Energy Transfers
Edexcel IGCSE PhysicsΒ· 4.2β4.10 (Section 4, 2017 Specification)Β· 25 min read
1. Energy Stores, Transfer Pathways and Conservation of Energyβ β ββββ± 6 min
Energy Store
A location where energy is held within a system, with 8 required types for Edexcel IGCSE: chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear.
Example:
The gravitational store of a book placed on a high shelf.
Energy Transfer Pathway
A mechanism that moves energy between stores. The 4 required pathways are: mechanical work, electrical work, heating, and radiation (light/sound).
Example:
Radiation (light) transferring energy from the Sun to a solar panel.
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred between stores or dissipated to the surroundings. The total energy in a closed system always remains constant, so no energy is ever lost during transfers.
A student drops a tennis ball from a height of 2m. Describe the energy transfers that occur as the ball falls to the ground, ignoring air resistance.
- 1
- Before the ball is dropped, all energy is held in the ball's gravitational store.
- 2
- As the ball falls, energy is transferred mechanically from the gravitational store to the ball's kinetic store.
- 3
- When the ball hits the ground, energy is transferred mechanically to the thermal store of the ball and ground, and to the surroundings via sound radiation.
- 4
- Total energy remains constant at all stages, complying with the principle of conservation of energy.
Exam tip:
Never refer to 'heat energy' or 'sound energy' as stores: heating is a transfer pathway, and sound is a radiation transfer pathway. Use only the 8 named stores and 4 named pathways in exam answers to earn full marks.
2. Efficiency Calculations and Sankey Diagramsβ β β βββ± 8 min
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Efficiency
The ratio of useful energy output from a device to the total energy input, expressed as a percentage (or decimal less than 1). The formula must be memorised:
Example:
A 15% efficient bulb converts 15J of every 100J input to useful light energy, with 85J wasted as thermal energy.
Useful energy is energy transferred to the desired store for the device's purpose. Wasted energy is energy dissipated to the surroundings, almost always to thermal stores, often via sound or heating pathways. Efficiency can never exceed 100% as this would violate conservation of energy.
Sankey Diagram
A scaled diagram representing energy transfers through a system, where the width of each arrow is proportional to the amount of energy it represents. Total energy input always equals total energy output (useful + wasted).
Example:
A 100J input Sankey diagram for a bulb has a 10J wide useful light arrow, and 90J wide wasted thermal energy arrow.
A hairdryer uses 800J of electrical energy input to produce 120J of useful kinetic energy to move air. Calculate the efficiency of the hairdryer, and sketch the key features of its Sankey diagram.
- 1
- Recall the efficiency formula:
- 2
- 3
- Substitute the given values: useful output = 120J, total input = 800J
- 4
- 5
- Sankey diagram features: Total input arrow (width scaled to 800J) splits into two branches: 120J wide useful kinetic energy branch, and 680J wide wasted thermal energy branch. Total output = 120 + 680 = 800J, matching input.
Exam tip:
Always show full working for efficiency calculations, and include the % sign in your final answer. If a question asks for efficiency as a decimal, divide the percentage value by 100 (e.g. 15% = 0.15).
3. Thermal Energy Transfer Mechanismsβ β ββββ± 6 min
Thermal energy transfers occur when there is a temperature difference between two objects, and always move from hotter to cooler regions. There are three distinct mechanisms: conduction, convection and radiation.
Conduction
Thermal energy transfer in solids, occurring when vibrating particles pass kinetic energy to neighbouring particles, or free electrons (in metals) carry energy through the material. Non-metals are generally poor conductors (insulators).
Example:
A metal spoon in hot tea getting hot at the handle via conduction.
Convection
Thermal energy transfer in fluids (liquids and gases), occurring when warmer, less dense regions of fluid rise, and cooler, denser regions sink, creating a convection current that transfers energy through the fluid.
Example:
Hot air rising above a radiator, circulating around a room.
Infrared Radiation
Thermal energy transfer via electromagnetic waves, which requires no medium (can travel through a vacuum, e.g. from the Sun to Earth). Dark, matt surfaces are the best absorbers and emitters of infrared radiation; light, shiny surfaces are poor absorbers and emitters (good reflectors).
Example:
A black electric grill emitting infrared radiation to cook food.
Explain the three thermal transfer mechanisms that cause a hot cup of tea to cool down on a kitchen table.
- 1
- Conduction: Thermal energy is transferred from the hot cup to the cooler table and surrounding air via direct contact, as particles in the cup pass energy to neighbouring particles in the table and air.
- 2
- Convection: Warm air next to the hot cup becomes less dense and rises, carrying thermal energy away, while cooler denser air replaces it, creating a convection current that transfers energy away from the cup.
- 3
- Radiation: The hot cup emits infrared radiation to the cooler surroundings, with the rate of emission dependent on the cup's surface (a dark matt cup cools faster than a shiny white cup).
Exam tip:
When explaining convection in exams, always mention the density change of the fluid when it heats or cools: this is a required marking point for full marks.
