The National Grid — AQA GCSE Combined Science
Test yourself on The National Grid with AQA GCSE practice questions.
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The National Grid explained
The National Grid is the network that carries electrical energy from power stations to homes, schools and factories.
Read the full explanation
It consists of cables, mostly overhead aluminium lines supported by pylons, and transformers that change the potential difference of the supply. A step-up transformer near a power station raises the potential difference to very high values, often 400 000 V, because for a given power a higher potential difference means a smaller current, and smaller currents reduce energy wasted by heating the cables. Near towns, step-down transformers lower the potential difference to safer domestic values such as 230 V. Transformers work only with alternating current, which is why the grid supply is a.c. The system links generation to consumers across the country.
Electrical power is transferred from power stations to consumers using the National Grid.
The National Grid is a nationwide network of cables and transformers that links power stations to homes, schools, hospitals and factories. Electrical power is generated at power stations, often at around 25 kV, and must travel long distances to consumers. Because the cables have resistance, some energy is transferred to the thermal store of the surroundings, reducing efficiency. To reduce this waste, the potential difference is increased using step-up transformers before transmission, so that a smaller current transfers the same power. The grid therefore transfers electrical power continuously from generators to consumers, matching supply to demand. Students should describe the grid as a system with input from power stations and output to consumers, and explain that transformers are essential for efficient transfer.
Step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use.
Transformers are used in the National Grid to change the potential difference of an alternating current supply. A step-up transformer increases the potential difference from the power station to the transmission cables, often to hundreds of kilovolts. This high potential difference means a smaller current is needed to transfer the same power, significantly reducing energy wasted as heating in the cables and increasing efficiency. Before electricity reaches homes, a step-down transformer decreases the potential difference to a much lower, safe domestic value, commonly around 230 V in the UK. Students should describe this sequence: generation, step-up, transmission, step-down, and domestic use, explaining the reason for each stage without needing to discuss primary or secondary coils.
Students should be able to explain why the National Grid system is an efficient way to transfer energy.
The National Grid is a network of cables and transformers linking power stations to consumers. Electrical power is transmitted at very high voltage and low current, because for a given power P = V × I, raising the voltage lowers the current. Lower current means less energy is wasted as thermal energy in the resistance of the transmission cables, since power wasted is I² × R. Transformers step the voltage up at the power station and down near homes and factories, so energy arrives at a safe voltage. This makes the system efficient: a high proportion of the energy generated reaches consumers rather than being dissipated in the cables.
potential difference across primary coil x current in primary coil = potential difference across secondary coil x current in secondary coil
The published AQA 8464 Combined Science specification includes this relationship for Higher Tier. For a 100% efficient transformer, the electrical power entering the primary coil equals the electrical power leaving the secondary coil. Since power equals potential difference multiplied by current, Vₚ × Iₚ = Vₛ × Iₛ. Higher Tier students should substitute values, rearrange the relationship and keep units consistent. For example, if Vₚ = 230 V, Iₚ = 4 A and Vₛ = 460 V, then Iₛ = (230 × 4) ÷ 460 = 2 A. A step-up transformer raises potential difference and lowers current for the same power; this lower current reduces thermal energy dissipation in National Grid cables.
Your focus
- Describe the structure of the National Grid as cables and transformers linking power stations to consumers.
- Explain how step-up and step-down transformers change potential difference and current during transmission.
- Explain why reducing current reduces energy wasted in the transmission cables.
Show all 15 objectives
- Describe the National Grid as a system that transfers electrical power from power stations to consumers.
- Explain why electrical power is transmitted at high potential difference to reduce energy losses.
- Identify the roles of step-up and step-down transformers in the National Grid.
- Describe how step-up and step-down transformers change potential difference in the National Grid.
- Explain why a step-up transformer is used before transmission and a step-down transformer before domestic use.
- Describe the sequence of electricity transmission from power station to consumer.
- Describe the role of step-up and step-down transformers in the National Grid.
- Use P = V × I to explain why transmitting at high voltage reduces current.
- Explain, using I² × R, why reduced current lowers energy wasted in transmission cables.
- Recall and use Vₚ × Iₚ = Vₛ × Iₛ for a 100% efficient transformer.
- Rearrange the relationship to calculate any one of the four quantities.
- Explain why a step-up transformer reduces current and transmission losses.
The National Grid exam tips
Marking Points
- Identify the National Grid as a network of cables and transformers connecting power stations to consumers.
- Describe the role of step-up transformers in raising potential difference and reducing current in transmission cables.
- Explain that reducing current reduces energy wasted as thermal energy in the cables, improving efficiency.
- Describe the role of step-down transformers in lowering potential difference to safe domestic values such as 230 V.
- State that the grid uses alternating current because transformers require a changing current to operate.
- Recognise that the grid allows electrical energy to be transferred over long distances from where it is generated to where it is used.
