What you'll learn
This revision guide covers the principle of conservation of energy and how energy is transferred between different stores. You'll learn to identify energy stores, describe transfer pathways, calculate efficiency, and interpret Sankey diagrams. These concepts form the foundation of energy topics in AQA GCSE Physics and appear frequently in exam questions worth 4-6 marks.
Key terms and definitions
Energy store — a form in which a system can possess energy, such as kinetic, gravitational potential, or thermal
Energy transfer pathway — the mechanism by which energy moves from one store to another (mechanical working, electrical working, heating, or radiation)
Conservation of energy — the principle that energy cannot be created or destroyed, only transferred from one store to another
Dissipation — the spreading out of energy into less useful forms, typically thermal energy transferred to the surroundings
Efficiency — the proportion of input energy that is usefully transferred, expressed as a decimal or percentage
Sankey diagram — a visual representation showing energy transfers, where arrow width represents the quantity of energy
Work done — energy transferred when a force moves an object through a distance; calculated as force × distance
Closed system — a system in which no energy transfers take place to or from the surroundings
Core concepts
Energy stores
Energy can be stored in various forms within a system. The AQA specification requires you to know these eight energy stores:
Kinetic energy stores hold energy in moving objects. The amount depends on mass and velocity. A car travelling at 30 m/s has more kinetic energy than the same car at 15 m/s.
Gravitational potential energy stores hold energy in objects raised above ground level. The amount depends on mass, height, and gravitational field strength. Water at the top of a waterfall has gravitational potential energy.
Elastic potential energy stores hold energy in stretched or compressed objects. A drawn bow, compressed spring, or stretched elastic band all store energy elastically.
Chemical energy stores hold energy in substances that can release it through chemical reactions. Food, batteries, and fuels (petrol, wood, natural gas) all contain chemical energy.
Thermal energy stores hold energy in hot objects. The hotter an object and the greater its mass, the more thermal energy it stores. This is sometimes called internal energy.
Magnetic energy stores hold energy in magnets that attract or repel. Two repelling magnets or a magnetic door catch store magnetic energy.
Electrostatic energy stores hold energy in charged objects that attract or repel. Thunderclouds before lightning or charged balloons stuck to walls store electrostatic energy.
Nuclear energy stores hold energy in atomic nuclei. This energy is released during nuclear fission in power stations or fusion in stars.
Energy transfer pathways
Energy moves between stores through four main pathways:
Mechanical working occurs when a force moves an object. When you lift a box, mechanical working transfers energy from your chemical stores to the box's gravitational potential store. When friction acts, mechanical working transfers energy to thermal stores.
Electrical working occurs when charge flows through a potential difference. Current in a kettle element transfers energy electrically from the mains supply to the thermal store of the water.
Heating occurs when energy flows from a hotter region to a cooler one due to temperature difference. This happens through conduction, convection, or radiation. A hot radiator heats a room through heating.
Radiation occurs when energy travels as electromagnetic waves or sound. The Sun transfers energy to Earth through radiation (light and infrared). Radio transmitters transfer energy through radio waves.
The principle of conservation of energy
The law of conservation of energy states that energy cannot be created or destroyed. In any process, the total energy before equals the total energy after. Energy can only be:
- Transferred from one store to another
- Dissipated (spread out to the surroundings)
- Both transferred and dissipated
In a closed system, the total energy remains constant because no energy enters or leaves. However, truly closed systems rarely exist in practice. Even well-insulated systems lose some energy to surroundings.
When a ball falls, chemical energy in your muscles transfers to gravitational potential energy in the ball. As it falls, this converts to kinetic energy. When it hits the ground, energy transfers to thermal and sound energy stores. The total energy remains constant throughout.
Energy dissipation and useful energy
Not all energy transfers are useful. Useful energy performs the intended task, while wasted energy does not contribute to the desired outcome.
In a light bulb, useful energy transfers to light radiation, but wasted energy dissipates as heat. This thermal energy spreads to the surroundings, becoming less concentrated and harder to use further. Eventually, all energy dissipates this way.
Reducing wasted energy improves efficiency. Methods include:
- Lubrication reduces friction, decreasing energy wasted as heat
- Thermal insulation reduces unwanted heating transfers
- Streamlining reduces air resistance
- Using more efficient devices or materials
Efficiency calculations
Efficiency measures how much input energy transfers usefully. It is calculated using:
Efficiency = useful energy transferred ÷ total energy supplied
Or for power:
Efficiency = useful power output ÷ total power input
Efficiency has no units when expressed as a decimal (0 to 1) but is often given as a percentage (0% to 100%).
To convert decimal to percentage: multiply by 100 To convert percentage to decimal: divide by 100
Example: A motor supplied with 500 J transfers 350 J usefully.
Efficiency = 350 ÷ 500 = 0.70 or 70%
No device is 100% efficient because some energy always dissipates. The most efficient devices waste the least energy.
You may also need to rearrange the equation:
- Useful energy = efficiency × total energy supplied
- Total energy supplied = useful energy ÷ efficiency
Sankey diagrams
Sankey diagrams visually represent energy transfers. The width of each arrow is proportional to the amount of energy it represents.
