What you'll learn
This guide covers the fundamental concept of energy in physics, focusing on how energy is stored in different ways and transferred between stores within systems. You'll learn to identify the eight energy stores, describe the four main pathways of energy transfer, and apply the principle of energy conservation to solve problems. These concepts form the foundation for understanding work, power, and efficiency in the AQA GCSE Physics specification.
Key terms and definitions
Energy store — a way in which energy can be held within a system or object, such as kinetic, gravitational potential, or thermal energy.
System — a defined object or group of objects that you're analysing; energy can be transferred into, out of, or within a system.
Energy transfer pathway — the process by which energy moves from one store to another, including mechanically, electrically, by heating, or by radiation.
Dissipation — the spreading out of energy into less useful stores, typically thermal energy in the surroundings, reducing the useful energy available.
Conservation of energy — the principle that energy cannot be created or destroyed, only transferred from one store to another; the total energy in a closed system remains constant.
Work done — energy transferred when a force moves an object through a distance; calculated as force × distance moved in the direction of the force.
Closed system — a system where no energy can enter or leave; the total energy remains constant even as it transfers between stores.
Useful energy — energy that is transferred to the store or form that we want or need for a particular purpose.
Core concepts
The eight energy stores
Energy can be stored in eight different ways. You must be able to identify and describe each:
Kinetic energy — energy in the kinetic store of a moving object. All moving objects possess kinetic energy; the faster the object moves or the greater its mass, the more kinetic energy it has.
Gravitational potential energy — energy stored when an object is raised above ground level within a gravitational field. A book on a shelf, water held behind a dam, or a lifted weight all have energy in this store.
Elastic potential energy — energy stored when an object is stretched, compressed, or deformed. Springs, elastic bands, and drawn bows store energy in this way.
Thermal energy — energy stored due to the temperature of an object. All objects above absolute zero possess thermal energy. The hotter an object, the more energy in its thermal store.
Chemical energy — energy stored in the bonds between atoms and molecules. Food, batteries, and fuels like petrol or natural gas store energy chemically.
Magnetic energy — energy stored when magnetic poles interact. Two magnets pushed together (repelling) or held apart (attracting) store energy in this way.
Electrostatic energy — energy stored when electric charges interact. Two charged objects positioned near each other store energy electrostatically.
Nuclear energy — energy stored in the nucleus of an atom. This energy is released during nuclear reactions such as fission or fusion.
Energy transfer pathways
Energy transfers between stores via four main pathways:
Mechanical transfer (by forces) — when a force does work on an object, energy is transferred mechanically. Examples include:
- Pushing a box across the floor (chemical energy in muscles → kinetic energy of box + thermal energy due to friction)
- A falling object (gravitational potential energy → kinetic energy)
- Compressing a spring (kinetic energy → elastic potential energy)
Electrical transfer — energy is transferred when charge flows through a circuit. A current carries energy from the power source to components:
- Battery powering a motor (chemical energy → kinetic energy)
- Mains electricity heating a kettle element (electrical energy → thermal energy)
Transfer by heating — energy flows from hotter objects to cooler ones due to a temperature difference. This occurs through conduction, convection, or radiation:
- A metal pan heating on a stove
- Warm air rising in a room
- Heat from the Sun reaching Earth
Transfer by radiation — energy is transferred by electromagnetic waves (light, infrared, microwaves, etc.) without requiring particles:
- Solar panels receiving energy from sunlight
- Food heating in a microwave oven
- Infrared radiation from a fire warming your hands
Describing changes in energy stores
When analysing a situation, you should:
- Define the system — decide what objects you're including in your analysis
- Identify initial energy stores — which stores contain energy at the start?
- Identify final energy stores — which stores contain energy at the end?
- Describe the transfer pathway — how did energy move between stores?
For example, when a car brakes to a stop:
- System: the car
- Initial: kinetic energy store
- Final: thermal energy store (in brakes and surroundings)
- Pathway: mechanical transfer by friction forces doing work
Conservation of energy
The principle of energy conservation states that energy cannot be created or destroyed. In any change, the total energy before equals the total energy after:
Total energy before = Total energy after
Energy may dissipate into less useful stores (often thermal energy in the surroundings), but the total amount never changes. This principle applies universally in physics.
In a closed system, the total energy remains constant. For example, a pendulum in a vacuum would swing forever, with energy constantly transferring between gravitational potential and kinetic stores. In reality, air resistance and friction at the pivot dissipate energy to thermal stores, causing the pendulum to stop.
Calculating energy changes
You need to perform calculations involving energy stores:
Kinetic energy: $$E_k = \frac{1}{2}mv^2$$
where:
- $E_k$ = kinetic energy (J)
- $m$ = mass (kg)
- $v$ = speed (m/s)
Gravitational potential energy: $$E_p = mgh$$
where:
- $E_p$ = gravitational potential energy (J)
- $m$ = mass (kg)
- $g$ = gravitational field strength (N/kg) — use 9.8 N/kg or 10 N/kg on Earth
- $h$ = height (m)
Elastic potential energy (for springs): $$E_e = \frac{1}{2}ke^2$$
where:
- $E_e$ = elastic potential energy (J)
- $k$ = spring constant (N/m)
- $e$ = extension (m)
This equation is required for Higher Tier only.
