Energy transfers in a system — AQA GCSE Physics
Test yourself on Energy transfers in a system with AQA GCSE practice questions.
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Energy transfers in a system explained
Conservation of energy is the rule underneath the whole topic: energy can be transferred usefully, stored for later or dissipated to the surroundings, but the total never changes.
Read the full explanation
Nothing is created and nothing is destroyed. That is why the word wasted is only ever shorthand. When a wind turbine turns, most energy goes usefully to the generator, but some is dissipated by friction in the bearings into the thermal store of the parts and the air. Oil the bearings and less work is done against friction, so a greater proportion is transferred usefully and the efficiency rises. The energy that was wasted has not disappeared; it has simply spread out, too thinly spread to be recovered.
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 matter or energy can enter or leave. Consequently, while energy can be transferred between different stores within the system, there is no net change to the total energy. For example, consider a cold spoon dropped into an insulated flask of hot soup. The flask acts as a closed system. Thermal energy is transferred from the hot soup to the cold spoon, causing the soup to cool and the spoon to heat up. However, the total thermal energy within the flask remains constant. If energy appears to vanish in a calculation, the system boundary has likely been drawn too tightly and must be widened to capture all transfers.
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.
Energy dissipation occurs when energy spreads out into less useful stores during a system change. Every real physical change dissipates some energy, usually into the thermal store of the surroundings, causing them to heat up. For example, when a filament lamp operates, it usefully transfers energy by light, but it also dissipates energy by heating the surroundings. This dissipated energy is not destroyed, but it becomes so thinly spread that it cannot be usefully recovered or gathered back by any device. This explains why mechanical systems require lubrication to reduce friction and why homes need thermal insulation to minimise the rate at which energy dissipates into the environment.
This energy is often described as being ‘wasted’.
Wasted is the everyday word for energy that has been dissipated into stores nobody can use. It is a label about usefulness, not about existence: when brakes get hot, the energy in the thermal store of the brake discs is every bit as real as the kinetic store it came from, but nothing in the car can collect it back. In a wind turbine the wasted share warms the bearings and the air; in a kettle it warms the body of the kettle and the kitchen; in a runner it warms the muscles. Because the wasted energy spreads into the surroundings at a temperature close to everything around it, it becomes impossible to concentrate again. So an exam answer should say the energy is wasted or dissipated to the thermal store of the surroundings, never that it is lost.
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 can be significantly reduced, though never entirely eliminated, using methods such as lubrication and thermal insulation. Friction between moving mechanical parts does work that transfers energy to thermal stores, causing dissipation. Applying a lubricant, like oil to bicycle gears, reduces this friction and decreases the rate of unwanted energy transfer. Similarly, thermal insulation, such as a fibreglass jacket around a hot water tank, lowers the thermal conductivity and cuts the rate at which energy passes to the surroundings. By reducing these unwanted transfers, a greater proportion of the input energy is transferred usefully, thereby increasing the overall efficiency of the system.
The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material.
Conduction carries energy through a material as vibrating particles jostle their neighbours and, in metals, as free electrons move through the structure. Thermal conductivity is the property that says how readily a material does this. A high thermal conductivity means a high rate of energy transfer for the same temperature difference and thickness, which is why a metal saucepan handle feels hot quickly. A low thermal conductivity means a low rate, which is what makes wool, foam and trapped air good insulators. In a comparison experiment the material that lets a beaker of water cool by 20 °C in 450 s has a higher thermal conductivity than one that takes 745 s, because the same temperature drop in a shorter time means energy is being transferred faster.
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 passes out through the walls, roof and windows by conduction, and the rate of that loss depends on two properties of the walls. Thermal conductivity comes first: brick and glass conduct more readily than foam, mineral wool or trapped air, so filling a cavity with foam lowers the rate of cooling. Thickness comes second: for the same material, a thicker wall means a lower rate of energy transfer, which is why loft insulation is sold by depth and why the standard practical compares thicknesses of the same material. The two work together, so a thin layer of a poor conductor can outperform a thick layer of a good one. The consequence is practical: the building stays warmer for longer and needs less heating.
Required practical activity 2 (physics only): investigate the effectiveness of different materials as thermal insulators and the factors that may affect the thermal insulation properties of a material.
