Edexcel IGCSE Physics 4(b) Energy Transfers — Notes
Written for Pearson Edexcel International GCSE Physics (4PH1), Science Double Award (4SD0) and Science Single Award (4SS0), linear and modular. This page also covers the 4(a) units for Topic 4.
Which parts do I need?
Single Award candidates need the units, energy stores and transfers, conservation of energy, efficiency and Sankey diagrams, and can skip every section tagged Double · Physics: conduction, convection and radiation, the thermal energy transfer practical, and reducing unwanted energy transfer (statements 4.6–4.10). Nothing on this page is Physics only.
What this page covers
| Spec | Modular no. | You need to be able to… | Route |
|---|---|---|---|
| 4.1 | 3.1 | use kg, J, m, m/s, m/s², N, s and W | Single · Double · Physics |
| 4.2 | 3.2 | describe how energy moves between the eight stores by the four transfer pathways | Single · Double · Physics |
| 4.3 | 3.3 | apply the idea that energy is never created or destroyed | Single · Double · Physics |
| 4.4 | 3.4 | recall and use the efficiency equation | Single · Double · Physics |
| 4.5 | 3.5 | explain where the input energy goes in real devices, and draw and read Sankey diagrams | Single · Double · Physics |
| 4.6 | 3.6 | describe conduction, convection and radiation | Double · Physics |
| 4.7 | 3.7 | explain everyday effects caused by convection | Double · Physics |
| 4.8 | 3.8 | explain how the surface and the temperature of an object affect how well it emits and absorbs radiation | Double · Physics |
| 4.9 | 3.9 | investigate conduction, convection and radiation (practical) | Double · Physics |
| 4.10 | 3.10 | explain how insulation and other methods cut down unwanted energy transfer | Double · Physics |
Modular candidates: Topic 4 is Topic 3 in the modular specifications, so 4.1–4.10 are statements 3.1–3.10 of 4XPH1 Unit 1 (4WPH1) and 4XSD1 Physics Unit 5 (4WSD5).
In one minute
- Energy is held in stores (chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear) and moves between them by four transfers: mechanically, electrically, by heating and by radiation (light and sound).
- Conservation of energy: energy cannot be created or destroyed. The total energy out of a device equals the total energy in.
- \(\text{efficiency}=\dfrac{\text{useful energy output}}{\text{total energy output}}\times100\%\). It can never be more than 100%.
- A Sankey diagram shows the energy transfers with arrow widths drawn to scale. Wasted energy usually ends up in the thermal store of the surroundings.
- Thermal energy moves by conduction (solids), convection (liquids and gases) and radiation (infrared, needs no medium). Matt black surfaces are the best emitters and absorbers. Double · Physics
Which units do I need for Topic 4? Single · Double · Physics
These are the units used across Topic 4. Two of them are new: the joule for energy and the watt for power.
| Quantity | Unit | Symbol | Route |
|---|---|---|---|
| mass | kilogram | kg | All |
| energy, work done | joule | J | All |
| distance, height | metre | m | All |
| speed | metre/second | m/s | All |
| acceleration | metre/second² | m/s² | All |
| force | newton | N | All |
| time | second | s | All |
| power | watt | W | All |
- Energy values are often large, so watch the prefixes: 1 kJ = 1000 J and 1 MJ = 1 000 000 J. For example, 2.5 kJ = 2500 J and 3.6 MJ = 3 600 000 J.
- The watt (1 W = 1 J/s) is used for power, which is taught on the 4(c) Work and power page.
What are energy stores and how is energy transferred? Single · Double · Physics
When anything happens — a ball is thrown, a kettle boils, a phone charges — energy moves from one store to another. The specification lists eight stores and four ways (pathways) of transferring energy between them.
