Energy transfers in a system — AQA GCSE Combined Science
Test yourself on Energy transfers in a system with AQA GCSE practice questions.
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Energy transfers in a system explained
The conservation of energy states that the total energy of a closed system stays constant: energy is not created and not destroyed, only transferred between stores or dissipated to the surroundings.
Read the full explanation
Useful energy transfer is the energy delivered to the intended store, such as a motor transferring energy to a gravitational potential store. Dissipated energy spreads into less useful stores, often warming the surroundings by heating, and cannot easily be recovered. For example, a falling ball transfers energy from its gravitational potential store to its kinetic store, while friction and air resistance dissipate some energy to the thermal store of the surroundings. The total energy before and after the change is the same.
Students should be able to describe with examples where there are energy transfers in a closed system, that there is no net change to the total energy.
A closed system is one where no energy enters or leaves, so the total energy stored inside stays constant. Energy moves between stores, but the total is conserved. For example, a pendulum swinging in a vacuum: gravitational potential energy transfers to kinetic energy and back again, while the total energy remains the same. In a closed system, energy transfers between kinetic, gravitational potential, thermal and other stores, but no energy is added or removed. 'No net change' means the sum of all energy stores before and after any transfer is equal. Describe specific transfers, such as a falling object where gravitational potential energy decreases as kinetic energy increases, and state that the total energy is unchanged.
Students should be able to describe, with examples, how in all system changes energy is dissipated, so that it is stored in less useful ways. This energy is often described as being ‘wasted’.
Dissipation occurs in every energy transfer because some energy spreads out into the surroundings, usually as thermal energy, making it less useful for further transfers. For example, a moving car transfers energy from its kinetic store to thermal energy in the brakes, tyres and air; this thermal energy is dissipated and cannot easily be used to make the car move again. Similarly, a light bulb transfers electrical energy to light and thermal energy; the thermal energy is dissipated to the surroundings. This energy is often called 'wasted' because it is not transferred to the intended useful store. Students should describe specific examples, identify the dissipated energy store, and explain that it is stored in less useful ways, often as thermal energy spread over a larger area.
Students should be able to explain ways of reducing unwanted energy transfers, for example through lubrication and the use of thermal insulation.
Unwanted energy transfers usually end up heating the surroundings, so reducing them improves efficiency. Lubrication puts a low-friction layer between moving surfaces, so less energy is transferred mechanically to the thermal store of the surfaces. Thermal insulation traps air or uses low-conductivity material, reducing the rate of conduction, convection and radiation from a hot object. For example, a hot drink in a vacuum flask: the vacuum stops conduction and convection, shiny walls reflect radiation, and the stopper reduces evaporation and convection. Explain each method by naming the pathway blocked and the store that stays warmer or cooler.
The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material.
Thermal conductivity measures how readily energy passes through a material by conduction. A high value means energy transfers quickly for a given temperature difference and thickness; a low value means it transfers slowly. Rate depends on conductivity, temperature difference, thickness and area. For example, a copper rod feels cold quickly because copper conducts at a high rate, while a foam cup keeps tea warm because foam conducts slowly. Compare materials by keeping other variables fixed: same thickness, same temperature difference, same area. Then the material with higher thermal conductivity shows the greater rate of energy transfer.
Students should be able to describe how the rate of cooling of a building is affected by the thickness and thermal conductivity of its walls.
A building cools because energy is transferred from its warmer interior to the cooler outside through its walls. The rate of this transfer depends on the wall's thermal conductivity and thickness. A material with high thermal conductivity, such as metal, transfers energy quickly, so the building cools rapidly. A material with low thermal conductivity, such as foam or brick, transfers energy slowly, so cooling is slower. Increasing wall thickness also slows cooling because energy must conduct through more material, reducing the rate of transfer for the same temperature difference. In practice, thick walls made from a low-conductivity material keep a building warmer for longer. Students should describe both variables together: for a given thickness, lower thermal conductivity gives slower cooling; for a given material, greater thickness gives slower cooling.
Your focus
- State the principle of conservation of energy.
