Your revision route
What you’ll learn
- Describe a system using named energy stores and transfer pathways.
- Calculate changes in kinetic, elastic, gravitational and thermal energy stores.
- Explain conservation, dissipation and ways to reduce unwanted transfers.
- Use power and efficiency to compare devices and processes.
- Plan and evaluate investigations involving specific heat capacity and thermal insulation.
- Name the main energy resources, connect them to transport, heating and electricity generation, and explain usage trends using reliability, environmental impact and cost.
Build the big picture
Key ideas
Energy changes address
Treat every energy question like detective work: define the system, then follow the transfer.
- Name the store that decreases and the one that increases: kinetic, thermal, chemical, nuclear, magnetic, electrostatic, elastic potential or gravitational potential.
- Name the pathway separately: mechanically by a force, electrically by a current, by heating across a temperature difference, or by radiation.
- A precise answer gives the complete journey—store → pathway → store.
The punchline: Energy is stored in stores; pathways describe how it is transferred.
Conserved can still mean wasted
A squealing brake has not destroyed energy; it has scattered it into warmer, less useful surroundings.
- In a closed system, the total energy stays constant even when it moves between stores.
- Friction and drag often transfer energy into thermal stores, where it becomes dissipated. Lubrication, streamlining and insulation reduce unwanted transfers.
- With area, thickness and temperature difference fixed, higher thermal conductivity raises the conduction rate. Thicker or lower-conductivity walls slow cooling.
The punchline: Track the total energy first, then judge how much remains useful.
Energy accounting has three columns
Define the system, name the store that shrinks, then state the pathway and the store that grows. Energy itself never vanishes from the ledger.
- Initial storeLocate the energy before the change, such as a chemical or kinetic store.
- Transfer pathwayName mechanical work, electrical work, heating or radiation.
- Final storeLocate the energy afterwards, including thermal stores of the surroundings.
The equation reveals what matters
A formula is a compressed story about the system. Read it before reaching for the calculator.
- Kinetic energy is unusually sensitive to speed because velocity is squared.
- Gravitational energy uses vertical height change; elastic energy uses extension, not the spring's full length.
- Heating calculations use mass, specific heat capacity and temperature change—not the final temperature by itself.
The punchline: Choose the equation from the physical story, then substitute values with units.
Fast is not the same as frugal
A powerful machine and an efficient machine may be answering completely different questions.
- Power measures how quickly energy is transferred or work is done.
- Efficiency compares useful output with total input, so a powerful device can still waste a large fraction of its input.
- Check whether the required efficiency is a decimal or a percentage.
- HT only: increase the efficiency of an intended transfer by reducing unwanted transfers, for example with lubrication or insulation.
The punchline: Power asks ‘how fast?’; efficiency asks ‘how much was useful?’
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Follow the joules
Build the energy story of a braking car
Arrange the stages from the moving car to the final, less-useful spread of energy.
Choose the first step below.
Braking transfers energy mechanically from the car's kinetic store into thermal stores; later heating spreads energy into the surroundings.
Turn the dials
Explore energy, power and time
Change both sliders. Watch how doubling power or time doubles the energy transferred.
E = P × t
Energy transferred6,000 J
Power is the rate of energy transfer. A device transferring more joules each second, or running for longer, transfers more energy.
Every resource has a job interview
Do not just memorise a list. Ask what each resource can do, where it works and what it demands in return.
- Non-renewables are coal, oil, natural gas and nuclear fuel. Renewables include biofuel, wind, hydroelectricity, geothermal, tides, sunlight and waves.
- Transport and heating can use resources directly or electricity generated from them; electricity is an energy carrier, not a primary energy resource.
- Electricity generation can use fossil fuels, nuclear fuel or renewable resources, but suitability depends on the situation.
The punchline: Name the resource, connect it to the use, then justify why it fits.
No power station is consequence-free
Energy choices are trade-offs disguised as headlines. Compare them with evidence, not slogans.
