Transformer Calculations (Cambridge O Level Physics)

• 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:

• 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)
  • n_p = number of turns on primary coil
  • n_s = number of turns on secondary coil

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

• 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

• Transformers have a number of roles:
  • They are used to increase the potential difference of electricity before it is transmitted across the national grid
  • They are used to lower the high voltage electricity used in power lines to the lower voltages used in houses
  • They are used in adapters to lower mains voltage to the lower voltages used by many electronic devices

Advantages of High Voltage Transmission

• When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
• To transmit the same amount of power as the input power the potential difference at which the electricity is transmitted should be increased
  • This will result in a smaller current being transmitted through the power lines
  • This is because P = IV, so if V increases, I must decrease to transmit the same power

• A smaller current flowing through the power lines results in less heat being produced in the wire
  • This will reduce the energy loss in the power lines

Model of High-Voltage Transmission

Electricity is transmitted at high voltage, reducing the current and hence power loss in the cables