Edexcel IGCSE Physics 2(b) Mains Electricity — Notes
Written by an examiner for Pearson Edexcel International GCSE Physics (4PH1), Science Double Award (4SD0) and Science Single Award (4SS0), linear and modular. This page also covers the 2(a) units for Topic 2.
Which parts do I need?
Single Award candidates need the units, a.c. and d.c., and \(P=I\times V\). They skip every section tagged Double · Physics: how the heating effect works and is used, \(E=I\times V\times t\), choosing a fuse, and the safety features (insulation, double insulation, earthing, fuses and circuit breakers).
What this page covers
| Spec | You need to be able to… | Route |
|---|---|---|
| 2.1 | use A, C, J, Ω, s, V and W | Single · Double · Physics |
| 2.2 | explain how insulation, double insulation, earthing, fuses and circuit breakers keep users and appliances safe | Double · Physics |
| 2.3 | explain why a current heats a resistor, and give household uses of this heating | Double · Physics |
| 2.4 | recall and use power = current × voltage; use it to choose a suitable fuse (fuse choice: Double · Physics only) | Single · Double · Physics |
| 2.5 | use energy transferred = current × voltage × time | Double · Physics |
| 2.6 | tell the difference between a.c. from the mains and d.c. from a cell or battery | Single · Double · Physics |
Modular candidates: these are statements 2.1–2.6 of 4XPH1 Unit 1 (4WPH1) and 4XSD1 Physics Unit 5 (4WSD5).
In one minute
- Mains electricity is a.c. — the current keeps reversing direction. Cells and batteries supply d.c. — the current flows in one direction only.
- Power = current × voltage, \(P=I\times V\), in watts (W). 1 W = 1 J/s.
- A current in a resistor transfers energy to it and makes it hotter — the basis of kettles, toasters and heaters. Double · Physics
- Energy transferred = current × voltage × time, \(E=I\times V\times t\), in joules (J). Double · Physics
- The fuse (in the live wire) and the earth wire work together: a fault sends a large current to earth, the fuse melts and the live supply is cut off. Choose the fuse rating just above the normal current. Double · Physics
Which units do I need for Topic 2? Single · Double · Physics
Every quantity in Topic 2 is measured in one of these seven units. Learn the name, the symbol and what it measures — and give a unit with every calculated answer.
| Quantity | Unit | Symbol |
|---|---|---|
| current | ampere (amp) | A |
| charge | coulomb | C |
| energy transferred | joule | J |
| resistance | ohm | Ω |
| time | second | s |
| voltage | volt | V |
| power | watt | W |
- All seven units are needed by every route. Charge (C) and resistance (Ω) are used in 2(c).
- Convert before you substitute: kW to W (× 1000), mA to A (÷ 1000), minutes to s (× 60). For example, 2.5 kW = 2500 W and 3 minutes = 180 s.
- “V” is both the symbol for voltage in an equation and the symbol for its unit, the volt. Read the context carefully.
What is the difference between a.c. and d.c.? Single · Double · Physics
- Cells and batteries supply d.c. One terminal is always positive and the other always negative, so the current always flows the same way round the circuit.
- The mains supply in homes is a.c. The voltage of the live terminal keeps changing between positive and negative, so the current flows one way, then the other, many times every second. In the UK the mains voltage is about 230 V.
- On a voltage–time graph, d.c. from a cell is a horizontal line on one side of the time axis; a.c. is a wave that goes above and below the axis.
How do you calculate electrical power? Single · Double · Physics
- \(P\) in watts (W), \(I\) in amps (A), \(V\) in volts (V).
- Rearranged: \(I=\dfrac{P}{V}\) and \(V=\dfrac{P}{I}\). The current an appliance draws from the mains is its power rating divided by 230 V.
- An appliance with a higher power rating draws a larger current from the same supply.
(a) A kettle connected to the 230 V mains takes a current of 9.0 A. Calculate its power.
(b) A car headlamp is rated 55 W and works from a 12 V battery. Calculate the current in the lamp.
(a) \(P=I\times V=9.0\times230=\) 2070 W (2.07 kW)
(b) \(I=\dfrac{P}{V}=\dfrac{55}{12}=4.58\ \text{A}\approx\) 4.6 A
Why does a current make a resistor hot? Double · Physics
Not required for Single Award (4SS0).
A metal is made of a lattice of positive ions with free electrons moving between them. When there is a current, the electrons drift through the lattice.
- The voltage of the supply transfers energy to the electrons.
- As they move through the resistor, the electrons collide with the ions of the metal.
- In each collision, energy is transferred from the electrons to the ions, so the ions vibrate more.
- Faster vibration means the resistor has more thermal energy — its temperature rises. Energy is then transferred to the surroundings by heating.
This is described as the electrical transfer of energy: energy is carried to the resistor electrically and stored there as thermal energy.
