⚡ High voltage
Small current for the same power. Little energy wasted as heat in the cables.
Electromagnetic Induction » Transformers and Electricity Transmission
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 6.18P
- Follow electrical energy from the power station to your home
- Explain why electricity is transmitted at very high voltage
- Explain why the voltage is stepped down again before it reaches homes
- Explain why the supply must be a.c.
This whole lesson is Paper 2 only content. It is not examined in Paper 1.
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Unlock This CourseFrom power station to home
A power station can be hundreds of kilometres from the people who use its electricity. The energy has to travel along cables, and that journey follows the same steps every time. You already know what step-up and step-down transformers do. Now you will see why the National Grid needs both.
| Stage | What happens |
|---|---|
| 1. Generator | The generator at the power station makes an a.c. voltage of a few tens of thousands of volts. |
| 2. Step-up transformer | The voltage is raised to a very high value. In the UK the main transmission lines run at up to 400 000 V. |
| 3. High-voltage transmission lines | The electricity travels long distances on pylons and underground cables at this high voltage. |
| 4. Step-down transformers | Near towns, transformers lower the voltage in stages. |
| 5. Homes and factories | Homes receive 230 V. Large factories may take a higher voltage. |
Key terms:
- Transmission lines: the cables that carry electrical energy over long distances from power stations to towns.
- National Grid: the network of power stations, transformers and cables that delivers electricity across the country.
Why use a very high voltage?
The power carried by a cable depends on both the voltage and the current. You met this earlier as power = current × voltage (P = I × V). The same power can be sent with a high voltage and a small current, or a low voltage and a large current.
A current in any wire has a heating effect, so some of the energy is transferred to the surroundings as heat. The larger the current, the more energy is wasted this way. Sending a smaller current means less energy is wasted heating the cables.
Worked example
A power station sends out 50 MW (50 000 000 W).
At the generator voltage of 25 000 V: I = P / V = 50 000 000 / 25 000 = 2000 A.
After a step-up transformer to 400 000 V: I = 50 000 000 / 400 000 = 125 A.
The voltage went up 16 times, so the current went down 16 times. The power sent is the same. The cables carry a much smaller current, so far less energy is wasted as heat.
This is why the step-up transformer sits right next to the generator. Raising the voltage lowers the current, and a lower current wastes less energy.
🔥 Low voltage
Large current for the same power. A lot of energy wasted as heat in the cables.
Why step the voltage down again?
If high voltage is so good, why not send it straight into our homes? There are two good reasons.
- Safety: a voltage of thousands of volts is far too dangerous in a house. A fault could be deadly.
- Appliances: home appliances are made to work at 230 V. A much higher voltage would destroy them.
So step-down transformers lower the voltage in stages, until it is a safe 230 V for homes.
Why the supply must be a.c.
A transformer only works when the current in its primary coil keeps changing. A changing current gives a changing magnetic field in the iron core, and that induces a voltage in the secondary coil. Mains electricity is a.c. (alternating current), which changes direction all the time, so it works perfectly. A steady d.c. supply gives a steady field, and no voltage is induced in the secondary coil. Without transformers the National Grid could not step the voltage up or down, so the supply has to be a.c.
Common mistakes
- Saying a higher voltage means more power is sent. The power stays the same. The current goes down.
- Saying a high voltage "makes the electricity travel faster". It does not. The reason is the smaller current.
- Saying the cables get hot because of the voltage. It is the large current that heats the cables.
- Forgetting the step-down stage, or forgetting to say why it is needed (safety).
- Saying transformers work with d.c. They need a changing current.
Exam-style question
Electrical energy is sent from a power station to homes through the National Grid. Electricity is transmitted at 200 000 V.
(a) A power of 4.0 MW is sent along a cable at 200 000 V. Calculate the current in the cable. [2 marks]
(b) Explain how step-up and step-down transformers are used to transmit electrical energy from a power station to homes, and why the supply must be a.c. [6 marks]
Model answer
(a) I = P / V = 4 000 000 / 200 000 (1) = 20 A (1)
(b) Any six from:
A step-up transformer raises the voltage from the generator (1). For the same power, a higher voltage means a smaller current (1). A smaller current means less energy is wasted as heat in the cables (1). The electricity travels along the high-voltage transmission lines (1). Step-down transformers near towns lower the voltage again (1). This makes the supply safe, and suitable for appliances, at 230 V in homes and factories (1). Transformers only work with a changing current / changing magnetic field (1), so the supply must be a.c. (1)
Exam tip
For a 6-mark "explain" question, plan a chain: step up, so smaller current, so less heating, then step down for safety. Add the a.c. point last. Each link in the chain earns a mark.