The national grid
The national grid distributes electrical power via cables and transformers. Electricity is generated at about 25 kV; a step-up transformer raises it to 400 kV (or 132 kV) for long-distance transmission on overhead cables, and step-down transformers reduce it in stages to 230 V for homes. The high transmission voltage is used specifically to cut energy losses in the cables — not for convenience.
High V → low I → low I²R loss
Cable power loss is ( = cable resistance, = current). For a fixed transmitted power (), raising the voltage lowers the current in the same proportion. Because loss depends on , cutting the current by a factor of 16 cuts the loss by . Doubling the voltage quarters the loss. The cable resistance is unchanged — only the current falls.
Transformer ratio
( = primary/secondary voltage, = turns). Step-up: , so ; step-down: , so . For an ideal transformer, power in = power out: , so stepping voltage up steps current down by the same ratio. Real transformers are 98–99% efficient, close to ideal.
Drawn from real examiner reports.
Explain loss via I²R, not just efficiency
The mark scheme wants the chain high voltage → lower current → lower loss. Writing only "high voltage is more efficient" or "less energy is wasted", without saying WHY (lower current, ), misses the key mark. State the current-reduction and the heating explicitly.
June 2024 Paper 2PR Q4(b): the connection between current and heating in the wires (I²R) was seen less often; "high voltage is efficient" without the mechanism was insufficient for the first marking point.
Draw transformers, not pylons
A national-grid diagram question wants transformer symbols, not pictures of pylons and cables. Drawing pylons instead of the step-up and step-down transformers scores zero for the transformer marks. Show two transformers with clearly different turn ratios.
June 2024 Paper 2PR Q4(a): many candidates drew pictures of pylons and transmission lines instead of transformers.
Don't swap step-up and step-down
Order matters: power station → step-up → high-voltage cables → step-down → consumers. Drawing two transformers but labelling them the wrong way round costs a mark (scores 2/3). Step-up sits by the power station (few primary, many secondary turns); step-down sits by homes (many primary, few secondary).
June 2024 Paper 2PR Q4(a): candidates who drew two transformers but swapped the step-up and step-down labels scored 2/3.
The transformer doesn't remove energy
A transformer changes voltage and current, not energy — an ideal one conserves it (). It does not "reduce energy loss" by itself. What reduces loss is the high voltage (low current) in the transmission cables, which the transformer merely provides. Never say "the transformer reduces the losses".
Cable loss uses P = I²R
For heat lost in a cable, use — not (that is the total power transmitted) or (needs the p.d. across the cable, not given). The cable has resistance and carries current , so is the right form for the heating loss.
Voltage up means current down
In the grid, high voltage goes with low current, not high current. From at fixed power, doubling the voltage halves the current. Writing "high voltage means high current" is backwards — it is exactly the low current that makes high-voltage transmission efficient.
Write the V↑ → I↓ → I²R chain
For "why high voltage reduces loss", write three explicit steps: (1) the step-up transformer raises the voltage; (2) for fixed power () the current falls; (3) , so the smaller current wastes much less heat in the cables. Each step is usually a mark.
Draw two labelled transformers
Draw both transformers left-to-right: step-up (few→many turns) by the power station, step-down (many→few) near the consumer, with clearly different turn ratios. If the ratios are ambiguous, add the step-up/step-down labels to score.
Use I²R for cable heating
When a question asks for the power lost in a transmission cable, use . First find the cable current from if needed, then square it and multiply by . Don't reach for , which gives the total power carried, not the loss.
Compare losses by the ratio squared
To compare losses at two voltages: if the voltage is multiplied by , the current divides by and the loss divides by . E.g. 25 kV to 400 kV () cuts the loss by . Show the step to justify the factor.
Transformer equation (voltage and turns ratio):
where = primary (input) voltage (V), = secondary (output) voltage (V), = number of turns on the primary coil, = number of turns on the secondary coil.
Ideal transformer (power conservation):
where = primary current (A), = secondary current (A). In an ideal transformer, power in equals power out.
Power loss in a cable:
where = current in the cable (A), = resistance of the cable (), = power dissipated as heat (W).
Define a step-up transformer.
A power station uses a step-up transformer to connect to the national grid. The primary coil has 200 turns and is connected to a 25 000 V supply. The secondary coil has 3200 turns.
Calculate the secondary (output) voltage of the transformer. State the unit of your answer.
