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GCSEPhysicsWJECRevision Notes1. Electricity, Energy & Waves1.9 ElectromagnetismTransformer Calculations

Transformer Calculations (WJEC GCSE Physics)

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Ann H

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Ann H

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The Transformer Equation

  • The output voltage of a transformer depends on:
    • The number of turns on the primary and secondary coils
    • The input voltage
  • When the number of turns on the secondary coil is lower than the primary coil
    • The output voltage is lower than the input
    • This is a step-down transformer
  • When the number of turns in the primary coil is lower than in the secondary coil
    • The output voltage is higher then
    • This is a step-up transformer
  • Input or output voltages, as well as the number of turns on the primary and secondary coils, can be calculated using the equation:

fraction numerator voltage space across space primary space coil over denominator voltage space across space secondary space coil end fraction space equals space fraction numerator number space of space turns space on space primary space coil over denominator number space of space turns space on space secondary space coil end fraction

V subscript 1 over V subscript 2 space equals space N subscript 1 over N subscript 2

  • The ratio of the voltage across the primary and secondary coils of a transformer is equal to the ratio of the number of turns on each coil

Efficiency of a Transformer

  • This equation assumes that the transformer is 100% efficient and no heat is dissipated into the surroundings
  • Heat energy losses are reduced through the use of a laminated iron core 

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
 

Answer: 

(a)

Step 1: List the known quantities

  • Number of turns in primary coil, N1  = 20
  • Number of turns in secondary coil, N2  = 800
  • Voltage in primary coil, V1  = 500 V

Step 2: Write the equation linking output voltage to the known quantities

N subscript 2 over N subscript 1 space equals space V subscript 1 over V subscript 2 

Step 3: Rearrange the equation to make V2  the subject

V subscript 2 space equals space fraction numerator V subscript 1 space cross times space N subscript 1 over denominator N subscript 2 end fraction 

Step 4: Substitute the known values into the equation

V subscript 2 space equals space fraction numerator 500 space cross times space 800 over denominator 20 end fraction 

Step 5: Calculate the output potential difference

bold italic V subscript bold 2 bold space bold equals bold space bold 20 bold space bold 000 bold space bold italic V 

(b)

  • This is a step-up transformer because the voltage on the secondary coil is higher than that on the primary coil

Examiner Tip

Remember this equation is a ratio, so you need to make sure you have voltage and number of turns for one coil on either the top or the bottom.

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 no coils.

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    Ann H

    Author: Ann H

    Expertise: Physics

    Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students no matter their schooling or background.