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A step-up transformer increases voltage because its secondary coil has more turns than its primary coil. Free GCSE Physics notes and 15 FAQs.
Transformers and Efficiency in the National Grid is part of Energy transfers in GCSE AQA Higher Physics.
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A step-up transformer increases voltage because its secondary coil has more turns than its primary coil. Free GCSE Physics notes and 15 FAQs.
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Use this lesson to stay inside Energy transfers while connecting the detail back to the wider Electricity topic.
Read the core explanations from Transformers and Efficiency in the National Grid before testing yourself from memory.
Picture electricity travelling from a power station to your home - it's like water flowing through pipes across hundreds of kilometres. But here's the problem: electricity loses energy as heat when it flows through wires, just like water loses pressure in long pipes.
The National Grid solves this using a clever trick. Instead of sending electricity at the voltage it's generated (around 25,000 V), it steps the voltage up to 400,000 V for transmission, then steps it back down to 230 V for your home. This isn't random - there's solid physics behind why high voltage saves energy.
The key insight is this: power losses in transmission lines depend on current squared (P = I²R). By using transformers to increase voltage, we can transmit the same power at much lower current, dramatically reducing these losses.
Transformers only work with alternating current (AC) because they rely on changing magnetic fields. The alternating current in the primary coil creates a changing magnetic field in the iron core, which induces a voltage in the secondary coil.
A transformer has two coils of wire wound around an iron core. The primary coil connects to the input voltage, and the secondary coil provides the output voltage. The iron core links the magnetic fields between the coils.
When AC flows through the primary coil, it creates a changing magnetic field in the iron core. This changing field cuts through the secondary coil, inducing a voltage by electromagnetic induction. The voltage ratio depends on the number of turns in each coil.
The transformer equation links the voltages and number of turns: Vs/Vp = Ns/Np. If the secondary has twice as many turns as the primary, the output voltage doubles. For an ideal transformer, power in equals power out, so VpIp = VsIs.
The National Grid uses a two-stage transformer system. First, step-up transformers at power stations increase the voltage from around 25,000 V to 400,000 V for transmission across the country. Then step-down transformers reduce this back to safe voltages for distribution.
The transmission lines carry electricity at 400,000 V, 275,000 V, or 132,000 V depending on the distance and power requirements. Local substations then step this down to 11,000 V for local distribution, and finally to 230 V for homes.
This system allows power stations to be located far from cities (near coal mines, rivers for cooling, or windy areas for wind farms) while still efficiently delivering electricity to where it's needed. Without transformers, we'd need power stations in every neighbourhood.
The efficiency of the National Grid comes down to one key physics principle: power loss in transmission lines equals I²R, where I is current and R is the resistance of the cables. This square relationship means that halving the current reduces power losses by a factor of four.
Since power equals voltage times current (P = VI), we can transmit the same power at high voltage and low current, or low voltage and high current. The National Grid chooses high voltage because it dramatically reduces the I² term in the power loss equation.
Worked example: Calculating Power Losses in Transmission
A power station transmits 100 MW of power. Compare the power losses when transmitting at 25,000 V versus 400,000 V, given that the transmission line resistance is 50 Ω.
Answer: At 25,000 V: The calculation yields an impractically high loss of 800 MW. This hypothetical result demonstrates that transmitting 100 MW at such a low voltage would require enormous currents and lead to unrealistic energy losses, hence low voltage transmission is not feasible. At 400,000 V: The loss is only 3.125 MW, illustrating how high voltage transmission drastically reduces losses.
This calculation shows why the National Grid is so efficient. By stepping up to high voltage, power losses drop from an impossible 800% to a manageable 3%. Without transformers, most of the electricity would be wasted as heat in the transmission lines.
Real transformers aren't 100% efficient, but they're close - typically 95-99% efficient. The small losses occur as heat in the coils and core. For exam calculations, we often assume ideal transformers where power in equals power out.
The key equations for transformers are the turns ratio (Vs/Vp = Ns/Np) and power conservation (VpIp = VsIs for ideal transformers). These let you calculate any unknown voltage, current, or turns ratio.