| Energy store | Energy is stored in… | Example |
|---|---|---|
| chemical | fuels, food and batteries, released in chemical reactions | petrol, a sandwich, a phone battery |
| kinetic | any moving object | a moving bus, a spinning fan blade |
| gravitational | an object raised above the ground | water in a raised tank, a lifted load |
| elastic | an object that is stretched, squashed or bent | a stretched bungee rope, a compressed spring |
| thermal | any object; the hotter it is, the more it stores | hot soup, warm air in a room |
| magnetic | magnets that attract or repel | two magnets held apart while they repel |
| electrostatic | charges that attract or repel | a charged balloon held near a wall |
| nuclear | the nuclei of atoms, released in nuclear reactions | uranium fuel in a reactor, the core of the Sun |
| Transfer pathway | What happens | Example |
|---|---|---|
| mechanically | a force moves something (work is done) | a hand pushing a trolley |
| electrically | an electric current flows | a battery driving current through a motor |
| by heating | energy passes from a hotter object to a cooler one | a hob heating a pan |
| by radiation | waves carry the energy — light and sound | the Sun warming your skin; a speaker |
To describe an energy transfer, name the store the energy starts in, the pathway, and the store it ends up in. Wasted energy almost always ends up in the thermal store of the surroundings, where it is spread out (dissipated) and cannot easily be used again.
A child uses a toy launcher with a stretched spring to fire a plastic ball straight upwards. Describe the energy transfers from the moment the spring is released until the ball reaches its highest point.
Energy is stored in the elastic store of the stretched spring.
When it is released, the spring pushes the ball, transferring energy mechanically to the kinetic store of the ball.
As the ball rises it slows down: its kinetic store decreases and its gravitational store increases. At the highest point the ball is momentarily at rest, so its kinetic store is zero.
Some energy is transferred to the thermal store of the surroundings because of air resistance, and some by sound when the spring is released.
What is the principle of conservation of energy? Single · Double · Physics
- For any device: total energy output = total energy input. If you know the input and all but one of the outputs, you can find the missing one by subtraction.
- Energy is not “used up” or “lost”. When we say energy is wasted, we mean it has gone into a store where it is not useful — usually as thermal energy spread thinly through the surroundings.
- Dissipated energy becomes less useful because it is spread out, not because there is less of it.
How do you calculate efficiency? Single · Double · Physics
\[ \text{efficiency}=\frac{\text{useful energy output}}{\text{total energy output}}\times100\% \]- The useful energy output is the energy transferred to the store you want (for a lamp, light; for a kettle, the thermal store of the water).
- By conservation of energy, the total energy output equals the total energy input, so you can use whichever one the question gives you.
- Efficiency has no unit. Leave out the ×100% and it is a decimal (0.75 instead of 75%). Both are accepted if your working is clear.
- No device can have an efficiency greater than 100% (or 1). If your answer is bigger, you have put the numbers the wrong way up.
- Rearranged: useful energy output = efficiency × total energy output (with efficiency as a decimal).
The motor in a desk fan is supplied with 2400 J of energy electrically. It transfers 1800 J to the kinetic store of the moving air. Calculate (a) the efficiency of the fan and (b) the energy wasted.
(a) efficiency = \(\dfrac{1800}{2400}\times100\%=\) 75%
(b) By conservation of energy, wasted energy = 2400 − 1800 = 600 J, mostly to the thermal store of the motor and the surroundings, and some as sound.
A car engine has an efficiency of 30%. During a short journey it transfers 4.5 MJ of energy usefully to the kinetic store of the car. (a) Calculate the energy transferred from the chemical store of the fuel. (b) Calculate the energy wasted. (c) Draw a Sankey diagram to scale.
(a) total energy = \(\dfrac{\text{useful energy output}}{\text{efficiency}}=\dfrac{4.5}{0.30}=\) 15 MJ
(b) wasted energy = 15 − 4.5 = 10.5 MJ
(c) Choose a scale, for example 1 MJ to 1 mm on paper. The input arrow is then 15 mm wide, the useful arrow 4.5 mm and the wasted arrow 10.5 mm.
Common error in (a): multiplying 4.5 by 0.30. Check: the total must be bigger than the useful part.
How do you draw and read a Sankey diagram? Single · Double · Physics
A Sankey diagram is a flow diagram for energy. The width of each arrow is proportional to the amount of energy it represents.
- The input arrow enters from the left. The useful output continues straight on; the wasted outputs bend away (usually downwards).
- The widths of the output arrows add up to the width of the input arrow — this is conservation of energy shown as a picture.
- On squared paper, choose a scale (for example 1 small square = 10 J) and state it. Count squares carefully across the width of each arrow, not along its length.
- Label every arrow with the store or transfer and its energy value.
