P1: Energy Stores and Transfers
Learn about the eight energy stores, how energy is transferred between stores, dissipation, Sankey diagrams and the principle of closed systems.
The Eight Energy Stores
Energy can be stored in different ways. There are eight energy stores you need to know for GCSE Physics:
1. Kinetic Energy Store
Any moving object has energy in its kinetic energy store. The faster an object moves and the greater its mass, the more energy it has in this store.
2. Gravitational Potential Energy Store
An object raised above the ground has energy in its gravitational potential energy store. The higher the object is lifted and the greater its mass, the more energy it stores.
3. Elastic Potential Energy Store
A stretched or compressed object (like a spring or rubber band) has energy in its elastic potential energy store. The more it is stretched or compressed, the more energy it stores.
4. Thermal Energy Store
All objects have energy in their thermal energy store. The hotter the object, the more energy it has in this store. This is the energy due to the random motion of particles.
5. Chemical Energy Store
Energy stored in chemical bonds. This is released during chemical reactions. Fuels, food, and batteries all store energy chemically.
6. Nuclear Energy Store
Energy stored in the nucleus of atoms. This is released during nuclear fission or fusion. Uranium and the Sun both release energy from the nuclear store.
7. Electrostatic Energy Store
Energy stored between two charged objects that are attracting or repelling each other due to their charges.
8. Magnetic Energy Store
Energy stored between two magnetic poles that are attracting or repelling each other. Like electrostatic but for magnets.
Energy Transfers
Energy is transferred from one store to another through four main pathways:
Mechanically
When a force does work on an object. For example, pushing a box along the floor transfers energy mechanically from the kinetic store of your hand to the kinetic and thermal stores of the box.
Electrically
When an electric current flows. For example, a kettle transfers energy electrically from the chemical store of the power station to the thermal store of the water.
By Heating
When energy is transferred from a hotter object to a colder object. For example, a hot coffee cup transfers energy by heating to the surrounding air.
By Radiation
When energy is transferred by electromagnetic waves (including light and infrared). For example, the Sun transfers energy by radiation to the Earth.
Worked Example: Identifying Energy Stores and Transfers
A ball is dropped from a height. As it falls, energy is transferred from the gravitational potential energy store to the kinetic energy store. When it hits the ground, some energy is transferred to the thermal energy store of the ball and the ground, and some energy is transferred by radiation (sound waves) to the surroundings.
Kinetic Energy
The energy in the kinetic energy store of a moving object depends on its mass and speed.
Kinetic Energy Equation
Ek = ½mv2
Ek = kinetic energy (J)
m = mass (kg)
v = speed (m/s)
Worked Example 1
A car of mass 1200 kg is travelling at 15 m/s. Calculate its kinetic energy.
Ek = ½mv2
Ek = ½ × 1200 × 152
Ek = ½ × 1200 × 225
Ek = 600 × 225
Ek = 135 000 J
Ek = 135 kJ
Worked Example 2
A cyclist and bicycle have a combined mass of 80 kg. The cyclist is travelling at 8 m/s. Calculate the kinetic energy.
Ek = ½mv2
Ek = ½ × 80 × 82
Ek = 40 × 64
Ek = 2560 J
Ek = 2.56 kJ
Remember that speed is squared in the kinetic energy equation. If the speed doubles, the kinetic energy increases by a factor of 4 (not 2). If the speed triples, kinetic energy increases by a factor of 9.
Gravitational Potential Energy
The energy in the gravitational potential energy store of an object depends on its mass, the height it is raised, and the gravitational field strength.
Gravitational Potential Energy Equation
Ep = mgh
Ep = gravitational potential energy (J)
m = mass (kg)
g = gravitational field strength (N/kg)
h = height (m)
Worked Example 3
A student of mass 55 kg climbs a flight of stairs of height 3.2 m. Calculate the gain in gravitational potential energy. (g = 9.8 N/kg)
Ep = mgh
Ep = 55 × 9.8 × 3.2
Ep = 1724.8 J
Ep = 1700 J (2 s.f.)
Worked Example 4
A book of mass 0.4 kg is on a shelf 1.8 m above the floor. Calculate the gravitational potential energy stored. (g = 9.8 N/kg)
Ep = mgh
Ep = 0.4 × 9.8 × 1.8
Ep = 7.056 J
Ep = 7.1 J (2 s.f.)
On Earth, g = 9.8 N/kg. On the Moon, g = 1.6 N/kg. The gravitational potential energy of an object on the Moon would be much less than on Earth because g is smaller.
