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AQA 6.1 / PHYSICS

Energy

Follow energy as it changes stores, calculate the amounts transferred and evaluate the resources we use.

Supported GCSE version · Clear examples, worked calculations and optional challenges.

Foundation & Higher35 main + 5 challenge questionsSave and return any time

Understand the science

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YOUR TOPIC GUIDE

Energy

Work through one section at a time. Higher-only material is labelled explicitly; optional challenges can also use Foundation content.

Words to know

System

The object or group of objects chosen for study.

Energy store

A way energy is held, such as kinetic or chemical.

Transfer pathway

How energy moves between stores: mechanically, electrically, by heating or radiation.

Work done

Energy transferred by a force or by an electric current.

Kinetic energy

Energy associated with movement.

Extension

The increase in length from an elastic object’s original length.

Specific heat capacity

Energy needed to raise 1 kg of a material by 1 °C.

Power

Energy transferred, or work done, each second.

Closed system

A system across whose boundary no energy is transferred.

Dissipation

Energy spreading into less useful stores, often thermal stores of the surroundings.

Efficiency

The fraction of total input transferred usefully.

Thermal conductivity

A property used to compare how readily materials transfer energy by conduction.

Renewable resource

An energy resource replenished on human timescales as it is used.

Reliable supply

A supply available when needed; predictability and controllability both matter.

1. Energy stores and transfer pathways

A system is the object or group of objects we choose to study. For a falling ball, we might study the ball and Earth together. Energy is measured in joules (J). It can be stored in different ways and transferred between stores.

StoreExample
KineticA moving bicycle
ThermalHot water; particles have energy associated with their motion and interactions
ChemicalFuel, food or a battery
Gravitational potentialA raised object in a gravitational field
Elastic potentialA stretched spring or compressed elastic object
MagneticTwo magnets arranged so that they can move when released
ElectrostaticSeparated electric charges
NuclearAtomic nuclei

Energy moves between stores mechanically (a force does work), electrically (a current flows), by heating or by radiation, such as light or infrared. Sound can also carry energy away through waves.

A store is where energy is held; a pathway describes how it moves. Avoid “a store of electricity” or “energy used up”. Say which store decreases, how energy is transferred and which store increases.

2. Describe the whole change

  • Ball thrown upwards: after it leaves the hand, its kinetic store decreases as the gravitational potential store of the ball–Earth system increases. Air resistance also transfers energy to thermal stores.
  • Moving trolley hits a barrier: its kinetic store decreases. Energy is transferred into deformation and thermal stores of the trolley, barrier and surroundings; sound carries some away.
  • Vehicle accelerates: chemical energy in fuel or a battery decreases; forces do work, increasing the vehicle’s kinetic store, with unwanted heating.
  • Vehicle brakes: friction transfers energy from its kinetic store to thermal stores of the brakes, tyres, road and air.
  • Electric kettle heats water: electrical work increases thermal energy in the water, kettle and surroundings. Before boiling, the water temperature rises.

Work done is energy transferred. For a constant force acting along a displacement, W = F × s (J = N × m). Electrical energy can be found using E = P × t, or E = V × I × t when voltage and current are measured. These links are developed further in Forces and Electricity.

Worked example: compare stores on the same scale

A falling object loses 120 J from its gravitational potential store. Its kinetic store gains 100 J. The remaining 20 J is transferred to thermal stores of the air and object. The total is still 120 J. A bar representing 120 J must equal bars representing 100 J and 20 J combined.

3. Kinetic energy: mass and speed

Ek = ½mv², where Ek is kinetic energy in J, mass m is in kg and speed v is in m/s. Square the speed before multiplying. Convert grams to kilograms by dividing by 1,000.

Worked example

A 500 g trolley moves at 4.0 m/s. Mass = 0.500 kg. Ek = 0.5 × 0.500 × 4.0² = 4.0 J.

For the same speed, doubling mass doubles kinetic energy. For the same mass, doubling speed multiplies kinetic energy by four, because speed is squared. A faster vehicle therefore needs to transfer much more energy when stopping.

To find mass, use m = 2Ek ÷ v². To find speed, use v = √(2Ek ÷ m). Keep the whole fraction inside the square root.

4. Gravitational and elastic potential energy

Raising a mass transfers energy to its gravitational potential store: Ep = mgh. Use mass in kg, gravitational field strength g in N/kg and vertical height h in m. Use the value of g given in the question.

Worked example: lifting

A 3.0 kg box rises 2.0 m where g = 9.8 N/kg. Ep = 3.0 × 9.8 × 2.0 = 58.8 J. If it falls back with negligible air resistance, this can become kinetic energy.