Features of Sankey diagrams:
- The input arrow shows total energy supplied
- Output arrows split to show useful and wasted energy
- Useful energy typically continues straight
- Wasted energy arrows branch off (usually downward)
- Arrow widths are drawn to scale
- Values in joules (J) or percentages label each arrow
To draw a Sankey diagram:
- Calculate wasted energy (total input – useful output)
- Draw the input arrow with appropriate width
- Split into useful and wasted outputs proportional to their values
- Label all arrows with energy values and store types
To interpret a Sankey diagram:
- Identify the input energy (left side)
- Identify useful and wasted outputs (right side)
- Check that input = useful output + wasted output
- Calculate efficiency if required
Energy calculations
Several equations relate to energy stores that you must know:
Kinetic energy (in joules, J): E_k = ½ × m × v²
Where:
- m = mass in kilograms (kg)
- v = velocity in metres per second (m/s)
Gravitational potential energy (in joules, J): E_p = m × g × h
Where:
- m = mass in kilograms (kg)
- g = gravitational field strength (9.8 N/kg on Earth, often approximated as 10 N/kg)
- h = height in metres (m)
Work done (in joules, J): W = F × s
Where:
- F = force in newtons (N)
- s = distance moved in the direction of the force in metres (m)
Power (in watts, W): P = E ÷ t or P = W ÷ t
Where:
- E or W = energy transferred or work done in joules (J)
- t = time in seconds (s)
Worked examples
Example 1: Energy store changes
Question: A 0.5 kg ball is thrown upward with a velocity of 10 m/s. Calculate: (a) the kinetic energy of the ball as it leaves the hand [2 marks] (b) the maximum height the ball reaches, assuming no energy is wasted [3 marks] (Use g = 10 N/kg)
Solution:
(a) E_k = ½ × m × v² E_k = ½ × 0.5 × 10² ✓ E_k = 0.25 × 100 = 25 J ✓
(b) At maximum height, all kinetic energy converts to gravitational potential energy ✓ E_p = 25 J (by conservation of energy) E_p = m × g × h 25 = 0.5 × 10 × h ✓ h = 25 ÷ 5 = 5 m ✓
Example 2: Efficiency calculation
Question: An electric motor is supplied with 2400 J of energy. It lifts a 20 kg mass through a vertical height of 8 m. (a) Calculate the useful energy transferred to the mass [2 marks] (b) Calculate the efficiency of the motor [2 marks] (Use g = 10 N/kg)
Solution:
(a) E_p = m × g × h ✓ E_p = 20 × 10 × 8 = 1600 J ✓
(b) Efficiency = useful energy ÷ total energy supplied ✓ Efficiency = 1600 ÷ 2400 = 0.67 or 67% ✓
Example 3: Sankey diagram interpretation
Question: A petrol engine in a car receives 10,000 J of chemical energy from fuel. The Sankey diagram shows 3000 J transfers usefully to kinetic energy, 6000 J wastes as thermal energy to surroundings, and 1000 J wastes as sound energy.
(a) State what the diagram shows about energy conservation [1 mark] (b) Calculate the efficiency of the engine [2 marks] (c) Suggest one way to improve the efficiency [1 mark]
Solution:
(a) The total energy input (10,000 J) equals the sum of all outputs (3000 + 6000 + 1000 = 10,000 J), showing energy is conserved ✓
(b) Efficiency = 3000 ÷ 10,000 ✓ Efficiency = 0.30 or 30% ✓
(c) Improve lubrication to reduce friction / Better cooling system / Reduce air resistance (any sensible answer) ✓
Common mistakes and how to avoid them
Confusing energy stores with transfer pathways. Remember: stores are forms energy takes (kinetic, thermal); pathways are how energy moves (heating, mechanical working). Don't say "kinetic energy is transferred" — say "energy is transferred to kinetic stores by mechanical working."
Forgetting to square velocity in kinetic energy calculations. E_k = ½mv² not ½mv. Doubling velocity quadruples kinetic energy. Always square the velocity value before multiplying.
Stating energy is lost or destroyed. Energy is never lost — it's dissipated or wasted. Use precise language: "energy is transferred to less useful thermal stores" not "energy is lost as heat."
Incorrect units in calculations. Always use standard SI units: mass in kg (not g), distance in m (not cm), time in s (not minutes). Convert before calculating.
Not showing working in calculations. Even if you use a calculator, write out the formula, substitute values, then give the answer with units. This earns method marks even if your final answer is wrong.
Drawing Sankey diagrams not to scale. Arrow widths must be proportional to energy amounts. An output of 30 J should have an arrow three times wider than one showing 10 J.
Exam technique for "Conservation of energy and energy transfers"
Command word awareness: "Describe" requires you to state features without explanation (2-3 marks). "Explain" requires reasons using scientific principles (2-4 marks). "Calculate" always requires working shown with units.
Multi-step calculations: Break complex questions into stages. Find what you can calculate first, then use that answer in the next step. Most 4-6 mark questions require two or three separate calculations.
Sankey diagram questions: Check the energy values add up correctly before submitting your answer. Input must equal total output. If values don't match, recheck your arithmetic.
Extended response questions: Structure answers using the point-evidence-explain format. State the energy transfer, identify the stores involved, then explain using conservation principles. Aim for 6-8 well-developed points for a 6-mark question.
Quick revision summary
Energy exists in eight stores (kinetic, gravitational potential, elastic potential, chemical, thermal, magnetic, electrostatic, nuclear) and transfers through four pathways (mechanical working, electrical working, heating, radiation). The principle of conservation states energy cannot be created or destroyed, only transferred or dissipated. Efficiency equals useful energy output divided by total energy input. Sankey diagrams visually represent energy transfers with proportional arrow widths. Key equations include E_k = ½mv², E_p = mgh, W = Fs, and efficiency = useful energy ÷ total energy supplied.