Worked example: Transformer Calculation
A step-up transformer has 100 turns on the primary coil and 2000 turns on the secondary coil. If the primary voltage is 230 V and primary current is 10 A, calculate the secondary voltage and current (assuming an ideal transformer).
Answer: Secondary voltage = 4,600 V, Secondary current = 0.5 A
Notice how the transformer trades voltage for current - the secondary voltage is 20 times higher, but the current is 20 times lower. This is exactly what makes high-voltage transmission efficient.
Transformer questions often test your understanding of the relationship between voltage, current, and power. The most common mistake is thinking that step-up transformers increase both voltage and current - they don't. Energy conservation means if one goes up, the other must go down.
When explaining why the National Grid uses high voltage, focus on the I²R relationship for power losses. Examiners want to see that you understand it's the square relationship that makes high voltage so effective, not just that 'high voltage reduces losses'.
For calculation questions, always check your answer makes sense. If a step-up transformer increases voltage by a factor of 10, the current should decrease by a factor of 10. If your calculation shows both increasing, you've made an error.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A step-up transformer increases voltage from the primary coil to the secondary coil, because the secondary coil has more turns than the primary coil. Step-up transformers at power stations raise the voltage from around 25,000 V to 400,000 V.
A step-down transformer decreases voltage from the primary coil to the secondary coil, because the secondary coil has fewer turns than the primary coil. Step-down transformers reduce the National Grid voltage to 11,000 V for local distribution and finally to 230 V for homes.
Transformers only work with alternating current because they rely on changing magnetic fields. The alternating current in the primary coil creates a changing magnetic field in the iron core, which induces a voltage in the secondary coil.
A step-up transformer does not increase both voltage and current: if voltage goes up, current must go down, because for an ideal transformer power in equals power out. If voltage increases by a factor of 10, the current decreases by a factor of 10.
The National Grid uses high voltage because power loss in transmission lines equals I²R. Since P = VI, the same power can be sent at high voltage and low current, and halving the current reduces power losses by a factor of four.
Power loss in transmission cables is calculated with P = I²R, after finding the current with I = P ÷ V. Sending 100 MW at 400,000 V gives 250 A, so through 50 Ω the loss is 250² × 50 = 3.125 MW, compared with 800 MW at 25,000 V.
Real transformers are typically 95-99% efficient, with small losses as heat in the coils and core. For exam calculations, ideal transformers are often assumed, where power in equals power out (VpIp = VsIs).
The National Grid needs transformers because sending electricity at the generated voltage of around 25,000 V would waste most of it as heat in the cables. Step-up transformers raise the voltage to 400,000 V so the same power flows at much lower current, then step-down transformers reduce it to 230 V for homes.
The National Grid is efficient because it transmits at high voltage and low current, reducing I²R losses. Transmitting 100 MW at 400,000 V loses about 3%, whereas at 25,000 V the losses would be an impossible 800%, and real transformers are typically 95–99% efficient.
For an ideal transformer, power in equals power out, so VpIp = VsIs. This means a transformer trades voltage for current: if the voltage goes up by a factor of 20, the current goes down by a factor of 20.
The secondary voltage is found from Vs/Vp = Ns/Np. With 100 primary turns, 2000 secondary turns and 230 V on the primary, the ratio is 20, so Vs = 230 × 20 = 4,600 V; with 10 A in the primary, VpIp = VsIs gives a secondary current of 0.5 A.
The iron core in a transformer links the magnetic fields between the primary and secondary coils. The changing magnetic field created by the primary coil passes through the core and induces a voltage in the secondary coil.
Halving the current reduces the power loss in transmission lines by a factor of four, because power loss equals I²R. This square relationship is why transmitting at high voltage and low current makes the National Grid efficient.
Transmitting 100 MW through 50 Ω cables at 25,000 V needs 4,000 A and would lose an impractical 800 MW. At 400,000 V the current is only 250 A, so the loss, P = I²R = 250² × 50, is just 3.125 MW.
National Grid transmission lines carry electricity at 400,000 V, 275,000 V or 132,000 V depending on the distance and power requirements. Local substations step this down to 11,000 V for local distribution, then to 230 V for homes.