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This lesson brings together all the content from the Energy topic (AQA GCSE Physics Section 4.1) and provides structured exam practice. You will review key equations, practise different question types, and learn the techniques needed to maximise your marks in the exam.
The following equations are essential for the Energy topic. Some are on the equation sheet; others must be memorised.
| Equation | Symbols | What It Calculates | On Equation Sheet? |
|---|---|---|---|
| E_k = 0.5 x m x v^2 | E_k = kinetic energy (J), m = mass (kg), v = speed (m/s) | Kinetic energy | Yes |
| E_p = m x g x h | E_p = gravitational potential energy (J), m = mass (kg), g = gravitational field strength (N/kg), h = height (m) | Gravitational potential energy | Yes |
| E_e = 0.5 x k x e^2 | E_e = elastic potential energy (J), k = spring constant (N/m), e = extension (m) | Elastic potential energy | Yes |
| E = m x c x change in temperature | E = energy (J), m = mass (kg), c = specific heat capacity (J/kg degC) | Energy for temperature change | Yes |
| P = E / t | P = power (W), E = energy (J), t = time (s) | Power | Yes |
| Efficiency = useful output / total input | Can use energy or power values | Efficiency | No (must learn) |
| F = k x e | F = force (N), k = spring constant (N/m), e = extension (m) | Hooke's law | No (must learn for combined) |
Exam Tip: Even though many equations are on the equation sheet, you will work faster and more confidently if you know them by heart. Practise writing them from memory until you can do it without hesitation.
These questions test your recall of key facts and your ability to apply concepts quickly.
Q1. Which of the following is NOT an energy store? (a) Kinetic (b) Thermal radiation (c) Gravitational potential (d) Chemical
Answer: (b) Thermal radiation — radiation is a transfer pathway, not a store.
Q2. A ball is thrown upwards. Describe the energy changes as it rises, reaches its highest point, and falls back down. Ignore air resistance.
Answer: As the ball rises, energy is transferred from the kinetic energy store to the gravitational potential energy store (the ball slows down and gains height). At the highest point, all the kinetic energy has been transferred to GPE (the ball is momentarily stationary). As it falls, energy is transferred back from the gravitational potential energy store to the kinetic energy store (the ball speeds up and loses height).
Q3. State what is meant by "specific heat capacity."
Answer: Specific heat capacity is the amount of energy required to raise the temperature of 1 kilogram of a substance by 1 degree Celsius.
Exam Tip: For "state" questions, keep your answer brief and precise. Use the exact definition from the specification where possible. You do not need to explain further unless the question says "explain."
Calculations are a major part of the Energy topic. Follow these steps for every calculation:
Q4. A car of mass 1500 kg is travelling at 20 m/s. Calculate its kinetic energy.
E_k = 0.5 x m x v^2 E_k = 0.5 x 1500 x 20^2 E_k = 0.5 x 1500 x 400 E_k = 300 000 J (or 300 kJ)
Q5. A crane lifts a load of mass 200 kg to a height of 15 m. Calculate the gravitational potential energy gained. (Use g = 9.8 N/kg.)
E_p = m x g x h E_p = 200 x 9.8 x 15 E_p = 29 400 J (or 29.4 kJ)
Q6. A spring with a spring constant of 50 N/m is stretched by 0.2 m. Calculate the elastic potential energy stored.
E_e = 0.5 x k x e^2 E_e = 0.5 x 50 x 0.2^2 E_e = 0.5 x 50 x 0.04 E_e = 1.0 J
Q7. 500 g of water is heated from 20 degC to 80 degC. Calculate the energy required. (c for water = 4200 J/kg degC.)
First convert mass: 500 g = 0.5 kg Change in temperature = 80 - 20 = 60 degC
E = m x c x change in temperature E = 0.5 x 4200 x 60 E = 126 000 J (or 126 kJ)
Q8. A heater has a power rating of 2000 W. How much energy does it transfer in 10 minutes?
First convert time: 10 minutes = 600 seconds
E = P x t E = 2000 x 600 E = 1 200 000 J (or 1200 kJ or 1.2 MJ)
Exam Tip: Always convert units before substituting into equations. Common conversions: g to kg (divide by 1000), minutes to seconds (multiply by 60), kW to W (multiply by 1000), cm to m (divide by 100).
These questions require you to link two or more equations together.
Q9. A ball of mass 0.4 kg is dropped from a height of 5 m. Calculate the speed of the ball just before it hits the ground. Assume no air resistance. (Use g = 10 N/kg.)
Step 1: Calculate GPE at the top. E_p = m x g x h = 0.4 x 10 x 5 = 20 J
Step 2: By conservation of energy, GPE = KE at the bottom. E_k = 20 J
Step 3: Rearrange E_k = 0.5 x m x v^2 to find v. 20 = 0.5 x 0.4 x v^2 20 = 0.2 x v^2 v^2 = 100 v = 10 m/s
Q10. An electric motor has a power rating of 500 W and is 80% efficient. It is used to lift a load. How much useful energy does it transfer in 30 seconds?
Step 1: Calculate total energy input. E = P x t = 500 x 30 = 15 000 J
Step 2: Calculate useful energy output. Useful output = efficiency x total input = 0.80 x 15 000 = 12 000 J
Q11. The motor in Q10 lifts a load to a height of 6 m. What is the maximum mass it can lift? (Use g = 10 N/kg.)
Useful output = E_p = m x g x h 12 000 = m x 10 x 6 12 000 = 60 x m m = 200 kg
graph TD
A["Total input: E = P x t = 15 000 J"] --> B["Useful output: 80% x 15 000 = 12 000 J"]
A --> C["Wasted: 3000 J (internal energy)"]
B --> D["GPE gained: E_p = m x g x h"]
D --> E["m = E_p / (g x h) = 200 kg"]
style C fill:#ffcccc,stroke:#cc0000
style E fill:#aaffaa,stroke:#006600
Exam Tip: Multi-step calculation questions carry the most marks. Break the problem into clear steps, show every line of working, and write the equation before substituting values. Even if your final answer is wrong, you can still get marks for correct method and intermediate answers.
Extended response questions require you to write a structured answer using scientific knowledge, often with a comparison or evaluation.
Q12. A school is choosing between installing solar panels and a wind turbine to generate electricity. Evaluate both options and recommend which the school should choose. [6 marks]
Model Answer:
Solar panels convert light energy directly into electrical energy using photovoltaic cells. They are renewable, produce no CO2 during operation, and can be installed on the school roof, requiring no extra land. However, they are intermittent — they produce no electricity at night and much less in winter or on cloudy days. The school would need to remain connected to the grid or have battery storage.
A wind turbine converts the kinetic energy of the wind into electrical energy. It is also renewable and produces no CO2 during operation. However, it requires a suitable location with consistent wind, which may not be available in a built-up area near a school. Wind is also intermittent — calm days mean no generation. Additionally, wind turbines can cause noise and visual impact, which may concern nearby residents.
Both options have high initial costs but low running costs over their lifespan. Solar panels are likely the better choice for a school because they can be placed on existing rooftops without additional land or planning issues, they have no moving parts requiring minimal maintenance, and they operate silently. However, the school should remain connected to the grid for times when solar generation is insufficient.
Exam Tip: For 6-mark extended response questions, quality of written communication is assessed. Use paragraphs, scientific terminology (stores, transfers, dissipation, intermittent, efficiency), and a clear logical structure. Always end with a reasoned conclusion that answers the question directly.
These questions test your knowledge of the required practicals — specific heat capacity and thermal insulation.
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