Transformers (Higher tier)

What is a transformer?

A transformer changes the size of an alternating voltage. It has two coils (primary and secondary) wound on the same iron core.

• Primary coil is connected to the input a.c. supply.
• Secondary coil is connected to the output.
• Both coils are wrapped on a soft iron core to concentrate the magnetic field.

How it works

1. A.c. in the primary coil produces a constantly changing magnetic field in the iron core.
2. The changing field induces a changing voltage in the secondary coil (electromagnetic induction).
3. Transformers only work with a.c. — d.c. produces a constant field that does not induce a voltage.

Turns ratio

The voltage ratio equals the turns ratio:

Vp / Vs = Np / Ns

• Vp = primary voltage; Vs = secondary voltage.
• Np = number of turns on primary; Ns = number of turns on secondary.

Step-up transformer: Ns > Np → Vs > Vp (voltage increased). Step-down transformer: Ns < Np → Vs < Vp (voltage decreased).

Power and current

For an ideal (100% efficient) transformer, power in = power out:

Vp × Ip = Vs × Is

So a step-up transformer increases voltage but decreases current by the same ratio. This is the key to the National Grid.

The National Grid

Electricity is generated at ~25 kV at the power station, then a step-up transformer raises it to 132–400 kV for transmission across pylons. At the consumer end, step-down transformers lower it back to 230 V for homes.

Why high-voltage transmission? Power loss in cables is P = I²R. Doubling the voltage halves the current; the I² term means power loss is reduced by a factor of 4. High-voltage transmission massively reduces wasted heat in the cables.

Edexcel exam tip

When asked "why is electricity transmitted at high voltage?" the marks are for: (1) at higher V the current is lower, (2) lower current means less energy lost as heat (because P = I²R), (3) so the grid is more efficient. Saying simply "less energy is lost" without the I²R reasoning is half marks.