- Describe energy transfers between stores and identify useful and dissipated energy.
- Apply conservation of energy to account for energy in a simple system.
Show all 18 objectives
- Define a closed system and state that the total energy within it remains constant.
- Describe energy transfers between named stores in a given closed system.
- Apply the principle of conservation of energy to show no net change in total energy.
- Describe how energy is dissipated in system changes, using specific examples.
- Explain why dissipated energy is stored in less useful ways and is often called wasted.
- Identify the useful and dissipated energy transfers in a given system change.
- Describe how lubrication reduces unwanted energy transfer by friction.
- Explain how thermal insulation reduces conduction, convection and radiation.
- Apply these ideas to a named everyday context and compare rates of cooling.
- State that thermal conductivity describes how well a material conducts energy.
- Explain why a higher thermal conductivity gives a higher rate of conduction.
- Compare two materials fairly by controlling thickness, area and temperature difference.
- Describe how the rate of cooling of a building depends on the thermal conductivity of its walls.
- Describe how the rate of cooling of a building depends on the thickness of its walls.
- Compare the cooling rates of buildings with walls of different thickness and thermal conductivity.
Energy transfers in a system exam tips
Marking Points
- States that energy cannot be created or destroyed, so the total energy of a closed system is constant.
- Describes useful energy transfer as energy delivered to the intended store or device.
- Explains that energy can be stored in stores such as kinetic, gravitational potential, elastic potential, thermal, chemical and nuclear stores.
- Describes dissipation as energy spreading to the surroundings, often by heating, becoming less useful.
- Applies conservation of energy to a system, showing that energy input equals useful output plus dissipated energy.
- Defines a closed system as one in which no energy enters or leaves, so the total energy stored remains constant.
- Describes a named example, such as a pendulum or falling object, identifying the stores involved and the direction of transfer.
- States that energy is conserved: the total energy before a transfer equals the total energy after, with no net change.
- Uses the idea that energy can transfer between stores but cannot be created or destroyed within a closed system.
- Applies the principle to a simple calculation or comparison, showing that the sum of energy stores is unchanged.
- States that in all system changes, some energy is dissipated to the surroundings, often as thermal energy.
- Describes a named example, such as a car braking or a light bulb, identifying the useful transfer and the dissipated transfer.
- Explains that dissipated energy is stored in less useful ways because it spreads out and is difficult to use for further useful transfers.
- Uses the term 'wasted' correctly to mean energy that is not transferred to the intended useful store.
- Recognises that dissipation occurs in every energy transfer, not just in inefficient devices.
- Lubrication reduces friction between moving surfaces, so less energy is dissipated to the thermal store of the surroundings.
- Thermal insulation reduces the rate of energy transfer by conduction because trapped air or foam has low thermal conductivity.
- Insulation can also reduce convection by preventing large-scale movement of fluid, and shiny surfaces reduce radiation.
- A valid explanation links each method to a named pathway and states that the useful store stays warmer or cooler for longer.
- Comparing rates, not just totals, shows understanding: insulation slows transfer, so the temperature difference falls more slowly.
- Thermal conductivity is a property of the material that indicates how well it conducts energy.
- A higher thermal conductivity gives a higher rate of energy transfer by conduction for the same temperature difference and thickness.
- Rate also increases with greater temperature difference and greater cross-sectional area, and decreases with greater thickness.
- Comparing materials fairly requires controlling thickness, area and temperature difference.
- Typical values: metals have high thermal conductivity; gases and foams have low thermal conductivity.
- State that energy is transferred from the warmer inside of the building to the cooler outside through the walls.
- Explain that a higher thermal conductivity means a faster rate of energy transfer, so the building cools more quickly.
- Explain that a lower thermal conductivity means a slower rate of energy transfer, so the building cools more slowly.
- Explain that increasing the thickness of a wall reduces the rate of energy transfer and therefore slows cooling.
- Compare walls correctly, for example a thick low-conductivity wall cools a building more slowly than a thin high-conductivity wall.
- Link the description to the temperature difference between inside and outside as the driving factor for the transfer.