- Fossil fuels can respond to demand but release carbon dioxide and pollutants; nuclear has low direct operational carbon emissions but creates radioactive waste.
- Wind and sunlight vary with conditions, tides are predictable, and hydroelectric or geothermal schemes need suitable locations.
- Dams and tides can alter habitats; wind and solar use land; biofuel can compete for land. The proposed site matters.
The punchline: A strong comparison weighs reliability, environmental effects and location together.
Every energy resource arrives with luggage
No single resource wins every category. Compare reliability, start-up time, environmental effects, location and cost for the stated use.
- Wind and solarRenewable with no fuel burning in operation, but output varies with conditions.
- Fossil fuelsReliable and dispatchable, but finite and release carbon dioxide when burned.
- Nuclear fuelReliable low-carbon operation, with radioactive waste and high construction costs.
- Hydroelectric and tidalRenewable and predictable to different degrees, but restricted by geography and habitat impacts.
An energy graph is history with axes
A trend is a clue, not an explanation. First describe what changed; only then explain why.
- State the direction, time period and size of the change, and distinguish an absolute amount from a percentage share.
- Demand, reserves, price, technology and infrastructure can all alter resource use.
- Environmental evidence, reliability, politics, economics, ethics and public choices affect which projects proceed. Use the data given, not a memorised current figure.
The punchline: Describe the pattern precisely before offering a cause.
Words worth knowing
Key definitions
- system
- An object or group of objects chosen for an energy analysis.
- energy store
- A way of describing where energy is held within a system.
- transfer pathway
- The route by which energy moves between stores or across a system boundary.
- closed system
- A system across whose boundary no energy is transferred.
- dissipated energy
- Energy that has spread into less useful stores, usually thermal stores in the surroundings.
- specific heat capacity
- The energy needed to raise the temperature of one kilogram of a material by one degree Celsius.
- power
- The rate at which energy is transferred or work is done.
- efficiency
- The useful fraction of the total energy or power supplied.
- renewable resource
- An energy resource that can be replenished while it is being used.
- thermal conductivity
- A material property that describes conduction rate: for the same temperature difference and geometry, a higher thermal conductivity gives a higher rate of energy transfer.
Calculate with confidence
Equations
Kinetic energy
E_k = 1/2 × m × v²
Energy in the kinetic store of a moving object.
| Symbol | Meaning | Unit |
|---|---|---|
| E_k | kinetic energy | J |
| m | mass | kg |
| v | speed | m/s |
Exam tip: Square the speed before multiplying, and convert the mass to kilograms.
Gravitational potential energy
E_p = m × g × h
The change in the gravitational potential store when an object changes height.
| Symbol | Meaning | Unit |
|---|---|---|
| E_p | gravitational potential energy | J |
| m | mass | kg |
| g | gravitational field strength | N/kg |
| h | vertical height change | m |
Exam tip: Use the value of g provided and use vertical height, not distance travelled along a slope.
Elastic potential energy
E_e = 1/2 × k × e²
Energy held by an elastically deformed spring, provided its limit of proportionality has not been exceeded.
| Symbol | Meaning | Unit |
|---|---|---|
| E_e | elastic potential energy | J |
| k | spring constant | N/m |
| e | extension | m |
Exam tip: Extension is the change in length and must be in metres.
Change in thermal energy
ΔE = m × c × Δθ
The energy transferred when the temperature of a mass changes without a change of state.
| Symbol | Meaning | Unit |
|---|---|---|
| ΔE | change in thermal energy | J |
| m | mass | kg |
| c | specific heat capacity | J/(kg °C) |
| Δθ | temperature change | °C |
Exam tip: Subtract the initial temperature from the final temperature and keep the mass in kilograms.