In this required practical you compare insulating materials. Pour a fixed volume of hot water into a beaker, wrap it in the material being tested, fit a lid with a thermometer through it, and record the temperature every minute for ten minutes, or time how long it takes to fall by a set amount. The independent variable is the type of material, or its thickness if that is what you change; the dependent variable is the temperature drop in a set time, or the time for a set drop. Control the starting temperature, the volume of water, the beaker and the room. The material that keeps the water hottest has the lowest thermal conductivity. Repeat and average, because a single run hides how much readings scatter.
Your focus
- State the principle of conservation of energy and apply it to a turbine, a motor or a vehicle.
- Explain how lubricating moving parts raises the proportion of energy transferred usefully.
- Identify the useful and the dissipated transfers in a named device and say where the dissipated energy ends up.
Show all 29 objectives
- Justify why no real device can transfer all of its input energy usefully.
- Define a closed system as one where no energy or matter can enter or leave.
- Identify whether a given situation can be treated as a closed system.
- Explain, using examples, that energy transfers within a closed system result in no net change to the total energy.
- Describe where energy is dissipated in everyday devices like a lamp, a kettle, or a motor.
- Explain that in all system changes, some energy is stored in less useful ways.
- Explain why dissipated energy cannot be collected and used again.
- Classify each transfer in a named device as useful or wasted and justify the choice.
- Explain why calling energy wasted does not break conservation of energy.
- Identify the store that receives wasted energy in a motor, a lamp and a braking car.
- Describe how reducing a wasted transfer changes the efficiency of a device.
- Explain how lubricating moving parts reduces the energy dissipated by a mechanical machine.
- Describe how thermal insulation lowers the rate of energy transfer from a hot object to its surroundings.
- Evaluate different methods of reducing unwanted energy transfers in a given context to improve efficiency.
- Explain how energy passes through a solid by conduction in terms of particles and free electrons.
- Deduce which of two materials has the higher thermal conductivity from times for the same temperature drop.
- Justify the choice of a low conductivity material for a saucepan handle or a loft.
- Rank three materials by the rate at which energy passes through them when their thermal conductivities are given.
- Predict how doubling the thickness of a wall changes the rate at which a building cools.
- Explain why a cavity filled with foam cools more slowly than one filled with air movement.
- Draw two conclusions from cooling curves for water wrapped in different insulating materials.
- Recommend an improvement to a building and justify it using thickness and conductivity.
- Identify the independent, dependent and control variables in an insulation investigation.
- Describe a method for comparing insulating materials that would give valid, repeatable results.
- Draw two supported conclusions from a graph of temperature against time for two materials.
- Explain why a lid and a fixed volume of water are needed for a fair comparison.
Energy transfers in a system exam tips
Marking Points
- one mark for stating that energy is transferred rather than created or destroyed
- one mark for naming the useful transfer and the dissipated transfer in the example given
- one mark for linking reduced friction to less energy dissipated to the thermal store
- one mark for concluding that the proportion of energy transferred usefully, and so the efficiency, increases
- identifying the initial energy stores before the change occurs
- describing the pathways of energy transfer between stores within the system
- stating that no energy or matter can enter or leave the defined system boundary
- concluding that the total energy of the closed system remains constant with no net change
- identifying the useful energy transfer in the specific device or system described
- identifying the wasted energy transfer, typically heating the surroundings
- explaining that the wasted energy is dissipated and stored in less useful ways
- stating that the dissipated energy spreads out and cannot be usefully recovered
- one mark for identifying which transfers in the device are wasted rather than useful
- one mark for naming the thermal store of the surroundings or of the component as the destination
- one mark for stating that the total energy is unchanged even though part is wasted
- one mark for linking a reduction in wasted energy to a rise in efficiency
- identifying the specific unwanted energy transfer and the store it fills
- describing a suitable method to reduce the unwanted transfer, such as lubrication or insulation
- explaining the mechanism, such as oil reducing friction or insulation lowering thermal conductivity
- stating the consequence, which is that less energy is dissipated and the system's efficiency increases
- one mark for stating that a higher thermal conductivity gives a higher rate of energy transfer
- one mark for comparing the times for the same temperature change in the two materials
- one mark for concluding which material has the higher thermal conductivity
- one mark for referring to conduction across the material rather than energy transfer in general
- one mark for stating that a thicker wall gives a lower rate of energy transfer
- one mark for stating that a lower thermal conductivity gives a lower rate of energy transfer
- one mark for linking the lower rate to a slower fall in the temperature inside the building
- one mark for supporting the conclusion with data, such as a longer time for the same temperature drop
- one mark for identifying the independent variable as the type of insulating material
- one mark for identifying the dependent variable, the temperature reached or the time taken
- one mark for two named control variables such as the volume and starting temperature of the water
- one mark for a conclusion linking the slowest cooling to the best insulator or lowest thermal conductivity
Examiner Tips
- 💡Use the word dissipated rather than lost; the mark schemes accept wasted but never lost energy as an explanation.