A spring stores elastic potential energy when stretched or compressed: Ee = ½ke². Spring constant k is in N/m; extension e is in m. Extension is the change from original length, not the whole stretched length. This equation assumes the limit of proportionality has not been exceeded.

Worked example: stretching

A spring with k = 200 N/m extends 5.0 cm. First convert: e = 0.050 m. Ee = 0.5 × 200 × 0.050² = 0.25 J. Doubling the extension would give four times the stored energy, if the spring remains within its proportional range.

Useful rearrangements: h = Ep ÷ (mg); e = √(2Ee ÷ k).

5. Heating and specific heat capacity

When a material changes temperature without changing state, its thermal energy change can be calculated using ΔE = mcΔθ. The symbol Δ means “change”. Temperature rise = final temperature − initial temperature.

  • ΔE: thermal energy change, J
  • m: mass, kg
  • c: specific heat capacity, J/(kg °C)
  • Δθ: temperature change, °C

Specific heat capacity tells us how much energy is needed to raise the temperature of 1 kg of a material by 1 °C. A larger c means that the same energy input produces a smaller temperature rise for the same mass.

Worked example: warming water

Heat 0.50 kg of water from 20 °C to 35 °C. Use c = 4,200 J/(kg °C). Δθ = 15 °C. ΔE = 0.50 × 4,200 × 15 = 31,500 J = 31.5 kJ. This is the water’s energy gain; a real heater may supply more because the container and surroundings also gain energy.

Rearrange to c = ΔE ÷ (mΔθ), or Δθ = ΔE ÷ (mc). This equation should not be used alone to calculate energy for melting or boiling at constant temperature.

Graph of temperature rise against energy supplied for a one kilogram block.
Read coordinates carefully. The graph uses kJ, so multiply by 1,000 before calculating c.

6. Required practical 14: determine specific heat capacity

An insulated metal block with an immersion heater and thermometer linked to a low-voltage power supply through a joulemeter.
The joulemeter measures electrical energy supplied. A balance measures the block’s mass.
  1. Measure the metal block’s mass with a balance. Record it in kg.
  2. Place a heater and thermometer into the appropriate holes. Use a little suitable contact fluid if instructed to improve thermal contact. Wrap the block in insulation and place it on a heat-resistant mat.
  3. Record the initial temperature. Connect the heater to a low-voltage supply through a joulemeter, then start heating.
  4. Record energy supplied and temperature at regular intervals. Alternatively measure voltage, current and time and calculate E = VIt.
  5. Find the temperature rise and calculate c = E ÷ (mΔθ). Repeat measurements and compare values. A graph can use several readings rather than only one pair.

Improve the measurement: insulation reduces energy transferred to the room; good contact helps the thermometer represent the block temperature. Wait for readings to stabilise where appropriate. Some supplied energy warms the heater, thermometer and insulation, so not all of it enters the block. Heat loss often makes the calculated c too high if all input energy is assumed to warm the block.

Safety: use the school’s low-voltage equipment and instructions. Hot blocks and heaters can burn skin; switch off and allow them to cool before handling. Keep liquids away from electrical connections.

This is the Energy required practical for Combined Science. Comparing insulation is a useful investigation, but is not an additional required practical for this course.

7. Power: how quickly energy is transferred

Power is energy transferred per second, or work done per second. P = E/t and P = W/t. Power is in watts (W), energy/work in joules (J), and time in seconds (s). One watt means one joule per second.

Two motors can lift the same load through the same height and do the same work. The motor that does it in less time has the greater useful power. A higher-power appliance does not automatically use more total energy: operating time matters.

Worked example: unit conversion

A 1.5 kW heater operates for 2.0 minutes. P = 1,500 W and t = 120 s. E = Pt = 1,500 × 120 = 180,000 J. To find time instead, use t = E/P.

The symbol W can mean work done in an equation, while the unit W means watt. Check the context: work is measured in J, not watts.
Two straight energy against time lines show motor A has twice the gradient of motor B.
A steeper energy–time graph means a larger energy transfer each second.

8. Conservation, dissipation and efficiency

Energy is conserved: it cannot be created or destroyed. In a closed system, no energy enters or leaves, so its total remains constant even when energy changes store. Choose the boundary carefully: a cooling cup alone loses energy, but the cup and its surroundings together can be treated as a larger system.

Dissipation means energy spreads into less useful stores, often thermal energy in the surroundings. “Wasted” energy has not disappeared; it is simply less useful for the intended task.