Elastic Potential Energy
When you stretch or compress a spring, energy is stored in its elastic potential energy store. The amount of energy depends on the spring constant and the extension.
Elastic Potential Energy Equation
Ee = ½ke2
Ee = elastic potential energy (J)
k = spring constant (N/m)
e = extension (m)
Worked Example 5
A spring has a spring constant of 200 N/m. It is compressed by 0.05 m. Calculate the elastic potential energy stored.
Ee = ½ke2
Ee = ½ × 200 × 0.052
Ee = 100 × 0.0025
Ee = 0.25 J
Worked Example 6
A spring with spring constant 500 N/m is stretched by 8 cm. Calculate the elastic potential energy stored.
First convert: e = 8 cm = 0.08 m
Ee = ½ke2
Ee = ½ × 500 × 0.082
Ee = 250 × 0.0064
Ee = 1.6 J
Always check the units of extension. The equation requires extension in metres, but exam questions often give it in centimetres. Convert cm to m by dividing by 100.
Dissipation of Energy
When energy is transferred between stores, not all of the energy is transferred usefully. Some energy is always dissipated (spread out) to the surroundings, usually as thermal energy.
What is Dissipation?
Dissipation is the transfer of energy to less useful stores. For example, when a car brakes, the kinetic energy of the car is transferred to the thermal energy store of the brakes and the surroundings. This thermal energy is dissipated and cannot easily be used again.
Causes of Dissipation
Friction between moving objects causes energy to be dissipated as thermal energy. Air resistance causes energy to be dissipated as thermal energy. Sound energy from vibrations is also a form of dissipation.
Worked Example: Energy Dissipation
A mechanical clock uses a wound spring. As the spring unwinds, energy is transferred from the elastic potential energy store to the kinetic energy store of the gears and hands. However, friction between the moving parts causes some energy to be dissipated to the thermal energy store of the gears and the surrounding air. This means the clock eventually stops unless the spring is rewound.
Sankey Diagrams
Sankey diagrams are used to show the energy transfers in a system. The width of each arrow represents the amount of energy in each store.
Features of Sankey Diagrams
The main arrow represents the total energy input. It splits into arrows showing useful and wasted energy outputs. The thickness of each arrow is proportional to the amount of energy it represents. Useful energy output goes straight ahead. Wasted energy turns downwards. The total energy output always equals the total energy input.
Worked Example: Drawing a Sankey Diagram
An electric motor uses 500 J of electrical energy. It transfers 350 J to kinetic energy and 150 J is wasted as thermal energy and sound.
The input arrow is 500 J wide. The useful output (kinetic) arrow is 350 J wide, which is 350/500 = 0.7 of the input width. The wasted output arrow is 150 J wide, which is 150/500 = 0.3 of the input width.
In a Sankey diagram, the total width of the output arrows must always equal the width of the input arrow. Energy is conserved, so energy in = energy out.
Closed Systems
A closed system is a system where neither matter nor energy can enter or leave. In reality, no system is perfectly closed, but we can model systems as closed for calculations.
Energy in Closed Systems
In a closed system, the total energy never changes. Energy can be transferred between different stores within the system, but the total amount of energy remains the same. This is because of the conservation of energy principle.
Worked Example: Closed System
A pendulum swinging back and forth can be modelled as a closed system. At the highest point, the pendulum has maximum gravitational potential energy and zero kinetic energy. At the lowest point, it has maximum kinetic energy and minimum gravitational potential energy. The total energy (kinetic + gravitational potential) remains constant throughout the swing if we ignore air resistance and friction.
Real Systems vs Closed Systems
In reality, energy is always dissipated to the surroundings through friction, air resistance, sound, and heating. A real pendulum gradually loses energy to the surroundings and eventually stops. This means no real system is truly closed, but modelling systems as closed makes calculations simpler and provides a good approximation.
Practice Questions
1. A car of mass 800 kg travels at 20 m/s. Calculate its kinetic energy.
Ek = ½mv2 = ½ × 800 × 202 = 400 × 400 = 160 000 J = 160 kJ
2. A rock of mass 2 kg is at the top of a cliff 50 m high. Calculate its gravitational potential energy. (g = 9.8 N/kg)
Ep = mgh = 2 × 9.8 × 50 = 980 J
3. A spring with spring constant 400 N/m is stretched by 0.1 m. Calculate the elastic potential energy stored.
Ee = ½ke2 = ½ × 400 × 0.12 = 200 × 0.01 = 2 J
4. Name all eight energy stores.
Kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, electrostatic, magnetic.
5. A hairdryer uses 2000 J of electrical energy. It transfers 1200 J as heat, 600 J as kinetic energy (air movement) and 200 J as sound. Draw a Sankey diagram to show these transfers.
