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Energy is the ability to do work. Whenever something happens anywhere in the universe, energy is transferred.
Energy is stored in objects. The main energy stores are: gravitational, kinetic, elastic, thermal, chemical, nuclear, electrostatic and magnetic.
Energy cannot be created or destroyed — it is only transferred between stores. This is the Law of Conservation of Energy.
The unit of energy is the Joule (J). Larger quantities are expressed in kJ (1 kJ = 1000 J) or MJ.
Gravitational potential energy (GPE) is stored when an object with mass is raised in a gravitational field.
Ep = gravitational potential energy (J); m = mass (kg); g = gravitational field strength (N/kg); h = height (m).
On Earth, g = 9.8 N/kg. GPE increases if mass, height, or g increases.
Example: A rock of mass 75 kg is lifted 4 m. Ep = 75 × 9.8 × 4 = 2940 J.
To find mass: m = Ep ÷ (g × h)
To find height: h = Ep ÷ (m × g)
To find g: g = Ep ÷ (m × h)
Use the VESSS method: Values → Equation → Substitute → Solve → State units.
A person slides down a zip wire from height h. Mass = 60 kg, g = 9.8 N/kg.
Kinetic energy (KE) is the energy stored in a moving object.
Ek = kinetic energy (J); m = mass (kg); v = velocity (m/s).
Velocity is used (not speed) because KE depends on both magnitude and direction of motion.
Tip: calculate v² first, then multiply by ½m. Using 0.5 for ½ on a calculator avoids errors.
To find mass: m = 2Ek ÷ v²
To find velocity: v = √(2Ek ÷ m)
A lorry moving at 30 mph has more KE than a car at the same speed because its mass is larger — KE ∝ m.
KE increases with the square of velocity — doubling speed quadruples KE.
KE and GPE are linked by conservation of energy. When an object falls, GPE converts to KE (ignoring air resistance).
If a 3 kg ball falls from 5 m: GPE lost = mgh = 3 × 9.8 × 5 = 147 J = KE gained.
The maximum KE at the bottom equals the GPE at the top (in a closed system).
A ball of mass 0.5 kg is dropped from a height of 8 m. g = 9.8 N/kg. Ignore air resistance.
In a closed system, the total energy remains constant. Energy is transferred between stores but the total never changes.
Example: A ball thrown upward — KE converts to GPE as it rises; GPE converts back to KE as it falls.
In reality, no system is perfectly closed. Some energy is always transferred to the surroundings as thermal energy.
When a ball hits the ground, KE is transferred to sound and thermal energy stores — both are dissipated.
Dissipation is when energy is transferred to the surroundings in a less useful form, usually thermal or sound.
Dissipated energy is "wasted" — it spreads into the surroundings and cannot be recovered easily.
Examples: friction in a car engine (thermal), air resistance on a cyclist (thermal), sound from brakes.
Reducing dissipation: lubrication reduces friction; streamlining reduces air resistance; insulation reduces thermal loss.
When describing energy transfers, state: initial store → mechanism of transfer → final store(s).
Zip wire: GPE → (mechanically) → KE + thermal (friction).
Bouncing ball: GPE → KE (falling) → elastic PE (squash) → KE + thermal + sound (bounce).
The total energy at the end equals the total at the start; only the distribution changes.
A person of mass 60 kg slides down a zip wire. The change in vertical height is 2.5 m. g = 9.8 N/kg.
Elastic potential energy (EPE) is stored when an elastic object (e.g. a spring) is stretched or compressed.
Ee = elastic potential energy (J); k = spring constant (N/m); e = extension (m).
The extension is the extra length stretched — not the total length.
The spring constant k measures the stiffness of the spring. A higher k means a stiffer spring.
This formula is on the AQA formula sheet — you do not need to memorise it.
To find spring constant: k = 2Ee ÷ e²
To find extension: e = √(2Ee ÷ k)
Example: A spring stores 18 J with k = 4 N/m. Find the extension. e = √(2 × 18 ÷ 4) = √9 = 3 m.
When a spring is released, elastic PE converts to KE (and some thermal).
A bungee cord stores elastic PE when stretched; when it pulls the jumper back, EPE converts to KE then GPE.
Elastic PE → KE conversions occur in trampolines, bows, catapults and musical instruments.
A bungee cord has spring constant k = 15 N/m. A student of mass 70 kg jumps from a bridge. The unstretched cord length is 20 m.
