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Physics Current electricity and magnetism
Class 10 Intermediate About 12 min Premium

The transformer: stepping voltage up and down

Change the number of turns on the primary and secondary coils of a transformer on 230 V mains, and see how the output voltage changes. Then try it on DC.

Core concept

A transformer has two coils on an iron core; an alternating current in the primary makes a changing magnetic flux that induces a voltage in the secondary, with Vs ÷ Vp = Ns ÷ Np; a steady DC makes no changing flux, so it gives no output

Topic: Current electricity and magnetism

Your phone charger, the electricity substation near your town and the power station all use transformers. A transformer has two coils of insulated wire wound on a soft iron core. The mains supply in Nepal is alternating current: The flow of electricity through a wire, measured in amperes (A). All key words (AC) at 230 V and 50 Hz: it changes direction 50 times a second. In the primary coil this makes a changing magnet: An object that pulls iron and steel towards it. All key words field (magnetic flux) in the core, and the changing flux induces a voltage: The push that makes electricity flow, measured in volts (V). All key words in the secondary coil: electromagnetic induction. The voltages are in the ratio of the turns: Vs ÷ Vp = Ns ÷ Np. A step-down transformer has fewer secondary turns and lowers the voltage; a step-up transformer has more and raises it. Direct current (DC) flows steadily one way, so after it is switched on the flux does not change and no voltage is induced.

Key words explained

What you will learn

  • Distinguish between alternating current and direct current.
  • Describe the parts and the working of a transformer.
  • Use Vs ÷ Vp = Ns ÷ Np to find the output voltage.
  • Explain why a transformer does not work with DC, and why power is sent at high voltage.

Before you start

  • Know that a changing magnetic field near a coil induces a current in it.

Materials

  • An old phone charger or a doorbell transformer (to look at, unplugged)

Equipment

  • A school transformer kit with a low-voltage AC supply, a voltmeter

Procedure

  1. Record the output with 1000 primary turns and 100 secondary turns.
  2. Record with 50, 1000 and 2000 secondary turns.
  3. Record with 500 primary turns and 2000 secondary turns.
  4. Switch to DC and record.

How this simulation is simplified

  • The transformer is ideal: no energy is lost.
  • The input is 230 V, either AC mains or DC.
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