Transformer Calculations | WJEC GCSE Physics Revision Notes 2018

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:

$\frac{voltage across primary coil}{voltage across secondary coil} = \frac{number of turns on primary coil}{number of turns on secondary coil}$

$\frac{V_{1}}{V_{2}} = \frac{N_{1}}{N_{2}}$

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

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

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

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

$\frac{N_{2}}{N_{1}} = \frac{V_{1}}{V_{2}}$

Step 3: Rearrange the equation to make V₂ the subject

$V_{2} = \frac{V_{1} \times N_{1}}{N_{2}}$

Step 4: Substitute the known values into the equation

$V_{2} = \frac{500 \times 800}{20}$

Step 5: Calculate the output potential difference

$V_{2} = 20 000 V$

(b)

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

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.