Energy ResourcesCambridge IGCSE Physics: Revision notes
Section 1
What are the different forms of energy storage?
Energy can be stored in several different forms. Understanding each form is essential for applying conservation of energy to real situations.
The seven main energy stores are:
| Energy Store | Description |
|---|---|
| Kinetic | Energy of a moving object; depends on mass and velocity |
| Gravitational potential | Energy stored due to an object's position in a gravitational field |
| Chemical | Energy stored in chemical bonds; released during chemical reactions |
| Elastic (strain) | Energy stored in a stretched or compressed spring or material |
| Nuclear | Energy stored in atomic nuclei; released during fission or fusion |
| Electrostatic | Energy stored due to separated electric charges |
| Internal (thermal) | Energy stored as the random motion of particles in a substance |
Each form can be converted to other forms during physical and chemical processes. The ability to identify which energy stores are involved in a given situation is crucial for energy transfer calculations.
Examiners expect you to identify and name the correct energy stores involved in a process. When answering, always state 'energy is transferred from [store name] to [store name]' — vague answers like 'energy is converted' will not gain full marks.
Section 2
How is energy transferred between stores?
Energy is never created or destroyed, but it is constantly transferred between different stores. There are four main methods of energy transfer:
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By forces (mechanical work): When a force acts on an object and moves it through a distance, energy is transferred. This includes pushing, pulling, and lifting objects. Work done equals force multiplied by distance in the direction of the force.
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By electrical currents: Electrical energy flows through circuits and is converted to other forms (heat in a resistor, light in a bulb, kinetic energy in a motor).
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By heating: Thermal energy transfers from a hotter object to a cooler object through conduction, convection, or radiation. The internal energy store increases in the cooler object.
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By waves: Electromagnetic waves (light, radio waves), sound waves, and other forms of radiation carry energy from a source to a receiver, transferring it between stores.
In any real process, energy is transferred through one or more of these methods. Identifying the correct transfer method is essential for constructing energy flow diagrams.
When describing energy transfers, always state the method: 'energy is transferred by [method] from [store] to [store]'. For example, 'energy is transferred by mechanical work from chemical energy to kinetic energy' is much stronger than just saying 'energy is transferred'.
Think of energy stores like bank accounts and transfer methods as different ways to move money between them — you can transfer by cheque (work), by direct debit (electrical), by cash (heating), or by post (waves). Each method moves the same 'money' (energy) but in different ways.
Section 3
What is the principle of conservation of energy?
The principle of conservation of energy states that:
Energy cannot be created or destroyed. In any closed system, the total energy remains constant. Energy is only transferred between different stores or converted from one form to another.
This means:
- In an isolated system, the total energy at the start equals the total energy at the end
- Energy may appear to disappear, but it has actually been transferred to another store (often internal/thermal energy)
- In real situations, some energy is often transferred to unwanted stores (e.g. heat loss), but the total is still conserved
Applying conservation of energy:
To solve conservation of energy problems:
- Identify all energy stores at the start of the process
- Identify all energy stores at the end of the process
- State that total energy at start = total energy at end (accounting for any external energy input or loss)
- Calculate unknown values using this principle
This principle applies to all physical and chemical processes, from simple motions to complex systems with multiple energy transfers.
Examiners want to see the equation 'total energy at start = total energy at end' or 'energy in = energy out'. Always write this statement explicitly in your answer — it demonstrates clear understanding of conservation of energy.
A ball is dropped from a height of 5 m. At the start, it has 250 J of gravitational potential energy and 0 J of kinetic energy (total = 250 J). At the bottom, conservation of energy tells us total energy must still equal 250 J. If 20 J is lost to air resistance, then kinetic energy = 230 J. Students must account for all energy transfers.
Section 4
How do you calculate kinetic and potential energy changes?
Two key equations allow you to calculate changes in mechanical energy stores:
Kinetic Energy:
Ek = ½mv²
Where:
- Ek = kinetic energy in joules (J)
- m = mass in kilograms (kg)
- v = velocity in metres per second (m/s)
Kinetic energy depends on both mass and the square of velocity. Doubling the velocity quadruples the kinetic energy.