The Sankey diagram above is for an electric motor that lifts a load. The input arrow is 20 squares wide and represents 500 J. (a) Determine the energy represented by each arrow. (b) Calculate the efficiency of the motor.
(a) Scale: 500 J ÷ 20 squares = 25 J per square.
useful (gravitational store of the load): 12 squares × 25 = 300 J
thermal: 6 squares × 25 = 150 J
sound: 2 squares × 25 = 50 J
Check: 300 + 150 + 50 = 500 J ✓
(b) efficiency = \(\dfrac{300}{500}\times100\%=\) 60%
Devices you should be able to explain
| Device or situation | Useful output | Main wasted output |
|---|---|---|
| electric kettle | thermal store of the water | heating the kettle body and the air; sound |
| filament lamp | light (radiation) | heating the surroundings — most of the input |
| LED lamp | light (radiation) | heating the surroundings — much less than a filament lamp, so an LED is more efficient |
| electric motor | kinetic store (or gravitational store of a lifted load) | heating, due to friction and the current in the coils; sound |
| loudspeaker | sound (radiation) | heating the coil and surroundings |
| car braking | — (the aim is to empty the kinetic store) | energy goes to the thermal store of the brakes and surroundings |
How is thermal energy transferred by conduction, convection and radiation? Double · Physics
Not required for Single Award (4SS0) — statements 4.6 to 4.10 are Double Award and Physics only.
Energy is transferred by heating whenever there is a temperature difference. It always goes from the hotter region to the cooler one, by one or more of three methods.
| Method | How it works | Where it happens |
|---|---|---|
| conduction | Particles at the hot end vibrate more. They pass energy to neighbouring particles, which then vibrate more too. In metals, free electrons also carry energy quickly through the material, so metals are good conductors. | mainly in solids; poor in liquids and very poor in gases (particles far apart) |
| convection | The heated part of a fluid expands, becomes less dense and rises; cooler, denser fluid sinks to take its place. This sets up a circulating convection current that carries energy with the moving fluid. | only in liquids and gases (fluids), because their particles can move from place to place |
| radiation | Energy is carried by infrared waves (electromagnetic radiation). All objects emit and absorb infrared; hotter objects emit more. | everywhere — it needs no particles, so it is the only method that works through a vacuum (for example from the Sun to Earth) |
How does convection explain everyday effects? Double · Physics
Every convection explanation follows the same chain. Learn it as four links:
- The fluid near the heat source is heated, so its particles move faster and spread out — the fluid expands.
- Its density decreases, so it rises.
- Cooler, denser fluid sinks to replace it and is heated in turn.
- This circulation is a convection current, which spreads the energy through the whole fluid.
| Everyday effect | Role of convection |
|---|---|
| a room radiator | warm air rises above the radiator and spreads across the ceiling; cooled air sinks and returns along the floor, so the whole room is warmed |
| freezer compartment at the top of a fridge | air cooled at the top becomes denser and sinks, pushing warmer air up to be cooled, so the whole fridge stays cold |
| heating element at the bottom of a kettle | water heated at the bottom rises and cooler water sinks to the element, so all the water is heated |
| sea breeze during the day | air over the warmer land rises; cooler, denser air from over the sea moves in to replace it |
How do surface and temperature affect emission and absorption of radiation? Double · Physics
| Surface | Emitter | Absorber | Reflector | Used for |
|---|---|---|---|---|
| matt black | best | best | worst | solar water-heating panels; cooling fins on the back of a fridge |
| shiny silver or white | worst | worst | best | survival blankets; white-painted houses in hot countries; vacuum flask walls |
- An object at a higher temperature than its surroundings emits more radiation than it absorbs, so it cools down. The bigger the temperature difference, the faster it cools.
- A good absorber is always a good emitter. Matt black is not “hot” — it simply transfers infrared well in both directions.
- A larger surface area also increases the rate of emission, which is why cooling fins are thin and spread out.
Practical: how do you investigate thermal energy transfer? Double · Physics
The specification asks you to investigate conduction, convection and radiation. Each method has a simple experiment; questions ask you to name variables, describe a method, interpret results and suggest improvements.
Part 1 — Conduction: which metal is the best conductor?
- Aim: compare how well different metals conduct thermal energy.