Using the heating effect at home
| Appliance | How the heating effect is used |
|---|---|
| kettle, immersion heater | a heating element in the water transfers energy to the water |
| electric fire, fan heater | a hot element heats the air in the room (a fan blows air over it) |
| toaster, electric oven | red-hot wires heat the food |
| hair dryer, clothes iron | an element heats air blown over it, or heats a metal plate |
| fuse | a thin wire that gets so hot it melts if the current is too large, breaking the circuit |
The same effect is often unwanted: connecting wires, chargers and computers warm up, and this energy is wasted. Thick cables are used for high-power appliances so that they have a low resistance and do not overheat.
How do you calculate the energy transferred by an appliance? Double · Physics
Not required for Single Award (4SS0).
\[ \text{energy transferred}=\text{current}\times\text{voltage}\times\text{time}\qquad E=I\times V\times t \]- \(E\) in joules (J), \(I\) in amps (A), \(V\) in volts (V), \(t\) in seconds (s).
- Since \(P=I\times V\), this is the same as energy = power × time.
- Energies from mains appliances are large: give them in J, or in kJ if you convert correctly (1 kJ = 1000 J).
A heater takes a current of 8.0 A from the 230 V mains. Calculate the energy it transfers in 3.0 minutes.
Convert the time first: \(t=3.0\times60=180\ \text{s}\).
\(E=I\times V\times t=8.0\times230\times180=331\,200\ \text{J}\approx\) 330 000 J (330 kJ, to 2 significant figures)
Common error: substituting 3.0 for the time gives an answer 60 times too small.
How do insulation, earthing, fuses and circuit breakers keep us safe? Double · Physics
Not required for Single Award (4SS0).
A mains cable has three wires. The live wire (brown) carries the alternating voltage, the neutral wire (blue) completes the circuit, and the earth wire (green and yellow) is a safety wire that normally carries no current. Touching the live wire can give a fatal electric shock, and a fault that lets a large current flow can overheat wires and start a fire. Each safety feature deals with one of these dangers.
| Safety feature | What it is | How it protects |
|---|---|---|
| insulation | plastic or rubber coating on each wire and around the cable | plastic is an electrical insulator, so a person touching the cable cannot touch a live conductor; it also stops the wires touching each other |
| double insulation | the appliance has a plastic (insulating) case and no exposed metal parts that could become live | even if a live wire comes loose inside, nothing the user can touch becomes live, so no earth wire is needed (a two-core cable is used) |
| earthing | the earth wire joins any metal case to the ground through a low-resistance path | if the live wire touches the case, the current flows through the earth wire instead of through the user; the current is large, so the fuse melts or the circuit breaker trips |
| fuse | a thin wire inside a cartridge, fitted in the live wire | if the current is larger than the fuse rating, the fuse wire heats up, melts and breaks the circuit, disconnecting the live supply; this protects the wiring and the appliance from overheating and fire |
| circuit breaker | an automatic switch in the live wire, usually in the consumer unit | it detects a current that is too large and switches off (trips), breaking the circuit; it acts faster than a fuse and can be reset rather than replaced |
- The fuse and circuit breaker go in the live wire so that, when they break the circuit, the appliance is disconnected from the high voltage. In the neutral wire the appliance would stay connected to the live supply.
- Earthing and the fuse work as a pair: the earth wire alone would leave the case connected to the live wire; the fuse alone would not melt if the fault current passed through a person.
A toaster has a metal case and is connected to the mains with a three-core cable and a fused plug. A fault makes the live wire touch the metal case. Explain how the earth wire and the fuse protect the user.
- The case is connected to earth by the earth wire, which has a very low resistance.
- So a very large current flows from the live wire, through the case and down the earth wire.
- This current is larger than the fuse rating, so the fuse wire heats up and melts.
- The live wire is now disconnected, so the case is no longer live and the user cannot get a shock by touching it.
Examiners look for a logical chain: low-resistance path → large current → fuse melts → live supply disconnected. “The earth wire takes the electricity away” does not score.
How do you choose the right fuse? Double · Physics
Single Award: you use \(P=I\times V\) but are not asked to select fuses.
- Calculate the normal working current: \(I=\dfrac{P}{V}\).
- Choose the fuse with the smallest rating that is greater than that current.
- If the rating is too low, the fuse melts when the appliance is working normally.
- If the rating is much too high, a fault current may not be large enough to melt the fuse, so the cable could overheat and catch fire.
- Plug fuses in the UK are commonly rated 3 A, 5 A and 13 A; a question will give you the ratings to choose from.
Fuses rated 3 A, 5 A and 13 A are available. Choose the correct fuse for each appliance, which is connected to the 230 V mains. (a) A hair straightener rated 60 W. (b) A clothes iron rated 1500 W.