Work done and energy transfer
When a force moves an object, work is done and energy is transferred:
$$W = Fs$$
where:
- $W$ = work done (J)
- $F$ = force (N)
- $s$ = distance moved in the direction of the force (m)
Work done equals the energy transferred. If you lift a 2 kg book through 1.5 m, the work done against gravity equals the increase in gravitational potential energy:
$W = Fs = (2 × 10) × 1.5 = 30$ J
This 30 J of chemical energy from your muscles transfers to the gravitational potential energy store of the book.
Worked examples
Example 1: Energy stores in a rollercoaster
Question: A rollercoaster car of mass 500 kg is at the top of a 25 m high slope. Calculate the energy in its gravitational potential energy store. Assume $g = 10$ N/kg. [3 marks]
Solution:
Step 1: Write the equation $$E_p = mgh$$
Step 2: Substitute values $$E_p = 500 × 10 × 25$$
Step 3: Calculate and give the answer with units $$E_p = 125,000\text{ J or }125\text{ kJ}$$
Mark scheme: 1 mark for correct equation, 1 mark for correct substitution, 1 mark for correct answer with unit.
Example 2: Energy transfers when a ball is thrown
Question: Describe the energy transfers that occur when a student throws a ball upwards from ground level until it reaches its maximum height. [4 marks]
Solution:
- Initially, the ball is stationary, so energy is stored chemically in the student's muscles [1 mark]
- As the student throws the ball, chemical energy transfers to the kinetic energy store of the ball (mechanical transfer/work done) [1 mark]
- As the ball rises, energy transfers from the kinetic store to the gravitational potential store [1 mark]
- At maximum height, the ball is momentarily stationary, with maximum gravitational potential energy and zero kinetic energy [1 mark]
Note: Some energy also dissipates to thermal stores due to air resistance throughout the motion.
Example 3: Conservation of energy calculation
Question: A 0.5 kg stone falls from a cliff 20 m high. Assuming no energy is dissipated, calculate: (a) The gravitational potential energy at the top [2 marks] (b) The kinetic energy just before it hits the ground [1 mark] (c) The speed just before impact [3 marks]
Use $g = 10$ N/kg.
Solution:
(a) $E_p = mgh = 0.5 × 10 × 20 = 100$ J [2 marks]
(b) By conservation of energy, all gravitational potential energy converts to kinetic energy = 100 J [1 mark]
(c) $E_k = \frac{1}{2}mv^2$
Rearranging: $v^2 = \frac{2E_k}{m} = \frac{2 × 100}{0.5} = 400$
$v = \sqrt{400} = 20$ m/s [3 marks: 1 for rearrangement, 1 for substitution, 1 for answer]
Common mistakes and how to avoid them
Confusing energy stores with energy transfers — "friction" is not an energy store; it's a force that causes mechanical energy transfer. Thermal energy is the store that increases due to friction.
Forgetting to square the velocity in kinetic energy calculations — in $E_k = \frac{1}{2}mv^2$, you must square the speed. Doubling the speed increases kinetic energy by a factor of four, not two.
Using incorrect units — mass must be in kilograms, distance in metres, and force in newtons. Always convert before calculating (e.g., 50 g = 0.05 kg, 2 cm = 0.02 m).
Saying energy is "lost" or "used up" — energy is never lost; it's dissipated or transferred to less useful stores. Use precise terminology: "energy is dissipated to thermal stores in the surroundings."
Forgetting that height is measured from a reference point — gravitational potential energy is always relative to a chosen zero level. Be clear about where you're measuring height from.
Not recognising that work done = energy transferred — these are equivalent concepts measured in the same unit (joules). If 500 J of work is done, 500 J of energy has been transferred.
Exam technique for "Energy stores and systems"
Command words matter — "Describe" requires you to state what happens; "Explain" requires you to say why it happens using physics principles. For energy questions, explanations often require mentioning specific energy stores and transfer pathways.
Show your working in calculations — even if your final answer is incorrect, you can earn method marks by writing the equation, substituting values correctly, and showing clear steps. Always include units with your final answer.
Be specific about energy stores — don't write "the ball has energy"; write "the ball has energy in its kinetic store" or "energy in the gravitational potential energy store of the ball increases."
Practice rearranging equations — many marks are lost because students can't rearrange $E_k = \frac{1}{2}mv^2$ to find speed or mass. Practice these algebraic skills regularly.
Quick revision summary
Energy is stored in eight ways: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic, and nuclear. Energy transfers between stores via four pathways: mechanically (by forces doing work), electrically, by heating, or by radiation. The principle of conservation of energy states that energy cannot be created or destroyed—the total energy in a closed system remains constant. Work done equals energy transferred and is calculated as force × distance. Master the equations for kinetic energy and gravitational potential energy, ensuring you use correct units throughout calculations.