Temperature is the average kinetic energy store of the particles in a system.
Two blocks of equal mass but different materials will heat up by different amounts when given the same energy.
Copper has SHC = 386 J/kg°C; gold has SHC = 126 J/kg°C. Gold heats up faster with the same energy input.
The material that is "easier to heat up" has the lower specific heat capacity.
ΔE = change in thermal energy (J); m = mass (kg); c = specific heat capacity (J/kg°C); ΔT = change in temperature (°C).
Example: Water (c = 4200 J/kg°C), mass 2 kg, heated by 10°C. ΔE = 2 × 4200 × 10 = 84 000 J.
The higher the SHC, the more energy needed to raise the temperature — water has one of the highest SHCs.
This equation is on the AQA formula sheet.
To find mass: m = ΔE ÷ (c × ΔT)
To find SHC: c = ΔE ÷ (m × ΔT)
To find temperature change: ΔT = ΔE ÷ (m × c)
Water has c = 4200 J/kg°C — this is why it is used in central heating systems (stores a lot of energy).
A student heats 0.5 kg of water using a 100 W immersion heater for 5 minutes. The specific heat capacity of water is 4200 J/kg°C.
Aim: to find the specific heat capacity of a metal block by measuring energy transferred and temperature change.
Equipment: metal block, immersion heater, thermometer, joulemeter (or ammeter + voltmeter + stopwatch), digital scales.
Calculate c using: c = ΔE ÷ (m × ΔT)
Plot a graph of temperature (y-axis) against energy supplied (x-axis). The gradient = 1 ÷ (mc).
Possible errors: heat loss to surroundings (c appears too high); poor thermal contact (uneven heating).
To improve: use lagging; allow system to reach thermal equilibrium; repeat and average.
Compare calculated c with the known value (e.g. aluminium: 900 J/kg°C). Discuss percentage error.
Sources of error: heat loss to surroundings; thermal lag (thermometer not reaching equilibrium); poor contact.
Systematic errors: if the joulemeter is not zeroed; if mass measurement is incorrect.
Reliability: repeat the experiment and average values; use a data logger for precision.
Variables: independent = energy supplied; dependent = temperature change; controlled = mass, material, insulation.
A student investigates the specific heat capacity of a metal block of mass 0.8 kg. The immersion heater supplies 3600 J of energy. The temperature rises from 20°C to 25°C.
Rearranging P = E ÷ t gives: E = P × t and t = E ÷ P
A motor lifts a 50 kg rock through a height of 4 m in 8 s. g = 9.8 N/kg. Calculate the power of the motor.
A crane lifts a 150 kg load through a height of 8 m in 12 s. g = 9.8 N/kg.
A pump raises 60 kg of water through a height of 5 m in 6 s. g = 9.8 N/kg.
A crane lifts a 400 kg steel beam through a height of 15 m. g = 9.8 N/kg.
The four main stores studied so far: Gravitational (E = mgh), Kinetic (E = ½mv²), Elastic (E = ½ke²), Thermal (E = mcΔT).
Other stores: Chemical (food, fuel, batteries), Nuclear (uranium), Electrostatic (charged objects), Magnetic (magnets).
Energy is transferred when something happens. Transfers can be shown as bar models or Sankey diagrams.
In a bar model, the total height of bars stays constant (conservation) while the energy distributes between stores.
Energy is transferred between stores by:
The rate of energy transfer is power (Watts).
Example — electric motor: electrical → (electrical working) → kinetic + thermal.
Example — burning fuel: chemical → (heating) → thermal + (radiation) → light.
A Sankey diagram shows energy transfers with arrow widths proportional to energy values.
Useful energy goes forwards (horizontal); wasted energy goes downward (usually thermal).
For a car engine: 1000 J input → 250 J kinetic (forward) + 750 J thermal (down).
Efficiency can be read from a Sankey diagram: useful output ÷ total input.
A student jumps off a bridge on a bungee cord. The cord has an unstretched length of 20 m.
Thermal insulation reduces the rate of energy transfer to the surroundings.
Methods: cavity wall insulation, loft insulation (foil or foam), double glazing, draught excluders.
Best insulating materials have low thermal conductivity (e.g. foam, wool, air gaps).
High thermal conductivity = faster energy transfer. Low thermal conductivity = slower transfer.
Metal has high thermal conductivity; foam has low thermal conductivity.
Lubrication (oil / grease) reduces friction between moving surfaces, reducing thermal energy waste.