- 💡For an explain mark, give the chain: less friction, less energy dissipated, higher proportion transferred usefully.
- 💡Do not answer with more electricity is generated on its own, as it is ignored without the efficiency link.
- 💡Always define a closed system by stating that no energy or matter can enter or leave it.
- 💡When asked to explain energy changes, explicitly state that the total energy remains constant to secure the final mark.
- 💡Use the term 'dissipated' in your answer and explicitly state that it transfers to the thermal store of the surroundings.
- 💡Remember that 'wasted' energy is still conserved; it has just become too spread out to be useful.
- 💡Decide what the device is for first, because the same thermal transfer is useful in a heater and wasted in a drill.
- 💡Pair the word wasted with a destination, such as wasted to the thermal store of the surroundings.
- 💡Unqualified references to friction are ignored, so always say less work is done against friction.
- 💡Always match the reduction measure to the correct mechanism; use lubrication for moving parts and insulation for heating issues.
- 💡When explaining insulation, mention that it reduces the rate of energy transfer, not just that it 'stops heat escaping'.
- 💡Anchor every comparison to the same temperature change, then talk about the time taken.
- 💡The converse answer is accepted, so arguing from the slower material is equally valid.
- 💡Say rate of energy transfer explicitly, since the word rate is usually the marking point.
- 💡Answer in terms of rate of cooling, since higher and lower rate are the phrases the mark scheme uses.
- 💡Where a graph is given, quote a pair of readings to support the conclusion rather than describing the shape.
- 💡Keep conduction and radiation separate; wall colour and shininess belong to the radiation questions.
- 💡Read the stem carefully before naming variables; the mark scheme rejects thickness where material was the variable.
- 💡Say how long the readings were taken for, because a method without a time span is incomplete.
- 💡Offer a conclusion in the form water wrapped in X cooled more slowly, then add the conductivity comparison.
Common Mistakes
- writing that energy is lost or destroyed instead of dissipated to the surroundings
- saying oiling the bearings creates more electricity rather than increasing the proportion usefully transferred
- naming friction on its own without saying that less work is done against it
- claiming a device can be made 100% efficient by removing friction entirely
- defining a closed system solely as one with constant energy, rather than one where no matter or energy enters or leaves
- describing a closed system as one where no energy transfers happen at all, rather than transfers occurring with no net change
- treating a system that is heating the room around it as closed, instead of widening the boundary to include the surroundings
- saying dissipated energy is destroyed rather than spread out into the surroundings
- naming sound or light as wasted energy without specifying which store or surroundings eventually receive it
- assuming that highly efficient devices do not dissipate any energy at all, whereas all real system changes involve some dissipation
- writing that wasted energy is lost or destroyed, which mark schemes do not credit
- calling all thermal energy wasted, even in a kettle or heater where heating is the useful output
- giving sound as the main waste from a motor when the thermal store takes far more
- treating wasted energy as a fixed property of a device rather than something lubrication or insulation can reduce
- naming oil or insulation without explaining the physical mechanism of how it changes the energy transfer
- claiming that lubrication removes friction completely, rather than just reducing it
- confusing the applications of insulation and lubrication, such as suggesting insulation reduces mechanical friction
- confusing a faster fall in temperature with a lower thermal conductivity, so reading the data backwards
- comparing times without checking that both materials cooled through the same temperature difference
- describing thermal conductivity as the amount of energy transferred rather than the rate
- using the word conduction for energy carried away by moving air, which is convection
- claiming thick insulation stops cooling completely rather than slowing it
- changing both thickness and material in an argument, so neither effect is isolated
- saying the thicker wall keeps more heat in, without mentioning the rate of energy transfer
- assuming a material feels warm because it produces energy rather than because it conducts poorly
- giving thickness of material as the independent variable when the investigation compared different materials
- changing the volume of water between trials so the comparison is invalid
- leaving the beaker without a lid, so evaporation and convection swamp the effect being tested
- concluding the material is best without referring to the cooling data that shows it