Efficiency = useful energy output ÷ total energy input. Alternatively use useful power output ÷ total power input. A decimal of 0.75 means 75%; multiply by 100 to convert to a percentage. Efficiency cannot exceed 1 (100%).

Sankey diagram with 100 joules input splitting into 80 joules useful and 20 joules dissipated.
Arrow thickness represents energy transferred on a common scale.
Worked example: using power

A motor takes 250 W electrically and transfers 180 W usefully. Efficiency = 180 ÷ 250 = 0.72 = 72%. Unwanted output = 250 − 180 = 70 W.

Higher-only: improving the intended transfer

Identify the unwanted pathway, then explain the change. Lubricating moving parts reduces frictional heating, leaving a larger fraction of input available for useful mechanical work. Insulating a hot-water tank reduces transfer to the room, so a larger fraction of the supplied energy remains useful in the water. Compare useful output with total input; reducing input alone does not prove efficiency improved.

9. Reducing unwanted transfers

Lubrication reduces friction between moving surfaces and so reduces unwanted energy transfer to thermal stores. Thermal insulation reduces unwanted energy transfer from a hotter object to colder surroundings.

A material with higher thermal conductivity transfers energy by conduction more rapidly under the same conditions. For a wall of the same material and area, increasing thickness reduces the rate of transfer. For walls of equal thickness, lower conductivity reduces transfer. Insulation slows cooling; it does not supply energy or stop all transfer.

To compare insulation fairly, keep water mass, starting temperature, container dimensions, surrounding temperature and measurement times the same. Change only the insulation being investigated. Use repeated measurements, calculate means and consider unusual results.

Temperature against time for insulated and uninsulated containers, both starting at eighty degrees Celsius.
Both cool. The insulated container has a smaller temperature fall over the same time.

10. National and global energy resources

An energy resource is a source that people use for heating, transport or generating electricity. Renewable resources are replenished on human timescales as they are used. Non-renewable resources have finite reserves and are not replaced quickly enough.

ResourceUses and reliabilityImportant impacts or limits
Coal, oil and natural gas — non-renewableFuel for heating, transport and electricity; can be stored and used when needed.Burning releases carbon dioxide. Extraction damages habitats; some fuels also release harmful pollutants.
Nuclear fuel — non-renewableProduces steady electricity; output is less flexible than some gas plants.Low operational carbon emissions, but radioactive waste needs long-term management; construction and decommissioning are costly.
Biofuel — renewable when crops are replaced sustainablyCan fuel transport, heating and generators; stored fuel can be used on demand.Growing crops uses land, possibly competing with food or habitats. Burning emits carbon dioxide; production and transport also have impacts.
Wind — renewableGenerates electricity when wind speed is suitable.Variable output; visual/noise impacts and effects on some wildlife. No fuel burning during generation.
Sun — renewableSolar cells generate electricity; solar heating warms water.Output depends on daylight and cloud; manufacturing and land use have impacts.
Hydroelectricity — renewableWater released from a reservoir drives turbines; can respond quickly if sufficient water is stored.Dams flood land and affect river ecosystems. Drought can reduce available water.
Tides — renewablePredictable periodic electricity production.Not continuous; suitable sites are limited and coastal ecosystems may be affected.
Water waves — renewableWaves drive generators; output depends on sea conditions.Equipment must withstand storms; effects on marine use and habitats must be considered.
Geothermal — renewableEarth’s internal thermal energy provides heating and, at suitable sites, electricity; often steady.Useful high-temperature sites are geographically limited; drilling is costly.

Renewable does not mean reliable every minute, and no energy resource has zero environmental impact. A varied supply, storage, interconnections and demand management can help balance variable output. Detailed construction of power stations is not required in this topic.

11. Compare evidence and justify decisions

Read the date, location, units and source before comparing national or global energy-use data. A percentage share can fall even while the total energy from a resource rises, if overall demand grows faster. Historical trends do not prove that the same trend must continue.

For a fair comparison, consider reliability, total energy available, cost, location, environmental effects and the intended use. A school’s heating choice and a country’s electricity supply need different evidence.

Science can measure emissions and model their effects, but decisions also depend on money, politics, jobs, land ownership, public acceptance and ethical choices. Explain which evidence supports a recommendation and acknowledge its disadvantages.

Worked example: interpreting a trend

In an invented region, renewable electricity rises from 20 TWh out of 100 TWh to 45 TWh out of 150 TWh. The renewable share rises from 20% to 30%. Renewable output increases by 25 TWh, not by 10 TWh. The ten-percentage-point change describes the share.