(a) \(I=\dfrac{60}{230}=0.26\ \text{A}\). The smallest rating above 0.26 A is 3 A.
(b) \(I=\dfrac{1500}{230}=6.5\ \text{A}\). A 5 A fuse is below the normal current and would melt in normal use, so fit the 13 A fuse.
A kettle is rated 2.3 kW and is connected to the 230 V mains.
(a) Calculate the current in the kettle. (b) Fuses rated 3 A, 5 A and 13 A are available. Determine which fuse should be fitted. (c) Calculate the time the kettle takes to transfer 690 kJ of energy to the water.
(a) \(I=\dfrac{P}{V}=\dfrac{2300}{230}=\) 10 A
(b) The fuse must be rated just above the normal current of 10 A, so the 13 A fuse. A 3 A or 5 A fuse would melt as soon as the kettle was switched on.
(c) \(t=\dfrac{E}{I\times V}=\dfrac{690\,000}{10\times230}=\) 300 s (5.0 minutes)
In (c), change 690 kJ to 690 000 J and 2.3 kW to 2300 W before you substitute.
Examiner tips for 2(b)
Questions often give a mark for stating \(P=I\times V\) or \(E=I\times V\times t\). Give it in words or in standard symbols. Writing units instead of quantities (W = A × V) does not earn the mark.
Most lost marks in this topic come from units, not physics. Change kilowatts to watts and minutes to seconds before substituting. “ms” means milliseconds, never minutes.
When you explain a safety feature, say which wire it is connected to: the fuse is in the live wire; the earth wire is connected to the metal case. Vague answers such as “it stops the electricity” score nothing.
A fuse choice needs the calculated current and the reason: “normal current 6.5 A, so 13 A — 5 A would melt during normal use”. A rating with no working rarely gets full credit.
For “explain why the resistor gets hot”, mention electrons colliding with ions (or atoms) in the metal, energy transferred to the ions, and the ions vibrating more. “Friction” and “resistance makes heat” are not credited.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| A fuse is fitted to stop a person getting a shock. | A shock current through a person is far too small to melt a fuse. | The fuse protects the wiring and appliance from overheating; with the earth wire it also disconnects a live case. |
| The earth wire carries current all the time. | In normal use the case is not live, so there is no current in it. | The earth wire carries current only when there is a fault. |
| Choose a fuse with a rating as close as possible to the current, even if below it. | A fuse rated below the normal current melts in normal use. | Choose the smallest rating that is above the normal current. |
| Alternating current means the electrons flow round the circuit and come back. | In a.c. the charges move backwards and forwards. | a.c. repeatedly reverses direction; d.c. flows in one direction. |
| Double-insulated appliances need an earth wire. | There is no metal case that could become live. | Double-insulated appliances use a two-core cable with no earth wire. |
Key equations
| Equation | Use it to find | Route | On the Equation List? |
|---|---|---|---|
| power = current × voltage \(P=I\times V\) | power, or the current an appliance draws | All | yes — all papers |
| energy transferred = current × voltage × time \(E=I\times V\times t\) | energy transferred by an appliance in a given time | Double · Physics | yes — all papers |
Pearson includes the Equation List with the question papers, but statement 2.4 asks you to know \(P=I\times V\) — learn it so you can write it without looking.
Check your understanding
1. State the unit of power and the unit of charge, with their symbols. All
Power: watt (W). Charge: coulomb (C).
2. Describe the difference between the current from a battery and the current from the mains. All
The battery gives direct current, which flows in one direction only. The mains gives alternating current, which keeps reversing direction.
3. A television takes a current of 0.40 A from the 230 V mains. Calculate its power. All
\(P=I\times V=0.40\times230=\) 92 W
4. Explain why the element of an electric fire gets hot when there is a current in it. Double · Physics
Electrons moving through the element collide with the metal ions (1). Energy is transferred to the ions, which vibrate more (1), so the temperature of the element rises (1).
5. A 12 V motor takes a current of 2.5 A for 2.0 minutes. Calculate the energy transferred. Double · Physics
\(E=I\times V\times t=2.5\times12\times120=\) 3600 J (1 mark for converting 2.0 minutes to 120 s.)
6. A 2000 W fan heater is used on the 230 V mains. Fuses of 3 A, 5 A and 13 A are available. Which should be fitted? Explain. Double · Physics
\(I=2000\div230=8.7\ \text{A}\). Fit the 13 A fuse — the smallest rating above 8.7 A; 3 A and 5 A fuses would melt in normal use.
7. A hair dryer has a plastic case and is connected with only two wires. Explain why it is still safe to use. Double · Physics
It is double insulated: the case is an insulator and there are no exposed metal parts, so even if a live wire came loose inside, the user could not touch anything live (1). So it does not need an earth wire (1).
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