4. Reducing Unwanted Energy Transfer and Practical Investigationsβ β β βββ± 5 min
Unwanted energy transfer (usually thermal energy loss) reduces the efficiency of devices and buildings. Insulation methods work by reducing conduction, convection or radiation transfers. Common domestic insulation methods include:
Cavity wall insulation: foam between brick layers reduces conduction and convection in the wall gap
Loft insulation: fibreglass wool traps pockets of air, reducing conduction and convection through the roof
Double glazing: air gap between two glass panes reduces conduction through windows
Shiny foil behind radiators: reflects infrared radiation back into the room, reducing radiation loss through walls
Draught excluders: stop warm air escaping through gaps around doors/windows, reducing convection losses
Describe a practical investigation to compare how well different materials act as thermal insulators.
- 1
- Identify variables: Independent variable = type of insulation material (e.g. felt, cotton, aluminium foil, bubble wrap). Dependent variable = temperature of water in a beaker after a set time. Control variables = volume of water, starting water temperature, thickness of insulation, measurement time, room temperature.
- 2
- Wrap equal thicknesses of each material around identical beakers, leave one beaker unwrapped as a control. Pour the same volume of hot water (e.g. 200ml, 80Β°C) into each beaker, and seal each with a lid with a thermometer inserted.
- 3
- Record the temperature of the water in each beaker every 2 minutes for 15 minutes. The material that results in the highest final water temperature is the best insulator, as it has reduced the most thermal energy loss.
Exam tip:
When describing practical investigations, always name the independent, dependent and control variables to earn full AO3 marks for practical skills questions.
5. Common Pitfalls
Wrong move:
Referring to sound or heat as energy stores
Why:
The exam board uses the modern stores and pathways framework; sound and heat are transfer pathways, not stores, so you will lose marking points.
Correct move:
Only use the 8 named energy stores, and refer to heating/sound as radiation or heating transfer pathways.
Wrong move:
Calculating efficiency by dividing total input by useful output
Why:
This reverses the ratio, leading to values over 100% which are physically impossible.
Correct move:
Always divide useful output by total input, multiply by 100 to get a percentage, and check your answer is less than 100%.
Wrong move:
Forgetting to mention density changes when explaining convection currents
Why:
Density change is a required marking point for convection questions, as it is the root cause of fluid movement.
Correct move:
Always state that warmer fluid becomes less dense and rises, while cooler fluid is more dense and sinks.
Wrong move:
Stating that shiny surfaces absorb infrared radiation well
Why:
Shiny surfaces reflect infrared radiation, so they are poor absorbers and emitters.
Correct move:
Remember that dark, matt surfaces are the best absorbers and emitters of infrared radiation; light, shiny surfaces are the worst.
Wrong move:
Drawing Sankey diagrams with unequal total input and total output arrow widths
Why:
This violates conservation of energy, so you will lose marks for incorrect scaling.
Correct move:
Always ensure the sum of the widths of output arrows equals the width of the input arrow.
6. Quick Reference Cheatsheet
Concept | Key Facts | Exam Quick Reminder |
|---|---|---|
Energy Stores | 8 types: chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear | Never list sound/heat as stores |
Transfer Pathways | 4 types: mechanical, electrical, heating, radiation (light/sound) | Sound/light are pathways, not stores |
Efficiency Formula | No formula sheet provided; answer is % < 100% | |
Sankey Diagrams | Arrow width proportional to energy; total in = total out | Label useful and wasted branches clearly |
Conduction | Solids only; via particles/free electrons | Metals are good conductors, air is a good insulator |
Convection | Fluids only; density changes cause currents | Always mention density in explanations |
Infrared Radiation | No medium needed; dark/matt = best emitters/absorbers | Shiny surfaces reflect radiation, reduce loss |
7. Frequently Asked
What is the difference between an energy store and a transfer pathway?
An energy store is where energy is held in a system (e.g. the kinetic store of a moving car). A transfer pathway is how energy moves between stores (e.g. mechanical work done by the car's engine). Sound and light are transfer pathways, not stores, so you should never list them as energy stores in exam answers.
Can efficiency ever be more than 100%?
No. The principle of conservation of energy means you can never get more useful energy out of a system than you put in. Any device claiming efficiency over 100% violates fundamental physical laws, and answers with values over 100% will always be marked incorrect.
Do I need to memorise the efficiency formula for the exam?
Yes, the efficiency formula is not provided on the exam formula sheet. You must recall: . You may also use power values instead of energy for the same calculation, as power is energy per unit time.
Going deeper
What's Next
Now that you have mastered core energy transfer concepts for Edexcel IGCSE Physics, you are ready to move on to related topics in the Energy unit. Next, you will learn to calculate values for energy stores including kinetic energy, gravitational potential energy and work done, as well as the definition and calculations for power, which is the rate of energy transfer. Following that, you will cover energy resources, including renewable and non-renewable sources for electricity generation, and their associated environmental impacts. This knowledge builds directly on the conservation of energy and efficiency principles you have learned here, and is frequently tested alongside energy transfer questions in both Paper 1 and Paper 2 of the exam. Make sure you practice efficiency calculations and thermal transfer explanation questions to solidify your understanding before moving on.