The National Grid
Build confidence in The National Grid for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The national grid uses step-up transformers to raise the voltage for efficient transmission, then step-down transformers before use.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
The National Grid is a system of cables and transformers linking power stations to consumers across the country.
Electricity is transmitted at very high potential difference and correspondingly low current. This is because the power wasted as heat in the cables is given by P = I²R, so reducing the current reduces the wasted power dramatically — halving the current cuts the losses to a quarter.
Two types of transformer are used. Step-up transformers increase the potential difference for transmission, reducing the current. Step-down transformers reduce the potential difference to a safe level for domestic use. This arrangement makes the grid an efficient way to distribute energy.
Revision notes
Why high potential difference is used
Transmitting at high potential difference means a low current for the same power.
The power wasted in the cables is P = I²R. Because the current is squared, halving it reduces the wasted power to a quarter. Low current therefore means far less energy dissipated as heat.
The two transformers
Step-up transformers increase the potential difference and decrease the current, before transmission.
Step-down transformers decrease the potential difference to a safe level, around 230 V, for use in homes and businesses.
Efficiency
The high transmission potential difference makes the grid efficient because very little energy is wasted heating the cables.
Transformers themselves are close to 100 per cent efficient, so the power in equals the power out — the potential difference and current change but the power does not.
Key points
- The National Grid links power stations to consumers.
- Transmission uses high potential difference and low current.
- Power wasted in cables is given by P = I²R.
- Low current greatly reduces wasted power.
- Step-up transformers increase the potential difference.
- Step-down transformers make it safe for domestic use.
Worked examples
Example 1
Explain why electricity is transmitted at high potential difference. [3 marks]
Model answer
High potential difference means a low current is needed to transmit the same power (1 mark). The power wasted heating the cables is given by P = I²R, in which the current is squared (1 mark), so a lower current greatly reduces the energy wasted and makes transmission more efficient (1 mark).
Example 2
State the function of a step-down transformer in the National Grid. [2 marks]
Model answer
It decreases the potential difference (1 mark) to a safe level of around 230 V for use in homes and businesses (1 mark).
Example 3
The current in a transmission cable is halved. Explain the effect on the power wasted. [2 marks]
Model answer
The power wasted becomes one quarter of its original value (1 mark), because power is proportional to the current squared in P = I²R (1 mark).
Common mistakes
Saying high potential difference reduces resistance.
The resistance of the cables is unchanged; it is the current that falls.
Saying transformers increase power.
They change potential difference and current; the power stays essentially the same.
Confusing step-up and step-down.
Step-up increases potential difference for transmission; step-down reduces it for use.
Not using P = I²R in the explanation.
The squared relationship is where the marks are.
Exam tips
- Always cite P = I²R when explaining transmission efficiency.
- Say the current is squared, so halving it quarters the loss.
- Distinguish the two transformer types by their function.
- Note that transformers change potential difference, not power.
Key terms
- National Grid
- The system of cables and transformers distributing electricity.
- Step-up transformer
- A transformer increasing potential difference.
- Step-down transformer
- A transformer decreasing potential difference.
- Transmission
- Carrying electricity over long distances.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.