- Apparatus: rods of copper, aluminium and steel of the same length and diameter, a container of hot water (or a heat-resistant block heated at one end), wax or petroleum jelly, drawing pins, stopwatch.
- Independent variable: the material of the rod. Dependent variable: the time taken for the pin to fall.
- Control variables: length and thickness of the rods, the distance of each pin from the heated end, the amount of wax, the starting temperature of the hot water.
- Method: fix a pin with a small blob of wax at the same distance along each rod. Put the other ends into the hot water at the same moment and start the stopwatch. Record the time for each pin to drop as its wax melts.
- Result: the shortest time shows the best conductor (copper, then aluminium, then steel).
Part 2 — Convection: showing a convection current
- Aim: show the path of a convection current in water.
- Apparatus: large beaker of cold water, a small crystal of a soluble coloured solid (such as potassium manganate(VII)) dropped down a tube to one bottom corner, Bunsen burner or small heater.
- Method: gently heat the beaker under the corner where the crystal sits. Do not stir. Observe and sketch the path of the coloured water.
- Observation: coloured water rises above the heat, moves across the top, sinks on the cooler side and returns along the bottom — a convection current.
Part 3 — Radiation: how does the surface affect the rate of cooling?
- Aim: compare how well a matt black surface and a shiny silver surface emit infrared radiation.
- Apparatus: two identical metal cans, one painted matt black and one shiny silver, lids with holes, two thermometers, measuring cylinder, kettle, stopwatch.
- Independent variable: the type of surface. Dependent variable: the temperature of the water after equal times.
- Control variables: the volume and starting temperature of the water, the size and shape of the cans, the room temperature, both cans away from draughts and the same distance from other objects.
- Method: pour the same volume of hot water into each can and fit the lids. Record the starting temperature, then the temperature every 4 minutes for 20 minutes. Plot both cooling curves on the same axes.
Errors and improvements
- Use temperature sensors and a data logger instead of thermometers so that both cans are read at exactly the same moment and reading errors are removed.
- Use lids on the cans so that less energy is transferred from the water surface by convection — then the difference between the cans is due to radiation from their sides.
- Repeat the experiment and swap the positions of the cans to check that a draught or a nearby warm object did not affect one can more than the other.
- Safety: handle hot water carefully and stand to pour it; let rods and the beaker cool before touching them; wear eye protection; use only a small crystal and avoid skin contact, as the solid stains and is harmful.
| time / min | matt black can temperature / °C | shiny silver can temperature / °C |
|---|---|---|
| 0 | 80 | 80 |
| 4 | 72 | 76 |
| 8 | 65 | 72 |
| 12 | 59 | 68 |
| 16 | 54 | 65 |
| 20 | 50 | 62 |
A student carries out Part 3 of the practical and obtains the results in the table above. (a) Calculate the fall in temperature of each can over the 20 minutes. (b) Explain the difference. (c) Explain why both cans cool more slowly as time goes on.
(a) matt black: 80 − 50 = 30 °C; shiny silver: 80 − 62 = 18 °C.
(b) The matt black surface is a better emitter of infrared radiation than the shiny surface, so energy is transferred from the black can to the surroundings at a greater rate and its temperature falls faster. The cans are otherwise identical, so the surface is the only cause of the difference.
(c) In the first 4 minutes the black can falls by 8 °C, but in the last 4 minutes by only 4 °C. As the water cools, the temperature difference between the can and the room gets smaller, and a cooler can emits less radiation each second, so the rate of energy transfer decreases.
How can unwanted energy transfer be reduced? Double · Physics
To reduce an unwanted energy transfer, work out which method (conduction, convection or radiation) is carrying the energy and block that method.
- Reduce conduction: use insulating materials. Trapped air is an excellent insulator, which is why many insulators (foam, fibreglass, fleece) are full of tiny air pockets.
- Reduce convection: stop the fluid from moving. Trapping air in small pockets prevents convection currents; lids and draught excluders stop warm air escaping.
