What you'll learn
This topic covers the fundamental principles of energy in physical systems. You'll learn how to identify different energy stores, describe energy transfers between them, calculate efficiency and power, and apply the principle of conservation of energy. These concepts underpin much of GCSE Physics and appear frequently across both Foundation and Higher tier papers.
Key terms and definitions
Energy store — a system or object that contains energy in a particular form (e.g. kinetic, gravitational potential, thermal)
Energy transfer — the process by which energy moves from one store to another, occurring through mechanical work, electrical work, heating or radiation
System — a defined object or group of objects that we analyse in terms of energy changes
Dissipation — the spreading out of energy to the surroundings, usually by heating, reducing the useful energy available
Efficiency — the fraction or percentage of input energy that is usefully transferred (efficiency = useful output ÷ total input)
Power — the rate of energy transfer, measured in watts (W), where 1 W = 1 J/s
Conservation of energy — the principle that energy cannot be created or destroyed, only transferred between stores
Specific heat capacity — the energy required to raise the temperature of 1 kg of a substance by 1°C, measured in J/kg°C
Core concepts
Energy stores
Energy can be stored in eight different ways. You must be able to identify these stores and give examples:
Kinetic energy stores contain energy due to motion. Any moving object has energy in its kinetic store. Examples include moving vehicles, running water, or a kicked football.
Gravitational potential energy stores contain energy due to an object's position in a gravitational field. When you lift an object, you increase its gravitational potential energy. Examples include an aircraft at altitude, water stored in a reservoir, or a book on a shelf.
Elastic potential energy stores (also called strain energy) contain energy when objects are stretched, compressed or deformed. Examples include stretched springs, compressed mattresses, or drawn catapults.
Thermal energy stores contain energy due to the temperature of an object. All objects above absolute zero have energy in their thermal stores. Hotter objects have more energy in this store.
Chemical energy stores contain energy in chemical bonds. This energy is released during chemical reactions. Examples include food, batteries, and fuels like petrol or natural gas.
Nuclear energy stores contain energy in atomic nuclei. This energy is released during nuclear reactions such as fission or fusion.
Electrostatic energy stores contain energy when charged objects interact. Examples include a charged capacitor or charged particles attracting or repelling each other.
Magnetic energy stores contain energy when magnetic objects interact. Examples include two magnets attracting or repelling, or the magnetic field in an MRI scanner.
Energy transfers
Energy transfers between stores through four main pathways:
Mechanical work — when a force moves an object through a distance. Examples: lifting an object (chemical → gravitational potential), a car accelerating (chemical → kinetic), friction slowing a bicycle (kinetic → thermal).
Electrical work — when charge flows through a potential difference. Examples: a battery powering a motor (chemical → kinetic), a kettle heating water (electrical → thermal).
Heating — energy transfer due to a temperature difference. Heat always flows from hotter to cooler objects. Examples: a radiator warming a room, a hot drink cooling down.
Radiation — energy transfer by electromagnetic waves. Examples: light from the Sun (nuclear → thermal, light), microwaves cooking food.
Calculations involving energy
Work done is the energy transferred when a force moves an object:
Work done (J) = force (N) × distance moved in direction of force (m)
W = F × d
Gravitational potential energy can be calculated using:
Change in gravitational potential energy (J) = mass (kg) × gravitational field strength (N/kg) × change in height (m)
ΔEₚ = m × g × Δh
On Earth, g = 9.8 N/kg (often approximated to 10 N/kg in calculations).
Kinetic energy is calculated using:
Kinetic energy (J) = ½ × mass (kg) × (speed)² (m/s)²
Eₖ = ½ m v²
Elastic potential energy for a stretched spring:
Elastic potential energy (J) = ½ × spring constant (N/m) × (extension)² (m)²
Eₑ = ½ k e²
This equation applies only when the spring obeys Hooke's law and is not permanently deformed.
Specific heat capacity
The specific heat capacity of a substance determines how much energy is needed to change its temperature. Different materials have different specific heat capacities. Water has a high specific heat capacity (4200 J/kg°C), meaning it takes a lot of energy to heat up.
Change in thermal energy (J) = mass (kg) × specific heat capacity (J/kg°C) × temperature change (°C)
ΔE = m × c × Δθ
This equation is used to calculate the energy transferred when heating or cooling substances.
Power calculations
Power measures how quickly energy is transferred:
Power (W) = energy transferred (J) ÷ time (s)
P = E ÷ t
Or when considering work done:
Power (W) = work done (J) ÷ time (s)
P = W ÷ t
A more powerful device transfers energy more quickly. For example, a 2000 W kettle transfers 2000 J of energy every second.
Efficiency
No energy transfer is perfectly efficient. Some energy is always dissipated to the surroundings, usually by heating due to friction or resistance.
Efficiency can be expressed as a decimal (0 to 1) or percentage (0% to 100%):
Efficiency = useful energy transferred by device ÷ total energy supplied to device
Efficiency = useful power output ÷ total power input
To convert a decimal to a percentage, multiply by 100.
Reducing unwanted energy transfers:
- Lubrication reduces friction between moving parts
- Thermal insulation (e.g. cavity walls, loft insulation) reduces energy loss by heating
- Making objects more streamlined reduces air resistance
- Using more efficient components (e.g. LED bulbs instead of filament bulbs)
Conservation of energy
The principle of conservation of energy states that energy cannot be created or destroyed. The total energy before and after any transfer remains constant.
