Energy — GCSE Physics Revision
Everything you need to revise energy for GCSE Physics: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Energy Stores and Transfers
Energy cannot be created or destroyed, only transferred from one store to another or dissipated. The main energy stores are chemical, kinetic, gravitational potential, elastic potential, thermal, magnetic, electrostatic, and nuclear. Energy transfers occur via mechanical work (forces), electrical work (currents), heating (temperature difference), and radiation (waves). Understanding these stores and how energy moves between them is fundamental to all physics topics, helping to explain processes from power generation to everyday actions.
Work Done and Energy Transfer
Work is done when a force causes an object to move. It is a measure of energy transfer. The work done (W) is calculated by multiplying the force (F) applied by the distance (s) moved in the direction of the force: W = F × s. The unit for work done is the joule (J). For example, lifting an object transfers energy from a chemical store (in your muscles) to a gravitational potential store (of the object), doing work against gravity. Work done is equivalent to the energy transferred.
Power as Rate of Energy Transfer
Power is defined as the rate at which energy is transferred or the rate at which work is done. It tells us how quickly energy is being used or converted. The formula for power (P) is energy transferred (E) divided by time taken (t): P = E / t. Alternatively, P = W / t, where W is work done. The unit for power is the watt (W), which is equivalent to one joule per second (J/s). A more powerful appliance transfers more energy in the same amount of time.
Conservation of Energy and Efficiency
The principle of conservation of energy states that the total energy of a closed system remains constant; energy cannot be created or destroyed. However, in most real-world transfers, some energy is dissipated, usually as heat, to the surroundings, making it less useful. Efficiency measures how well a device transfers energy into useful forms. It's calculated as (useful energy output / total energy input) × 100%, or (useful power output / total power input) × 100%. Improving efficiency reduces wasted energy.
Key facts
- Energy cannot be created or destroyed.|Work done = Force × Distance.|Power = Energy transferred / Time.|Efficiency = (Useful energy output / Total energy input) × 100%.|Unit of energy and work done is the joule (J).|Unit of power is the watt (W).
Key terms
- Energy Store::A place where energy is held, such as kinetic, gravitational potential, or chemical.|Energy Transfer::The movement of energy from one store to another, or from one place to another.|Work Done::Energy transferred when a force moves an object through a distance.|Power::The rate at which energy is transferred or work is done.|Efficiency::The proportion of total energy input that is transferred into useful energy output.|Dissipated Energy::Energy that is spread out and becomes less useful, often as heat to the surroundings.
Common mistakes
- Confusing power with energy; power is the rate of energy transfer, not energy itself.|Forgetting that work done only occurs when a force causes movement in its direction.|Not including '× 100%' when calculating efficiency as a percentage.|Assuming energy is 'lost' when it's just transferred to a less useful store (dissipated).
Exam tips
- Always state the units (J for energy/work, W for power) in your answers.|Remember the conservation of energy principle: total energy input always equals total energy output (useful + wasted).|When calculating work done, ensure the distance is moved in the direction of the force.|Practice rearranging the energy and power formulas to find different variables.
Quick quiz
1. Which of the following is NOT an energy store?
- Thermal
- Kinetic
- Electricity
- Gravitational Potential
Show answer
Electricity — Electricity is a method of energy transfer, not an energy store itself. Thermal, kinetic, and gravitational potential are all recognised energy stores.
2. A force of 20 N moves an object by 5 m. How much work is done?
- 4 J
- 15 J
- 100 J
- 25 J
Show answer
100 J — Work done = Force × Distance. So, 20 N × 5 m = 100 J.
3. What is the unit of power?
- Joule
- Newton
- Watt
- Volt
Show answer
Watt — The unit of power is the Watt (W). Joule is for energy, Newton for force, and Volt for potential difference.
4. An appliance uses 100 J of energy in 2 seconds. What is its power?
- 50 W
- 200 W
- 0.02 W
- 10 W
Show answer
50 W — Power = Energy / Time. So, 100 J / 2 s = 50 W.
5. Which statement correctly describes the conservation of energy?
- Energy is always lost as heat.
- Energy can be created but not destroyed.
- The total energy in a closed system remains constant.
- Useful energy output is always greater than total energy input.
Show answer
The total energy in a closed system remains constant. — The principle of conservation of energy states that energy cannot be created or destroyed, only transferred, meaning the total energy in a closed system stays the same.
Exam-style questions
Describe the principle of conservation of energy. [2 marks]
Show mark scheme
- Energy cannot be created or destroyed.
- Only transferred from one store to another / dissipated.
A lift motor does 30 000 J of useful work in 15 seconds. Calculate the useful power output of the motor. [3 marks]
Show mark scheme
- State formula P = E / t or P = W / t (1 mark)
- Substitute values: P = 30 000 J / 15 s (1 mark)
- Calculate answer: P = 2000 W (1 mark)
Explain why no energy transfer is 100% efficient. [2 marks]
Show mark scheme
- Some energy is always dissipated / transferred to less useful stores (1 mark)
- Often as heat / sound to the surroundings (1 mark)
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