Streamlining reduces air resistance, reducing thermal energy wasted in vehicles.
Insulation around pipes and tanks reduces thermal energy loss by conduction.
In electrical devices, thicker wires reduce resistance and therefore reduce heating losses.
A copper hot water tank loses energy quickly because copper has high thermal conductivity.
Insulation (foam jacket) around the tank reduces the rate of energy transfer to the room.
An insulated tank keeps water hot for longer, reducing the need to reheat it.
The electric immersion heater inside the tank converts electrical energy to thermal energy.
A copper hot water tank with insulation is shown.
Efficiency is the fraction of input energy that is transferred to a useful output.
Efficiency has no units. It is between 0 and 1 (or expressed as a percentage 0–100%).
No machine is 100% efficient — some energy is always dissipated as thermal or sound.
Example: A lightbulb uses 470 J, emits 180 J as heat and 290 J as light. Efficiency = 290 ÷ 470 = 0.617.
To find wasted energy: wasted = input − useful output.
Increasing efficiency: lubrication, streamlining, better insulation, using waste heat (CHP).
CHP (Combined Heat and Power) stations recapture waste steam to heat homes, greatly increasing overall efficiency.
To express as percentage: multiply the decimal by 100 (e.g. 0.67 → 67%).
Wasted energy is usually thermal. Reducing thermal losses increases efficiency.
Environmental argument: more efficient appliances use less fuel → lower CO₂ emissions.
Economic argument: more efficient appliances cost less to run.
A fuel burning power station uses 2000 J of fuel energy to generate 600 J of electrical energy.
Fossil fuels (coal, oil, natural gas) formed from ancient organisms over millions of years. They are non-renewable.
Fossil fuels store chemical energy. When burned: chemical → thermal → electrical (via turbine / generator).
Advantages: reliable, high energy density, existing infrastructure.
Disadvantages: non-renewable (will run out ~50 years); produce CO₂ (greenhouse gas / climate change); coal also produces SO₂ (acid rain).
They provide a consistent, controllable supply — useful for meeting peak demand.
Nuclear fuels (uranium, plutonium) are non-renewable but will last ~80 years.
Nuclear fission: heavy nucleus absorbs a neutron and splits, releasing large amounts of energy as heat.
Advantages: no CO₂ emissions during operation; very high energy density; reliable.
Disadvantages: expensive to build; produces radioactive waste (difficult to dispose of safely); risk of contamination if accident occurs.
Scientists research nuclear fusion (joining light nuclei) which would be cleaner but is not yet commercially viable.
The UK uses a mix of energy resources. In 2018, gas and nuclear provided the majority of electricity.
The UK government plans to phase out coal-fired power stations to reduce CO₂ emissions.
Electricity demand varies throughout the day — non-renewables are used to meet peak demand because they are controllable.
Renewables are increasingly replacing non-renewables as technology improves and costs fall.
Figures show how different energy resources generated electricity in the UK on one day in June 2018, and how electricity demand varies with time of day.
Wind power: Wind turns turbine blades → generator → electricity. Zero emissions, renewable. Disadvantage: intermittent; visual impact; harms birds.
Solar power: Photovoltaic cells convert sunlight to electricity. Renewable, no emissions. Disadvantage: no output at night or in cloudy weather; requires large area.
Geothermal: Water pumped into hot rocks underground, returns as steam → drives turbine. Renewable, no emissions. Disadvantage: only available in geologically active areas.
Hydroelectric: Water in reservoir flows through turbines. Renewable, zero emissions, reliable. Disadvantage: habitat destruction; needs suitable geography.
Tidal power: Turbines in sea turn with incoming / outgoing tides. Renewable, predictable. Disadvantage: few suitable sites; affects marine life.
Wave power: Floating devices harness wave motion to drive generators. Renewable. Disadvantage: unreliable, easily damaged by storms.
Biofuels (wood, bioethanol, manure) are produced from living things and are considered renewable.
Biofuels are theoretically carbon-neutral (CO₂ absorbed growing = CO₂ released burning), but in practice still contribute to climate change.
Disadvantage: land used for biofuel crops cannot grow food — a concern where food shortages exist.
Global primary energy consumption has risen sharply since 1800, driven by industrialisation and population growth.
Renewables are a small but growing fraction; fossil fuels still dominate globally.
An electric car has a motor powered by a battery. A diesel car has an engine powered by diesel fuel.