The National Grid

The National Grid is a network of cables and transformers that links power stations to homes, schools and businesses across the country.

• Step-up transformers at power stations increase the pd to hundreds of thousands of volts before the electricity travels along the transmission cables.
• Step-down transformers near where the electricity is used decrease the pd to a safe 230 V for homes.

For a given power, P = VI. So if the pd is high, the current can be low to carry the same power.

A current heats the cables, wasting energy. The power wasted is P = I²R, so it depends on the current squared. A lower current means much less energy is lost by heating, which makes the grid an efficient way to transfer energy.

Example: 1 000 000 W is sent along cables with a resistance of 10 Ω.

• At 25 000 V: I = 1 000 000 ÷ 25 000 = 40 A, so power lost = 40² × 10 = 16 000 W
• At 400 000 V: I = 1 000 000 ÷ 400 000 = 2.5 A, so power lost = 2.5² × 10 = 62.5 W

The journey from power station to home:

1. The power station generates electricity.
2. A step-up transformer raises the pd and lowers the current.
3. Transmission cables, often on pylons, carry the electricity across the country with little energy wasted.
4. A step-down transformer lowers the pd and raises the current.
5. Homes get the supply at about 230 V.