The Transformer Equation (OCR GCSE Physics A (Gateway)): Revision Note

The Transformer Equation

Higher Tier 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:

$\frac{V_p}{V_s}=\frac{N_p}{N_s}$

• Where

  • V_p = potential difference (voltage) across the primary coil in volts (V)

  • V_s = 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:

$\frac{V_s}{V_p}=\frac{N_s}{N_p}$

• 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 (N_s > N_p)

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 (N_s < N_p)

Using the Transformer Equation

Higher Tier 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