P4.1.1 Energy Stores & Systems — Physics with Kate
AQA GCSE Physics — 4.1.1
⚡ Energy Stores & Systems 🌪
From kinetic energy crackling through a lightning bolt to gravitational potential stored in towering storm clouds — energy is everywhere, always transferring, never lost.
⚡ stormy physics • energy unleashed • power of the storm 🌪
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Energy Stores (4.1.1.1)
⚡ The Eight Energy Stores
Energy can be stored in different ways. You need to know all eight energy stores:
Energy Store
Description
Example
Kinetic
Energy of a moving object
A car driving, wind blowing
Thermal
Energy related to temperature
A hot cup of tea
Chemical
Energy stored in chemical bonds
Food, fuel, batteries
Gravitational Potential
Energy of a raised object
A book on a shelf
Elastic Potential
Energy of a stretched/compressed object
A stretched spring or rubber band
Nuclear
Energy stored in atomic nuclei
Uranium fuel rods
Magnetic
Energy of magnets attracting/repelling
Two repelling magnets
Electrostatic
Energy of electric charges
A charged balloon
🌪 Systems and Energy Transfers
A system is an object or group of objects. When a system changes, energy is transferred between stores.
Energy can be transferred between stores by four pathways:
Mechanically — by a force doing work (pushing, pulling, stretching)
Electrically — by charges moving through a circuit
By heating — energy flows from hot to cold objects
By radiation — light, sound, or other waves carrying energy
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Kinetic Energy (4.1.1.2)
⚡ Kinetic Energy Equation
The kinetic energy of a moving object depends on its mass and its speed.
Ek = ½ m v²
Ek = kinetic energy in joules (J)
m = mass in kilograms (kg)
v = speed in metres per second (m/s)
Speed is squared — so doubling the speed gives four times the kinetic energy!
🌪 Worked Example — Kinetic Energy
A 70 kg runner moves at 8 m/s. Calculate their kinetic energy.
Ek = ½ × 70 × 8² = 0.5 × 70 × 64 = 2240 J
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Gravitational Potential Energy (4.1.1.2)
⚡ GPE Equation
The gravitational potential energy of a raised object depends on its mass, gravitational field strength, and height.
Ep = m g h
Ep = gravitational potential energy in joules (J)
m = mass in kilograms (kg)
g = gravitational field strength = 9.8 N/kg on Earth
h = height in metres (m)
🌪 Worked Example — GPE
A 2 kg book is lifted 1.5 m above the ground. Calculate the gravitational potential energy gained.
Ep = 2 × 9.8 × 1.5 = 29.4 J
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Elastic Potential Energy (4.1.1.2)
⚡ Elastic Potential Energy Equation
The energy stored in a stretched or compressed spring depends on the spring constant and the extension.
Ee = ½ k e²
Ee = elastic potential energy in joules (J)
k = spring constant in newtons per metre (N/m)
e = extension in metres (m)
This equation is only valid up to the limit of proportionality — beyond this point, the spring deforms permanently and the equation no longer applies.
🌪 Worked Example — Elastic PE
A spring with a spring constant of 40 N/m is stretched by 0.2 m. Calculate the elastic potential energy stored.
Ee = ½ × 40 × 0.2² = 0.5 × 40 × 0.04 = 0.8 J
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Conservation of Energy (4.1.1.2)
⚡ The Law of Conservation of Energy
Energy cannot be created or destroyed — it can only be transferred between stores. This is the Law of Conservation of Energy.
In a closed system, the total energy before a change = the total energy after the change.
Energy transfers can be shown using energy transfer diagrams (showing which stores energy moves between).
🌀 Dissipation — Wasted Energy
In every energy transfer, some energy is dissipated (spread out to the surroundings).
Dissipated energy is usually transferred to thermal energy stores by friction or air resistance.
This energy is often called "wasted" energy because it is not usefully transferred.
The energy has not been destroyed — it has just been spread out and is harder to use.