Gravitational Potential Energy Change:
ΔEp = mgΔh
Where:
- ΔEp = change in gravitational potential energy in joules (J)
- m = mass in kilograms (kg)
- g = gravitational field strength (9.8 m/s² on Earth)
- Δh = change in height in metres (m)
This equation calculates the change in potential energy when an object moves vertically. A positive change means the object has moved upward and gained potential energy; a negative change means it has moved downward and lost potential energy.
Important notes:
- Always use consistent units (SI units: kg, m/s, m, J)
- The velocity in Ek = ½mv² must be squared — common errors include forgetting this
- The height change in ΔEp = mgΔh is the vertical distance only, not the total distance travelled
Students often forget to square the velocity in Ek = ½mv². Remember: if velocity doubles, kinetic energy increases by a factor of four (2² = 4). Check your calculator is squaring the value.
A 2 kg ball is thrown upward with velocity 10 m/s. Calculate the kinetic energy: Ek = ½ × 2 × 10² = ½ × 2 × 100 = 100 J. If it rises to a height of 3 m above the starting point, the potential energy gain is ΔEp = 2 × 9.8 × 3 = 58.8 J. Note that kinetic energy lost (100 J) exceeds potential energy gained (58.8 J) — the difference (41.2 J) has been lost to air resistance.
Section 5
How do you apply conservation of energy to complex, multi-stage processes?
Complex energy problems often involve multiple stages, conversions, and transfers. These are best analysed using flow diagrams and Sankey diagrams.
Solving multi-stage problems:
- Identify all stages: Break the process into clear steps (e.g. chemical energy → kinetic energy → sound energy + heat)
- Apply conservation at each stage: The energy leaving one store must equal the energy entering the next store(s)
- Account for losses: In real systems, some energy is always dissipated as heat or sound; ensure total energy out = energy in at each stage
- Use algebraic equations: Set up equations using conservation of energy and the mechanical energy formulas
- Solve systematically: Calculate unknown values step-by-step, checking that energy is conserved throughout
Sankey diagrams are a powerful visual tool:
- The width of each arrow represents the amount of energy
- Energy flows from left to right through different stores
- The diagram clearly shows where energy is wasted (usually as heat)
- Reading a Sankey diagram requires identifying energy inputs, transfers, useful outputs, and wasted energy
Example structure for a complex problem:
Chemical energy (from fuel) → Kinetic energy (of vehicle) + Heat energy (engine inefficiency) + Sound energy (engine noise)
Total chemical energy = Kinetic energy + Heat energy + Sound energy (conservation applies)
If chemical energy input = 1000 J, and you know heat loss = 700 J and sound = 50 J, then kinetic energy = 250 J.
When reading or drawing a Sankey diagram, always check that all energy arrows add up correctly: the total width entering must equal the total width leaving (conservation of energy). Examiners expect you to identify wasted energy and explain where it goes.
A light bulb receives 100 J of electrical energy. It converts 10 J to light energy and 90 J to heat energy. A Sankey diagram would show one arrow of width 100 entering from the left, then splitting into two arrows exiting: one of width 10 (light) and one of width 90 (heat). Total energy in = 100 J; total energy out = 10 + 90 = 100 J. Energy is conserved.
Must Know
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Seven energy stores: kinetic, gravitational potential, chemical, elastic, nuclear, electrostatic, and internal (thermal). Be able to identify which stores are involved in any given situation.
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Four energy transfer methods: by mechanical work (forces), by electrical currents, by heating, and by waves (electromagnetic, sound, or other). Always name the method when describing an energy transfer.
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Conservation of energy principle: Total energy in a closed system is constant. Energy is transferred between stores or converted to different forms, but the total never changes. Always write the equation 'energy in = energy out' in your answers.
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Kinetic energy equation Ek = ½mv²: Use this to calculate the energy of moving objects. Remember to square the velocity — if velocity doubles, kinetic energy increases four times. Always use SI units (kg, m/s, joules).
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Gravitational potential energy change ΔEp = mgΔh: Use this for vertical height changes only (g = 9.8 m/s²). The change in potential energy is mass × gravitational field strength × vertical distance.
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Complex multi-stage problems and Sankey diagrams: Break processes into stages, apply conservation of energy at each stage, and use diagrams to visualise energy flows. Account for energy losses (usually as heat) and verify that total energy in equals total energy out.
That's the notes covered.
Carry on to the next subtopic.