreasing and reversing.
3
The iron core carries this changing field through the centre of the secondary coil.
4
The changing field cuts the turns of the secondary coil and induces an alternating potential difference across it.
5
The output has the same frequency as the input, and a direct current would not work because the field would not change.

Section 3 of 5Step-Up and Step-Down

Step-Up and Step-Down
Step-up transformer
  • The secondary coil has more turns than the primary, so the output voltage is greater than the input voltage.
Step-down transformer
  • The secondary coil has fewer turns than the primary, so the output voltage is smaller than the input voltage.
Power
  • A transformer cannot create energy, so a step-up transformer gives a smaller current at the higher voltage.
Efficiency
  • Good transformers are close to 100% efficient, with small losses as heat in the coils and core.
TypeTurnsOutput voltage
Step-upNs > NpHigher than input
Step-downNs < NpLower than input

Section 4 of 5Transformers in the National Grid

Step-up at power stations
  • Voltage is raised to hundreds of thousands of volts before the electricity is sent along transmission lines.
Why high voltage
  • For the same power, a higher voltage means a lower current, so less energy is wasted as heat in the cables.
Step-down near homes
  • Substations reduce the voltage in stages until it reaches a safe 230 V for use in houses.
Lower cost
  • Because less energy is lost, thinner and cheaper cables can be used for long distances.

Section 5 of 5Exam Checklist & Tips

1.9.6 – Key Points for the Exam

  • Transformer A device that changes the size of an alternating voltage using electromagnetic induction.
  • Primary coil The coil connected to the input supply.
  • Secondary coil The coil from which the output is taken.
  • Step-up transformer A transformer that increases the voltage because the secondary has more turns.

Common Mistakes

  • Connecting a transformer to a battery and expecting an output;
    • only a changing current produces a changing field.
  • Thinking a step-up transformer increases the energy; it increases voltage but decreases current.
  • Confusing which coil is primary and which is secondary; primary is the input.
  • Forgetting that the iron core must be laminated or explaining it as reducing 'friction' rather than induced currents.
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.