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Edexcel International GCSE Physics · Section 2 Electricity · 2 (b) Mains electricity
Electrical Safety and Energy Transfer in Resistors
In this lesson you'll learn to: sort the safety features into the ones that protect you from a
shock (insulation, double insulation, earthing) and the ones that protect your device
from overheating and fire (fuses, circuit breakers), explain how the earth wire and fuse work
together, explain why a resistor heats up using electrons and ions, and spot which appliances use
that heating on purpose.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. On tab 1, make a fault in the toaster and try each kind of protection: when is there a p.d. across you? On tab 2, plug appliances in until the current is too high, first with no protection, then with a fuse, then a circuit breaker. On tab 3, turn up the current, then sort the appliances. One of them is a trick.
Safety
Potential of the case
0 V
p.d. across you
0 V
Current through you
none
The rule: you are at 0 V. You only get a shock if something you touch is at a high voltage, because then there is a p.d. across you and a current flows through you to earth. Every “protect you” feature stops that p.d.
Protection
Plug in
Current in the cable
0 A
Cable temperature
20 °C
Status
off
The rule: a current that is too high overheats the wires and can start a fire. A fuse melts and a circuit breaker trips: both break the circuit so the current stops. The cable here can safely carry 13 A. Current = power ÷ voltage, and the mains is 230 V.
Sort it: why does this appliance get hot?
right 0 / 0
—
Choose a box.
▲ Slowed right down. In a real fault the fuse melts or the breaker trips in a fraction of a second.
Two jobs: protect you, protect your device
Every safety feature does one of two jobs. The notes sort them, show how each one works, and explain why some resistors are meant to get hot.
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Revision notes
2.2 Protect YOU or protect your DEVICE?
🧍
Protect YOU
Insulation, double insulation and earthing stop you getting an electric shock.
They make sure nothing you touch is at a high voltage, so there is no p.d. across you.🔌
Protect your DEVICE
Fuses and circuit breakers stop a current that is too high.
A high current overheats the wires and can start a fire. Both break the circuit so the current stops.| Feature | Protects | How it works | Key word |
|---|---|---|---|
| Insulation | you | A plastic coating around each wire. It doesn't conduct, so the live wire can't touch anything you might touch. | non-conductive barrier |
| Double insulation | you | The whole case is an insulator (plastic), with no metal parts you can touch, so the case can never become live. | no earth wire needed |
| Earthing | you | The earth wire joins a metal case to the ground. If the live wire touches the case, a large current flows down the earth wire and melts the fuse. | low-resistance path |
| Fuse | device | A thin wire on the live side. If the current is too high it heats up and melts, breaking the circuit. It has to be replaced. | melts |
| Circuit breaker | device | An automatic switch that trips (opens) if the current is too high. Faster than a fuse, and you just reset it. | faster / resettable |
⭐ Why you get a shock
You are at 0 V, the same as the ground. Touch something at 230 V and there is a 230 V p.d. across you, so a current flows through you to earth. No p.d. across you, no current, no shock. That is what every “protect you” feature is for.
The earth wire and fuse work together
This is the most-asked safety question. It needs three links, in this order:
1
Low-resistance path
The live wire touches the metal case. The earth wire is a low-resistance path to earth, so a large current flows down it.
2
Fuse melts
The large current melts the fuse.
3
Live disconnected
This disconnects the live wire, so the case is no longer live and the user is safe.
💡 Choosing a fuse
Work out the normal current with I = P ÷ V, then choose the smallest fuse rated just above it. Common fuses are 3 A, 5 A and 13 A.
A 1000 W toaster on 230 V: I = 1000 ÷ 230 = 4.3 A, so a 5 A fuse. A 3 A fuse would melt in normal use; a 13 A fuse would let the current get dangerously high before it melted.
2.3 Energy transfer in resistors
When a current flows through a resistor, it heats up. Explain it in three steps, using these key words: resistance, electrons transfer energy, electron collisions, (lattice) ions.
⭐ The heating effect in three steps
1 The resistor has resistance; as the free electrons flow through it, they transfer energy.
2 This is because of electron collisions with the (lattice) ions in the metal.
3 The ions vibrate more, so the temperature rises.
Heating on purpose, or heating wasted?
| Heating is the job | Heating is wasted energy |
|---|---|
| kettle · toaster · hair dryer · tumble dryer · clothes iron · electric heater: each has a heating element, a resistor designed to get hot | fridge · fan · vacuum cleaner (their job is done by a motor) · LED lamp (barely warms) · filament bulb |
⚠ The trick one: the filament bulb
A filament bulb certainly uses the heating effect: the thin filament gets so hot that it glows. But its job is to give out light. All the energy that goes into heating the room is wasted, which is why filament bulbs are so inefficient and have been replaced by LEDs.
How to write it in the exam
“Explain how the earth wire and the fuse work together to protect the user.”
The earth wire is a low-resistance path to earth, so a large current flows,
which melts the fuse, which disconnects the live wire.
the path what the current does why you're safe
These are the exam-style questions from pages 4 and 5 of the workbook.
Page 4 (a) · A fault makes the live wire touch the metal case of a toaster. Explain how the earth wire and the fuse work together to protect the user. [3]
- The earth wire is a low-resistance path to earth, so a large current flows (1)
- This large current melts the fuse (1)
- Which disconnects the live wire, so the case is no longer live (1)
Page 4 (b) · A different toaster has a plastic case and is double insulated. State why it does not need an earth wire. [1]
The case is an insulator / there are no exposed metal parts, so the case cannot become live.
Page 5 (a) · A hair dryer's coil heats the air. Explain, in terms of electrons, how the coil transfers energy. [3]
- The coil has resistance (1)
- The electrons transfer energy as they flow through the coil (1)
- Because of electron collisions with the (lattice) ions in the coil (1)
Page 5 (b) · The hair dryer has a fuse. Explain how a fuse protects a domestic appliance. [3]
- If the current becomes too high (1)
- The fuse wire melts (1)
- Which breaks the circuit and stops the current (1)
Where marks go missing: “the fuse stops you getting a shock”. A fuse protects the device and the wiring; it is the earth wire working with the fuse that protects you.
✅ Check your answer
For “protect you” questions, say what stops the p.d. across you. For “protect the device”, say what happens when the current is too high. For heating, use all three: resistance, electrons transfer energy, collisions with ions.
📘 Now do it in your workbook
Sort the safety features, complete the table with their key words, answer the toaster question, then explain the heating effect, tick the appliances that use it, and answer the hair dryer question.
Edexcel iGCSE Physics Workbook · ELECTRICITY · pages 4 & 5Free sample = the page for this lesson. The full workbook covers the whole of Section 2 Electricity, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 4 and 5 of the Electricity workbook are free: the safety features table, the toaster question, the heating effect and the hair dryer question.
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