Power
P = E / t or P = W / t
Energy transferred or work done per second.
| Symbol | Meaning | Unit |
|---|---|---|
| P | power | W |
| E or W | energy transferred or work done | J |
| t | time | s |
Exam tip: One watt means one joule transferred each second.
Efficiency
efficiency = useful energy output / total energy input or useful power output / total power input
The useful proportion of an energy transfer, calculated using energy with energy or power with power.
| Symbol | Meaning | Unit |
|---|---|---|
| useful output | useful energy transferred or useful power delivered | matching energy units such as J or kJ, or W when using power |
| total input | total energy supplied or total input power | the same quantity and unit as the useful output (for example kJ with kJ or W with W) |
Exam tip: Do not divide energy by power. Use matching quantities, then multiply the decimal by 100 only when a percentage is requested.
Follow it step by step
Processes to remember
How to describe an energy transfer
- Name the object or objects in the system.
- State which energy store decreases.
- Name the mechanical, electrical, heating or radiation pathway.
- State which store increases and identify any energy dissipated to the surroundings.
Exam tip: Avoid saying that energy disappears; identify its destination instead.
How to compare energy resources
- Name the resource and identify the intended use: transport, heating or electricity generation.
- Compare availability, controllability and reliability, including whether output varies or depends on location.
- Compare environmental effects such as greenhouse-gas or pollutant emissions, radioactive waste, land use and habitat change.
- Consider construction, fuel and running costs, then use any graph to describe how usage changes over time before reaching a supported conclusion.
Exam tip: A conclusion scores best when it follows from the factors and trend data in the question.
See the thinking
Worked example
Worked example: kinetic energy and efficiency
An 800 kg electric car accelerates from rest to 15 m/s. Calculate the increase in its kinetic energy store. During the acceleration, 120 kJ is supplied by the battery. Calculate the efficiency of this transfer as a percentage.
- Write the kinetic-energy equation: E_k = 1/2 × m × v².
- Substitute the values: E_k = 1/2 × 800 × 15².
- Calculate the store increase: E_k = 90,000 J = 90 kJ.
- Use matching units for efficiency: efficiency = 90 kJ / 120 kJ.
- Convert 0.75 to a percentage: 0.75 × 100 = 75%.
Answer: The kinetic energy store increases by 90 kJ and the transfer is 75% efficient.
The speed must be squared. Converting the kinetic energy to kilojoules before the efficiency calculation keeps both energy values in the same unit; the remaining 30 kJ is not transferred usefully and moves into less useful stores.
Protect the marks
Common mistakes
Watch out: Writing that energy is lost or used up.
Do this instead: State where it is transferred, including any energy dissipated into thermal stores.
Watch out: Forgetting the square on speed or extension.
Do this instead: Calculate v² or e² before completing the multiplication.
Watch out: Using a final temperature instead of a temperature change.
Do this instead: Calculate Δθ from final temperature minus initial temperature.
Watch out: Using grams, centimetres or kilojoules without converting.
Do this instead: Convert to the SI units expected by the equation before substituting.
Watch out: Treating power and energy as the same quantity.
Do this instead: Energy is measured in joules; power is the transfer rate in watts.
Watch out: Calling every renewable resource reliable or harmless.
Do this instead: Compare availability, environmental effects and costs for the particular use.
Plan it like the exam
Required practicals
Determine the specific heat capacity of a material
Combined Science and separate Physics
Aim: Link a measured electrical energy input to the temperature rise of a known mass and calculate the material's specific heat capacity.
Method
- Measure the block's mass, then place a suitable low-voltage laboratory heater and thermometer in their correct holes under teacher supervision.
- Insulate the block, place it on a heatproof mat and record its initial temperature.
- Measure the electrical energy supplied to the heater with a joulemeter or from current, potential difference and heating time.
- Record temperature at regular intervals while the heater operates.
- Find Δθ and calculate c = E / (m × Δθ).
- Repeat readings and compare the result with an accepted value only after considering heat loss.
