How does the National Grid transmit electricity efficiently, and why does it use transformers?
The National Grid, step-up and step-down transformers, and why high-voltage transmission reduces energy losses in the cables.
A focused answer to WJEC GCSE Physics topic 1.2 on the National Grid, covering how electricity is transmitted, the role of step-up and step-down transformers, and why a high transmission voltage and low current reduce the energy lost as heat in the cables.
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What this topic is asking
WJEC wants you to describe the National Grid, explain the roles of step-up and step-down transformers, and explain why a high transmission voltage reduces energy losses. This is part of topic 1.2 Generating electricity in Unit 1 of WJEC GCSE Physics (3420).
The National Grid
Why high voltage is used
Step-up and step-down transformers
The grid carries electricity at several stages of voltage. Power stations generate at around twenty-five thousand volts, a step-up transformer raises this to the very high voltages used on the transmission lines that cross the country, and a series of step-down transformers in substations then lowers it in stages for industry and finally to the mains voltage of about used in homes. Using alternating current is essential because transformers need a continually changing magnetic field to induce a voltage in the secondary coil; a steady direct current would produce no change and so no output. This is one of the main reasons the grid uses a.c. rather than d.c. The overhead lines themselves have a small but non-zero resistance, so keeping the current low is what makes nationwide transmission practical without wasting most of the energy as heat in the cables.
Try this
Q1. State why a low current reduces energy loss in transmission cables. [1 mark]
- Cue. The heat lost is , which falls sharply as the current falls.
Q2. State the job of a step-down transformer in the grid. [1 mark]
- Cue. It lowers the high transmission voltage to a safe value for consumers.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20184 marksExplain why electricity is transmitted across the National Grid at a high voltage rather than at mains voltage.Show worked answer →
A topic 1.2 Explain question. The power delivered is , so transmitting at a high voltage allows the same power to be carried at a low current (1 mark). The energy wasted as heat in the cables depends on the current, , so a smaller current means far less heat lost in the resistance of the lines (1 mark). This makes transmission much more efficient over long distances (1 mark). A step-up transformer raises the voltage for transmission and a step-down transformer lowers it for safe use (1 mark). Markers reward the low current, the loss, the efficiency and the transformers.
WJEC 20213 marksState the function of the step-up and step-down transformers in the National Grid, and why both are needed.Show worked answer →
A topic 1.2 question. The step-up transformer raises the generated voltage to a very high value for efficient transmission at low current (1 mark). The step-down transformer lowers the voltage again to a safe value (about ) for homes and businesses (1 mark). Both are needed because high voltage is efficient for transmission but dangerous to use directly, so it must be reduced before reaching consumers (1 mark). Markers reward the raising for transmission, the lowering for safety, and the reason both are required.
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Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)