Transformers (HT only)

4.1.3 National and global energy resources


A basic transformer consists of a primary coil and a secondary coil wound on an iron core.

Iron is used as it is easily magnetised.

Knowledge of laminations and eddy currents in the core is not required.

The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on the ratio of the number of turns on each coil, np and ns.

potential difference, Vp and Vs in volts, V

In a step-up transformer Vs > Vp

In a step-down transformer Vs < Vp

If transformers were 100 % efficient, the electrical power output would equal the electrical power input.

Vs × Is = Vp × Ip

Where Vs × Is is the power output (secondary coil) and Vp × Ip is the

power input (primary coil).

power input and output, in watts, W

Students should be able to:

  • explain how the effect of an alternating current in one coil in inducing a current in another is used in transformers
  • explain how the ratio of the potential differences across the two coils depends on the ratio of the number of turns on each
  • calculate the current drawn from the input supply to provide a particular power output
  • apply the equation linking the pds and number of turns in the
  • two coils of a transformer to the currents and the power transfer involved, and relate these to the advantages of power transmission at high potential differences.

MS 3b Change the subject of an equation

MS 3c Substitute numerical values into algebraic equations using appropriate units for physical quantities

MS 1c Use ratios, fractions and percentages







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