Welcome! When you switch on a light, the electricity may have come from a power station hundreds of kilometres away. In this lesson you’ll find out how the National Grid gets it to you without wasting it – then run the grid control room for a day. It takes about one hour. Work through the steps in order.
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: The National Grid – Keep the Country Running.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. describe the National Grid
2. explain why step-up and step-down transformers are used
3. use P = VI and P = I²R to show why a high p.d. wastes less energy
4. explain how the grid meets changing demand.
Step 1: Starter (5 minutes)
The plug sockets in your home are at about 230 V. But the big pylon cables carry electricity at up to 400 000 V. Why do you think it’s sent at such a high potential difference? Write your best guess.
Show answer
A high potential difference means a low current is needed to transfer the same power. A low current heats the cables less, so less energy is wasted. You’ll prove this with a calculation in Step 4!
📝 Copy into your book
High p.d. → low current → less energy wasted heating the cables.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- What is the National Grid?
- What does a step-up transformer do, and where is it?
- What does a step-down transformer do, and why is it needed?
📖 Read: Getting electricity from power station to home
The National Grid is a system of cables and transformers that links power stations to homes, schools and businesses (consumers) all over the country.
Electricity leaving a power station goes through a step-up transformer. This increases the potential difference to a very high value – up to 400 000 V – and decreases the current. A lower current means much less energy is lost heating the transmission cables, so the system is more efficient. Before the electricity reaches homes, step-down transformers decrease the potential difference to about 230 V, which is much safer to use.
Show answers
(a) A system of cables and transformers linking power stations to consumers.
(b) It increases the potential difference (and decreases the current) – between the power station and the transmission cables.
(c) It decreases the potential difference to a safer level for homes (about 230 V).
📝 Copy and complete
The National Grid is a system of __________ and __________ that links __________ __________ to consumers. A step-__________ transformer increases the p.d. A step-__________ transformer decreases it to about __________ V for homes.
Check your gaps
cables · transformers · power stations · up · down · 230
Step 3: Key facts (5 minutes)
This is the explanation examiners want – copy it carefully, arrows and all:
📝 Copy into your book
Step-up transformer increases p.d.
→ so the current in the cables is smaller (P = VI)
→ so less energy is wasted heating the cables (P = I²R)
→ so transmission is more efficient.
Then read: demand for electricity changes during the day. It’s lowest overnight and highest in the early evening. Some power stations (like nuclear) produce a steady output and are slow to change; others (like hydro-electric and pumped storage) can respond in minutes.
Step 4: Prove it with a calculation (10 minutes)
First, copy the two equations you’ll use:
📝 Copy into your book
power = potential difference × current P = V I
power wasted in a cable = current² × resistance P = I² R
A village needs 100 000 W of power. The cable to the village has a resistance of 5 Ω. Show every step.
Q1 If the power is sent at 1000 V, what is the current in the cable?
Show answer
P = V I, so 100 000 = 1000 × I
I = 100 A
Q2 How much power is wasted heating the cable?
Show answer
P = I² R = 100² × 5 = 50 000 W – half of the power is wasted!
Q3 Now a step-up transformer increases the p.d. to 25 000 V. Work out the new current and the power wasted.
Show answer
I = 100 000 ÷ 25 000 = 4 A
P = I² R = 4² × 5 = 80 W wasted – instead of 50 000 W!
Q4 In one sentence, explain what your answers show.
Show answer
Increasing the p.d. makes the current much smaller, and because the power wasted depends on the current squared, far less energy is wasted heating the cables.
✅ Score out of 4.
Step 5: Main task – Grid Control Room (12 minutes)
You’re in charge of the grid control room today! Here is how demand changes on a winter weekday:
| Time | Demand | What’s happening |
|---|---|---|
| 4 am | 20 GW | Most people asleep |
| 8 am | 32 GW | Breakfast, heating, getting ready |
| 6 pm | 38 GW | Cooking, lights, TVs – the daily peak |
| 8:45 pm | +1 GW in a few minutes | Half-time in a big football match – millions of kettles go on at once! |
You can use these power stations:
| Type | How fast can it change its output? | Other notes |
|---|---|---|
| Nuclear | Very slowly – best run steadily | No CO₂; reliable |
| Gas | About an hour to start | Releases CO₂; reliable |
| Hydro-electric / pumped storage | In minutes | No CO₂; limited amount of stored water |
| Wind | Can’t be controlled – depends on the wind | No CO₂; unreliable |
Your task: write your control-room plan with three parts:
- All day (the “base load”): which power stations run all the time? Why?
- Morning and evening peaks: which do you add? When must you switch them on?
- Half-time surge: which do you use? Why is it the only option?
Stuck? Match each power station’s speed to how quickly the demand changes.
Show a model plan
1. Base load: nuclear power stations run steadily all day and night, because they can’t change their output quickly and produce no CO₂. Wind is used whenever it’s available, because it’s free and produces no CO₂ – but we can’t rely on it.
2. Peaks: gas power stations are switched on about an hour before the 8 am and 6 pm peaks, because they take about an hour to start. They’re reliable, though they release CO₂.
3. Half-time: hydro-electric / pumped storage, because it’s the only option that can respond within minutes. Gas would take an hour – by then half-time is over!
📝 Copy into your book
Demand for electricity changes during the day. Steady power stations (like nuclear) supply the base load. Hydro-electric and pumped storage can respond to sudden surges within minutes.
Step 6: Exam practice (12 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (2 marks) Why is a step-down transformer used before electricity reaches homes?
Show model answer
It decreases the potential difference (1) to a safer level for use in homes (about 230 V) (1).
Q2 (3 marks) A power station supplies 500 MW to the grid at 400 000 V. Calculate the current in the cables.
Hint: convert MW to W first.
Show model answer
500 MW = 500 000 000 W (1)
P = V I, so I = 500 000 000 ÷ 400 000 (1)
I = 1250 A (1)
Q3 (4 marks) Explain how the National Grid uses transformers to make the transmission of electricity efficient.
Hint: use the chain in your 📝 box from Step 3!
Show model answer
Step-up transformers increase the potential difference from the power station (1). For the same power, this means the current in the cables is smaller (1). A smaller current means less energy is transferred to the surroundings by heating the cables (1). Step-down transformers then reduce the p.d. to a safe level for consumers (1).
✅ Add up your marks out of 9 and write the score in your book.
Step 7: Exit ticket (5 minutes)
- Electricity is sent at a high p.d. because…
- At half-time, the grid uses… because…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Circuits and mains electricity questions and the P = I²R practice page.
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