Variables
- Independent
- electrical energy supplied to the heater, usually varied by changing heating time
- Dependent
- temperature change of the block
- Controls
- mass and material of the block
- heater power
- insulation
- starting conditions
Analysis: Plot electrical energy supplied against temperature change and use the straight-line region. Ideally, gradient = m × c, so c = gradient / m. Heat loss usually makes the estimate too large.
Safety
- Use a heatproof mat and allow the heater and block to cool before moving them.
- Switch the power off before adjusting the apparatus.
- Keep electrical connections dry and secure.
- Use only the teacher-approved low-voltage laboratory supply and follow the school's risk assessment.
Improvements
- Add effective insulation and improve thermal contact around the thermometer and heater.
- Measure current and potential difference throughout rather than assuming they remain constant.
- Repeat the run and use a best-fit line or mean to reduce random variation.
Investigate thermal insulation
Separate Physics only
Aim: Compare how material type or thickness affects the cooling of a container of hot water.
Method
- Place equal volumes of hot water at the same starting temperature into identical lidded containers.
- Wrap each container with the same thickness of a different material, or vary only the thickness of one material.
- Measure the temperature at equal time intervals for the same total time.
- Compare the temperature falls; the smallest fall indicates the most effective insulation under those conditions.
- Repeat each condition and calculate a mean.
Variables
- Independent
- insulating material or its thickness
- Dependent
- temperature decrease over a fixed time
- Controls
- water volume
- starting temperature
- container and lid
- room conditions
- timing intervals
Analysis: Plot temperature against time or compare the mean fall over the chosen interval. A smaller cooling rate indicates a lower rate of unwanted energy transfer.
Safety
- Use hot rather than boiling water and pour it carefully.
- Keep containers stable and use eye protection where required.
- Handle thermometers through the lid without forcing them.
Improvements
- Use lids and identical containers to reduce uncontrolled differences.
- Use a temperature sensor and data logger for consistent intervals.
- Test several thicknesses before drawing a conclusion about thickness.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
A bicycle slows when its brakes are applied. Which store decreases, and where does most of that energy go?
Answer: The bicycle and rider's kinetic store decreases; energy is transferred mechanically into thermal stores of the brakes, wheels and surroundings.
Friction does not destroy energy. It transfers it and spreads it into less useful stores.
Calculate the kinetic energy of a 2 kg object moving at 3 m/s.
Answer: 9 J
E_k = 1/2 × 2 × 3² = 9 J.
A 0.50 kg block has a specific heat capacity of 900 J/(kg °C). How much energy raises its temperature by 4 °C?
Answer: 1,800 J
ΔE = m × c × Δθ = 0.50 × 900 × 4.
A device receives 800 J and transfers 600 J usefully. What is its efficiency as a percentage?
Answer: 75%
600 / 800 = 0.75, then multiply by 100.
Why might electricity from wind or sunlight still need storage or a backup source?
Answer: Wind speed and sunlight vary, so output may not match demand at every moment.
Wind and sunlight are renewable, but renewable describes replenishment rather than constant availability.
Good questions, clear answers
Frequently asked questions
What is the difference between an energy store and a transfer pathway?
A store describes where energy is held, such as a kinetic or chemical store. A pathway describes how it moves, such as mechanically, electrically, by heating or by radiation.
Is wasted energy destroyed?
No. It has usually been dissipated into thermal stores in the surroundings, making it less useful for the intended task.
How are energy and power different?
Energy is an amount measured in joules. Power is the rate of transfer, measured in watts, where one watt is one joule per second.
Can efficiency be greater than 100%?
No. The useful output cannot exceed the total input. A result above 100% means the data, units or calculation need checking.
Does renewable mean environmentally harmless?
No. Renewable resources are replenished, but their construction, land use, reliability and local environmental effects still need to be assessed.
See the whole topic
Illustrated notes
Use the guide above for searchable explanations, then revise from the visual note sheets below.
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