GCSEPhysicsOCR GatewayRevision Notes4. Magnetism & Magnetic Fields4.2 Uses of Magnetism4.2.7 The Transformer Equation

The Transformer Equation (OCR Gateway GCSE Physics)

Revision Note

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The Transformer Equation (HT only)

  • The output potential difference (voltage) of a transformer depends on:
    • The number of turns on the primary and secondary coils
    • The input potential difference (voltage)

  • It can be calculated using the equation:

  • This equation can be written using symbols as follows:

V subscript p over V subscript s equals N subscript p over N subscript s

  • Where
    • Vp = potential difference (voltage) across the primary coil in volts (V)
    • Vs = potential difference (voltage) across the secondary coil in volts (V)
    • Np = number of turns on primary coil
    • Ns = number of turns on secondary coil

  • The equation above can be flipped upside down to give:

V subscript s over V subscript p italic equals N subscript s over N subscript p

  • The equations above show that:
    • The ratio of the potential differences across the primary and secondary coils of a transformer is equal to the ratio of the number of turns on each coil

Step-up Transformer

  • A step-up transformer increases the potential difference of a power source
  • A step-up transformer has more turns on the secondary coil than on the primary coil (Ns > Np)

Step-down Transformer

  • A step-down transformer decreases the potential difference of a power source
  • A step-down transformer has fewer turns on the secondary coil than on the primary coil (Ns < Np)

Using the Transformer Equation (HT only)

  • The transformer equation is used to calculate the potential difference (V), or number of turns (N), in the primary or secondary coil of a transformer

Worked example

A transformer has 20 turns on the primary coil and 800 turns on the secondary coil. The input potential difference across the primary coil is 500 V.

a) Calculate the output potential difference

b) State what type of transformer this is

Exam Tip

When you are using the transformer equation make sure you have used the same letter (p or s) in the numerators (top line) of the fraction and the same letter (p or s) in the denominators (bottom line) of the fraction. 

There will be less rearranging to do in a calculation if the variable which you are trying to find is on the numerator (top line) of the fraction.

The individual loops of wire going around each side of the transformer should be referred to as turns and not coils.

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    Katie M

    Author: Katie M

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.