The input arrow is 2000 J wide. The useful heat output is 1200/2000 = 0.6 of the width. The useful kinetic output is 600/2000 = 0.3 of the width. The wasted sound output is 200/2000 = 0.1 of the width. Total outputs: 1200 + 600 + 200 = 2000 J, which equals the input.
6. Explain what is meant by dissipation of energy and give two examples.
Dissipation is when energy is transferred to less useful stores. Examples: (1) A car braking transfers kinetic energy to thermal energy of the brakes, which is dissipated. (2) A light bulb transfers some electrical energy to thermal energy instead of light energy, which is dissipated.
🔢 Maths Skills
Mathematical Skills for this Topic
Converting between energy units: Energy values in physics can be very large or very small, so unit conversion is essential. 1 kJ = 1000 J, 1 MJ = 1 000 000 J. To convert from joules to kilojoules, divide by 1000. To convert from kilojoules to joules, multiply by 1000. Always check what unit the question asks for and give your final answer in that unit.
Rearranging energy equations: You may need to rearrange Ek = ½mv² to find mass or speed. To find mass: m = 2Ek / v². To find speed: v = √(2Ek / m). For gravitational potential energy, Ep = mgh can be rearranged to find height: h = Ep / (mg). For elastic potential energy, Ee = ½ke² can be rearranged to find extension: e = √(2Ee / k). Always show your working step by step.
Standard form for very large/small energies: Nuclear energy stores involve extremely large values (e.g. millions of joules) while atomic-scale energies can be tiny. Standard form (scientific notation) expresses these compactly: 135 000 J = 1.35 × 10⁵ J; 0.0025 J = 2.5 × 10⁻³ J. When multiplying in standard form, multiply the coefficients and add the powers of 10.
Working with squared terms: In Ek = ½mv², remember that the speed is squared. If speed doubles, kinetic energy increases by a factor of 4 (2²). If speed triples, kinetic energy increases by a factor of 9 (3²). This is a common exam trap.
⚠️ Common Misconceptions
Watch Out!
Students often think that energy is "used up" when a device operates. Wrong: Energy is used up and disappears when a machine does work. Correct: Energy is never created or destroyed — it is transferred from one store to another. Some energy is dissipated to less useful stores (usually thermal), but it still exists.
Students often think that kinetic energy is the only useful energy store in real-world devices. Wrong: Kinetic energy is always the only useful output in any energy transfer. Correct: Many different energy stores can be the useful output depending on the device. A heater's useful output is thermal energy, a lamp's is light, and a battery's is chemical energy stored in its products.
✍️ 6-Mark Question
Extended Answer Question
6 marks: Explain the energy transfers that occur when a ball is thrown vertically upwards and then falls back down.
When the ball is thrown upwards, it has energy in its kinetic energy store. As it rises, energy is transferred from the kinetic energy store to the gravitational potential energy store. The ball slows down as it rises because kinetic energy is being transferred away. At the highest point, the ball momentarily stops, so its kinetic energy store is at its minimum and its gravitational potential energy store is at its maximum. As the ball falls back down, energy is transferred from the gravitational potential energy store back to the kinetic energy store. The ball speeds up as it falls because kinetic energy is being transferred in. Throughout the motion, some energy is dissipated to the thermal energy store of the ball and the surrounding air due to air resistance. This means the ball will not quite reach the same height on each bounce and will eventually come to rest. In a closed system with no air resistance, the total energy (kinetic + gravitational potential) would remain constant at every point in the motion.
Mark scheme: 2 marks for describing the upward journey (kinetic to gravitational potential), 2 marks for describing the downward journey (gravitational potential to kinetic), 1 mark for mentioning energy at the highest point, 1 mark for discussing dissipation due to air resistance
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A Sankey diagram for an electric motor shows a total input of 800 J. The useful kinetic energy output arrow has a width representing 520 J, and the wasted energy arrow turns downward.
(a) Calculate the efficiency of the motor.
(b) The motor is replaced with a more efficient model that wastes only 120 J for the same 800 J input. Calculate the new efficiency and state whether the improvement is worthwhile if the new motor costs £50 more.
Answer: (a) Efficiency = (520 / 800) × 100% = 65%. (b) New useful output = 800 − 120 = 680 J. New efficiency = (680 / 800) × 100% = 85%. The efficiency has increased by 20 percentage points, meaning significantly less energy is wasted as heat. Over time, the energy savings would offset the £50 additional cost, so the improvement is worthwhile for a motor that is used frequently.
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