- The National Grid is a system of cables and transformers connecting power stations to consumers.
- Electrical power is generated at power stations and transferred through the grid to homes, industry and other consumers.
- The grid operates at high potential difference during transmission to reduce energy losses in the cables.
- Step-up transformers increase potential difference from the power station to the transmission cables.
- Step-down transformers reduce potential difference to safe domestic levels for consumers.
- Energy losses in transmission cables are reduced because a lower current is used for the same power transfer.
- A step-up transformer increases the potential difference from the power station to the transmission cables.
- Transmission at a high potential difference reduces the current needed for the same power.
- A lower current reduces energy losses due to heating in the transmission cables, improving efficiency.
- A step-down transformer decreases the potential difference to a much lower, safer value for domestic use.
- The sequence is: power station, step-up transformer, transmission cables, step-down transformer, domestic consumer.
- States that the National Grid uses transformers to step voltage up for transmission and down for domestic use.
- Links high voltage to low current for the same transmitted power, using P = V × I.
- Explains that reduced current reduces energy wasted as thermal energy in the cables, because wasted power is I² × R.
- Concludes that a greater proportion of the generated energy reaches consumers, so the system is efficient.
- Uses the correct terms: step-up transformer, step-down transformer, transmission cables, thermal energy.
- States the ideal-transformer relationship Vₚ × Iₚ = Vₛ × Iₛ.
- Identifies Vₚ and Iₚ as primary-coil quantities and Vₛ and Iₛ as secondary-coil quantities.
- Substitutes values with volts and amperes consistently before rearranging for the unknown.
- Explains that increasing potential difference reduces current when transmitted power is unchanged.
- Links lower transmission current to reduced thermal energy dissipation in cables.
Examiner Tips
- 💡Use the terms step-up and step-down precisely, and link each to its position in the grid and its effect on potential difference.
- 💡When explaining reduced waste, refer to current and heating in the cables rather than vaguely saying 'less energy is lost'.
- 💡Include the reason transformers need a.c. whenever a question asks why the grid uses alternating current.
- 💡Define the National Grid clearly as a network of cables and transformers linking power stations to consumers.
- 💡When explaining efficiency, link high potential difference to low current and reduced heating in the cables.
- 💡Use the correct terms: step-up transformer, step-down transformer, transmission cables and consumers.
- 💡State clearly that step-up transformers increase potential difference and step-down transformers decrease it.
- 💡Link the high transmission potential difference to reduced current and reduced heating in the cables.
- 💡Use the phrase 'much lower value' when describing the domestic potential difference after step-down transformation.
- 💡Use the equation P = V × I to show numerically that doubling voltage halves current for the same power.
- 💡Refer to I² × R when explaining why a smaller current wastes less energy in the cables.
- 💡Structure the answer as: what the Grid does, why high voltage is used, why that reduces waste, and why this is efficient.
- 💡Write VₚIₚ = VₛIₛ before substituting so each value stays attached to the correct coil.
- 💡Check the direction: a step-up transformer should give a lower secondary current for the same power.
Common Mistakes
- Saying transformers change the power supplied; correct this by explaining that an ideal transformer changes potential difference and current while power is conserved.
- Claiming the grid uses direct current; correct this by stating that transformers need alternating current, so the grid supply is a.c.
- Thinking step-up transformers are used near homes; correct this by placing step-up transformers at power stations and step-down transformers near consumers.
- Thinking the National Grid stores electricity: it transfers electrical power continuously; energy is not stored in the cables.
- Confusing power stations with the grid: power stations generate electricity, while the grid is the network that distributes it.
- Assuming electricity travels instantly with no energy loss: cables have resistance, so some energy is transferred thermally to the surroundings.
- Reversing the transformer order; correction: remember that step-up transformers are at the power station end, and step-down transformers are near consumers.
- Thinking step-down transformers increase current to dangerous levels; correction: they reduce potential difference to a safe domestic value, and the focus should be on the safe voltage rather than current.
- Believing transformers work with direct current; correction: they require alternating current to change potential difference.
- Saying high voltage directly causes less heating without linking it to reduced current; correction: explain that for the same power, higher voltage gives lower current, and heating depends on current squared.
- Confusing step-up and step-down transformers; correction: step-up raises voltage at the power station, step-down lowers it before supply to homes.
- Claiming the cables have no resistance; correction: cables do have resistance, and it is this resistance that causes some energy to be wasted as thermal energy.
- Pairing a primary potential difference with a secondary current: correct this by keeping Vₚ with Iₚ and Vₛ with Iₛ.
- Assuming a step-up transformer increases both potential difference and current: correct this by using equal input and output power for an ideal transformer.
- Dividing before writing the full equality: correct this by write VₚIₚ = VₛIₛ, substitute, then isolate the required quantity.