Examiner Tips
- 💡Use the words transferred, stored and dissipated precisely rather than saying energy is lost.
- 💡For a system question, list the energy stores before and after the change and account for any dissipated energy.
- 💡Check that any energy account balances: input energy equals useful energy plus energy dissipated.
- 💡Name the system and state clearly that it is closed before describing the transfers.
- 💡Use the phrase 'no net change to the total energy' to show understanding of conservation.
- 💡Give a specific example with named stores and describe the direction of each transfer.
- 💡Check that your example does not allow energy to leave the system, otherwise it is not closed.
- 💡Name the useful energy transfer and the dissipated transfer in your example.
- 💡Use the phrase 'stored in less useful ways' to match the specification wording.
- 💡Explain why the dissipated energy is less useful, for example because it spreads out to the surroundings.
- 💡Avoid saying energy is 'lost'; instead say it is dissipated or transferred to less useful stores.
- 💡Name the pathway: conduction, convection, radiation or friction, then say how the method reduces it.
- 💡Use comparative language such as 'reduces the rate' rather than 'stops' or 'no energy is lost'.
- 💡Link each method to a familiar context, such as a vacuum flask or lubricated engine parts, to make the explanation concrete.
- 💡Use the phrase 'for the same temperature difference and thickness' when comparing two materials.
- 💡State the direction of transfer: from the hotter region to the cooler region.
- 💡If a graph is given, describe the gradient as the rate and link a steeper gradient to higher thermal conductivity.
- 💡Name both variables explicitly in your answer: thermal conductivity and thickness of the walls.
- 💡Use comparative language such as 'faster', 'slower', 'higher' and 'lower' to make the direction of each effect clear.
- 💡Apply the ideas to a real example, such as a cavity wall with insulating foam, to show understanding rather than repeating the statement.
Common Mistakes
- Saying energy is used up or lost: correct this by stating that energy is transferred or dissipated, not destroyed.
- Treating dissipated energy as destroyed: correct this by explaining that it spreads to the surroundings and remains in the total energy account.
- Confusing energy stores with energy transfers: correct this by naming the store, such as thermal, and the pathway, such as heating or mechanical work.
- Saying energy is 'used up' or 'lost' in a closed system; correct this by stating energy is transferred to other stores and the total remains constant.
- Assuming no energy transfers occur in a closed system; correct by explaining transfers still happen between stores inside the system, but no energy enters or leaves.
- Thinking that energy dissipated to the surroundings can happen in a closed system; correct by remembering that if energy transfers to the surroundings, the system is not closed.
- Saying energy is 'destroyed' or 'lost' when it is dissipated; correct this by stating energy is transferred to less useful stores, often thermal energy in the surroundings.
- Assuming dissipation only happens in old or inefficient devices; correct by explaining it happens in all system changes.
- Confusing 'wasted' with energy that no longer exists; correct by stating the energy still exists but is spread out and less useful.
- Saying lubrication 'creates energy' or 'stops energy transfer completely'; correct by stating it reduces friction and so reduces the rate of unwanted heating.
- Confusing insulation with a source of heat; correct by explaining insulation only slows transfer, it does not warm the object.
- Claiming trapped air stops conduction entirely; correct by saying air is a poor conductor, so it reduces the rate of conduction.
- Saying a high-conductivity material is 'hotter'; correct by saying it transfers energy at a higher rate, not that it has a higher temperature.
- Ignoring thickness or area; correct by stating that rate depends on these as well as conductivity.
- Confusing thermal conductivity with specific heat capacity; correct by noting conductivity is about rate of transfer, while specific heat capacity is about energy needed per kilogram per degree.
- Saying that thick walls 'absorb cold' or 'keep cold out' rather than reducing the rate of energy transfer by conduction; correct this by describing energy transfer from hot to cold and the effect of thickness on that rate.
- Confusing thermal conductivity with thickness, for example claiming a thicker wall has higher thermal conductivity; correct this by treating them as separate variables that each affect the cooling rate.
- Stating that a high thermal conductivity keeps a building warm; correct this by explaining that high conductivity means faster energy transfer out of the building, so it cools faster.