- Reduce radiation: use shiny surfaces, which are poor emitters and reflect infrared back.
| Method in the home | How it reduces energy transfer |
|---|---|
| loft insulation (fibreglass) | air trapped between the fibres is a poor conductor and cannot circulate, so conduction and convection through the ceiling are reduced |
| cavity wall insulation | foam fills the gap between two walls, trapping air in small pockets so convection currents cannot form in the cavity |
| double glazing | a layer of gas (or a vacuum) between two panes of glass reduces conduction |
| shiny foil behind a radiator | reflects infrared back into the room instead of letting the wall absorb it |
| draught excluders and carpets | reduce convection (draughts) and conduction through the floor |
Exam tips for 4(b)
Write “kinetic store of the car” or “thermal store of the surroundings”, not “movement energy” or “heat”. Questions on energy stores reward only the stores that actually change in the situation given — if the speed is constant, the kinetic store does not change.
Before substituting into the efficiency equation, decide which number is useful and which is the total. Write them down with labels. An efficiency above 100% always means they are swapped.
When you complete a Sankey diagram, the widths must match the energies and add up to the input. Count squares across the width of the arrow and quote the scale you used.
An explanation of convection that says only “heat rises” scores nothing. Say that the heated fluid expands, becomes less dense and rises, and that cooler, denser fluid sinks to replace it.
For each insulating feature, name the transfer it reduces and say how — for example “the vacuum has no particles, so there is no conduction or convection across it”. Naming the feature alone usually earns no credit.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| Energy is used up when a device works. | Energy cannot be destroyed. | It is transferred to other stores, often wasted as thermal energy in the surroundings. |
| Efficiency can be more than 100%. | That would mean more energy coming out than going in. | Efficiency is always less than (or at most equal to) 100%. |
| “Heat rises.” | Energy does not rise; hot fluid rises because it is less dense. | Heated fluid expands, its density falls and it rises. |
| Cold flows in through an open window. | There is no such thing as a flow of “cold”. | Energy is transferred out from the warmer room to the cooler outside. |
| Shiny surfaces are good emitters because they look bright. | Shiny surfaces reflect infrared; they emit very little. | Matt black surfaces are the best emitters and absorbers. |
| Insulation produces warmth. | An insulator supplies no energy. | Insulation only reduces the rate of energy transfer. |
Key equations
| Equation | Use it to find | Route | On the Equation List? |
|---|---|---|---|
| \(\text{efficiency}=\dfrac{\text{useful energy output}}{\text{total energy output}}\times100\%\) | how much of the energy supplied is transferred usefully | All | yes — all papers |
| useful energy output = efficiency × total energy output | the useful energy when the efficiency is known (efficiency as a decimal) | All | — (rearrangement) |
| total energy output = total energy input | a missing energy value (conservation of energy) | All | — (principle) |
Pearson includes the Equation List with the question papers, but the first mark in many questions is for writing the relationship yourself — learn it anyway.
Check your understanding
1. A ball is dropped and bounces several times before coming to rest on the floor. Where has the energy from its gravitational store gone? All
It has been transferred to the thermal store of the ball, the floor and the air (some by sound at each bounce). No energy is destroyed; it is dissipated to the surroundings.
2. A kettle is supplied with 200 000 J electrically and transfers 170 000 J to the thermal store of the water. Calculate its efficiency. All
\(\dfrac{170\,000}{200\,000}\times100\%=\) 85%
3. A filament lamp has an efficiency of 15%. It is supplied with 600 J. Calculate the energy transferred as light and the energy wasted. All
light = 0.15 × 600 = 90 J; wasted = 600 − 90 = 510 J (to the thermal store of the surroundings).
4. In a Sankey diagram the input arrow is 40 mm wide and the useful arrow is 6 mm wide. Calculate the efficiency. All
Width is proportional to energy, so efficiency = 6 ÷ 40 × 100% = 15%.
5. Explain why the freezer compartment of a fridge is placed at the top. Double · Physics
Air next to the freezer is cooled, becomes denser and sinks. Warmer, less dense air rises to the top to be cooled. This convection current cools all the air in the fridge.
6. A solar water-heating panel on a roof is painted matt black. Explain why. Double · Physics
Matt black is the best absorber of infrared radiation, so the panel absorbs energy from the Sun at the greatest rate and transfers more energy to the water.
7. Explain how cavity wall insulation reduces energy transfer from a house. Double · Physics
The foam traps air in small pockets. Air is a poor conductor, so conduction is reduced, and because the air cannot circulate, convection currents in the cavity are prevented.
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