⚡ Exam Tip — The Equations
All three equations (KE, GPE, and EPE) are given on the equation sheet. However, you must be able to use them — select the right equation, substitute values correctly, rearrange if needed, and give the correct units (Joules).
🚨 Common Exam Mistake
Students often forget to square the speed in the KE equation or the extension in the EPE equation. Always apply the square before multiplying by the other values. For example, v = 8 m/s means v² = 64, not 8 × 2 = 16.
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Question 01 [2 marks]
Name four of the eight energy stores.
⚡ Model Answer
Any four from: kinetic, thermal, chemical, gravitational potential, elastic potential, nuclear, magnetic, electrostatic (1 mark for any two correct, 2 marks for four correct).
Question 02 [2 marks]
State four ways that energy can be transferred between stores.
⚡ Model Answer
Mechanically (by forces), electrically (by charges in a circuit), by heating, and by radiation (e.g. light, sound waves) (1 mark for two correct, 2 marks for all four).
Question 03 [3 marks]
A car has a mass of 1200 kg and is travelling at 15 m/s. Calculate the kinetic energy of the car. Use the equation Ek = ½mv².
⚡ Model Answer
Ek = ½ × 1200 × 15² (1 mark for substitution) Ek = 0.5 × 1200 × 225 (1 mark for squaring speed correctly) Ek = 135,000 J (or 135 kJ) (1 mark for correct answer with units).
Question 04 [3 marks]
A 500 g ball is thrown upwards to a height of 4 m. Calculate the gravitational potential energy gained. (g = 9.8 N/kg)
⚡ Model Answer
First convert mass: 500 g = 0.5 kg (1 mark for correct unit conversion) Ep = 0.5 × 9.8 × 4 (1 mark for correct substitution) Ep = 19.6 J (1 mark for correct answer with units).
Question 05 [3 marks]
A spring with a spring constant of 60 N/m is stretched by 0.15 m. Calculate the elastic potential energy stored. Use Ee = ½ke².
⚡ Model Answer
Ee = ½ × 60 × 0.15² (1 mark for substitution) Ee = 0.5 × 60 × 0.0225 (1 mark for squaring extension) Ee = 0.675 J (1 mark for correct answer with units).
Question 06 [2 marks]
State the Law of Conservation of Energy.
⚡ Model Answer
Energy cannot be created or destroyed (1 mark). It can only be transferred from one store to another (1 mark).
Question 07 [3 marks]
Describe the energy transfers that take place when a ball is thrown upwards and then falls back down.
⚡ Model Answer
When thrown upwards, energy is transferred from the kinetic energy store to the gravitational potential energy store (1 mark). At the highest point, the ball has maximum GPE and zero KE (1 mark). As it falls, energy transfers back from the gravitational potential store to the kinetic energy store (1 mark).
Question 08 [2 marks]
Explain what is meant by energy being "dissipated".
⚡ Model Answer
Dissipated means the energy is spread out to the surroundings (1 mark), usually transferred to thermal energy stores by friction, making it less useful (1 mark).
Question 09 [4 marks]
A 0.5 kg ball falls from a height of 10 m. Calculate the speed of the ball just before it hits the ground. Assume all GPE is transferred to KE. (g = 9.8 N/kg)
⚡ Model Answer
First calculate GPE: Ep = mgh = 0.5 × 9.8 × 10 = 49 J (1 mark) All GPE transfers to KE, so Ek = 49 J (1 mark) Rearrange: v² = 2Ek / m = 2 × 49 / 0.5 = 196 (1 mark) v = √196 = 14 m/s (1 mark).
Question 10 [3 marks]
Under what condition is the elastic potential energy equation Ee = ½ke² valid? Explain what happens if this condition is exceeded.
⚡ Model Answer
The equation is only valid up to the limit of proportionality (1 mark). Beyond this point, the spring is permanently deformed / does not return to its original length (1 mark), and the extension is no longer directly proportional to the force applied, so the equation no longer gives accurate results (1 mark).