When a ball is dropped:
- Initially: energy mainly in gravitational potential store
- During fall: gravitational potential energy → kinetic energy
- On hitting ground: kinetic energy → thermal energy (and some sound)
- Total energy throughout = constant
This principle applies to all energy transfers and is fundamental to energy calculations. If energy seems to "disappear," it has been dissipated to the surroundings, often by heating.
Energy resources
Energy resources are classified as renewable or non-renewable:
Non-renewable resources will eventually run out:
- Fossil fuels (coal, oil, natural gas) — formed over millions of years from ancient organisms
- Nuclear fuel (uranium, plutonium)
Renewable resources are replenished naturally:
- Solar power
- Wind power
- Hydroelectric power
- Geothermal power
- Tidal power
- Wave power
- Biofuels
Each resource has advantages and disadvantages regarding cost, reliability, environmental impact, and energy output. You should be able to evaluate different energy resources for specific contexts.
Worked examples
Example 1: Gravitational potential energy and kinetic energy
Question: A 60 kg student climbs stairs of vertical height 5.0 m. Calculate the increase in their gravitational potential energy. (g = 10 N/kg) [3 marks]
Solution:
- Use ΔEₚ = m × g × Δh [1 mark for correct equation]
- ΔEₚ = 60 × 10 × 5.0 [1 mark for correct substitution]
- ΔEₚ = 3000 J [1 mark for correct answer with unit]
Example 2: Efficiency calculation
Question: An electric motor is supplied with 500 J of energy. It lifts a load, doing 350 J of useful work. Calculate the efficiency of the motor as a percentage. [3 marks]
Solution:
- Efficiency = useful energy output ÷ total energy input [1 mark for correct equation]
- Efficiency = 350 ÷ 500 = 0.70 [1 mark for correct calculation]
- Efficiency = 0.70 × 100 = 70% [1 mark for conversion to percentage]
Example 3: Power calculation
Question: A 2.0 kW electric heater is switched on for 5 minutes. Calculate the total energy transferred. [3 marks]
Solution:
- Convert time to seconds: 5 minutes = 5 × 60 = 300 s [1 mark]
- Rearrange P = E ÷ t to give E = P × t [1 mark for correct rearrangement]
- E = 2000 × 300 = 600 000 J (or 600 kJ) [1 mark for correct answer]
Example 4: Specific heat capacity
Question: Calculate the energy required to heat 2.0 kg of water from 20°C to 100°C. (Specific heat capacity of water = 4200 J/kg°C) [3 marks]
Solution:
- Temperature change = 100 – 20 = 80°C [1 mark]
- ΔE = m × c × Δθ [1 mark for correct equation]
- ΔE = 2.0 × 4200 × 80 = 672 000 J (or 672 kJ) [1 mark for correct answer]
Common mistakes and how to avoid them
Confusing energy stores with energy transfers. Remember: stores are "where" energy is (kinetic, gravitational potential, etc.); transfers are "how" energy moves (mechanically, electrically, by heating, by radiation). Never say "light energy" or "sound energy" as stores — these are transfer pathways.
Forgetting to square the velocity in kinetic energy calculations. The equation is Eₖ = ½mv², not ½mv. Doubling the speed quadruples the kinetic energy.
Using the wrong units. Energy must be in joules (J), power in watts (W), time in seconds (s), mass in kilograms (kg), and distance in metres (m). Convert before calculating: remember 1 kW = 1000 W, 1 kJ = 1000 J, and time in minutes must be multiplied by 60.
Not stating efficiency as a percentage when asked. If the question asks for percentage efficiency, you must multiply your decimal answer by 100 and include the % symbol.
Assuming all energy is useful. Remember that some energy is always dissipated (usually as thermal energy to the surroundings). Total energy in = useful energy out + wasted energy.
Sign errors in temperature change. ΔT is always final temperature minus initial temperature. The answer should be positive when heating and you're calculating energy required.
Exam technique for P8: Energy
Command words matter. "Calculate" requires numerical working and an answer with units. "Describe" requires you to state what happens. "Explain" requires reasons using physics principles. "State" needs a concise answer without explanation.
Show your working clearly. In calculation questions, write the equation, substitute values, then give the answer with units. This allows you to gain method marks even if your final answer is incorrect.
Check units and conversions. If given power in kW, convert to W (multiply by 1000). If given time in minutes, convert to seconds (multiply by 60). If mass is in grams, convert to kg (divide by 1000).
Use energy principles systematically. For complex problems, identify: (1) the system, (2) initial energy stores, (3) final energy stores, (4) transfer pathways, (5) any dissipated energy. Apply conservation of energy: total energy before = total energy after.
Quick revision summary
Energy exists in eight stores: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, electrostatic, and magnetic. Energy transfers between stores via mechanical work, electrical work, heating, or radiation. Energy is always conserved — it cannot be created or destroyed. Key equations: W = Fd, ΔEₚ = mgΔh, Eₖ = ½mv², ΔE = mcΔθ, P = E/t, and efficiency = useful output ÷ total input. Some energy is always dissipated, usually by heating, reducing efficiency.