The National Grid
The National Grid is the network of cables, pylons and substations that distributes electricity from power stations to homes and businesses across the UK. To minimise energy lost as heat, electricity is transmitted at very high voltage and stepped down before reaching consumers.
Why high-voltage transmission?
Power $P = VI$. If you want to deliver the same power, you can either use high $V$ and low $I$, or low $V$ and high $I$.
Heat dissipated in a cable is $P_{\text{loss}} = I^2 R$. So doubling the current increases heat loss by a factor of four. To minimise loss, transmit at low current — which means high voltage.
The grid transmits at 132 kV, 275 kV or 400 kV. Domestic supply is 230 V.
Transformers — how voltage is changed
A transformer changes ac voltage. It has:
- A primary coil (input).
- A secondary coil (output).
- A laminated iron core that links them magnetically.
The ratio of voltages equals the ratio of turns:
$\frac{V_p}{V_s} = \frac{n_p}{n_s}$
A step-up transformer has $n_s > n_p$, so $V_s > V_p$ — used at the power station to raise voltage for transmission.
A step-down transformer has $n_s < n_p$, used in the substation near consumers.
For an ideal (100% efficient) transformer: $V_p I_p = V_s I_s$
So when voltage is stepped up, current is stepped down by the same factor — keeping power constant.
The grid journey
- Power station — generates electricity at ~25 kV.
- Step-up transformer — raises to 132 / 275 / 400 kV.
- Transmission lines — long-distance pylons.
- Step-down transformer — local substation lowers to 11 kV (industry) or 230 V (homes).
- Consumer — uses domestic mains.
Why ac, not dc?
Transformers only work with changing magnetic fields, which require ac. dc generates a constant field, so the secondary coil sees no flux change and no induced voltage. ac is therefore the standard for grid transmission.
✦Worked example
A step-up transformer has 200 turns on the primary and 4000 turns on the secondary. The primary is connected to 25 kV. Find the secondary voltage.
- $V_s = V_p \times \frac{n_s}{n_p} = 25,000 \times \frac{4000}{200} = 25,000 \times 20 = 500,000$ V = 500 kV.
⚠Common mistakes
- Confusing step-up and step-down — step-up has more turns on the secondary.
- Forgetting that current goes down when voltage goes up (in an ideal transformer).
- Saying high V is dangerous in the home — it is, which is exactly why we step down before delivering to consumers.
- Believing transformers waste no power — real transformers have small losses (mainly in the core).
AI-generated · claude-opus-4-7 · v3-deep-physics