EnergyCambridge IGCSE Physics: Revision notes
Section 1
What are the different stores of energy?
Energy can be stored in seven main forms, each relevant to different physical situations:
| Energy Store | Description | Example |
|---|---|---|
| Kinetic | Energy of a moving object | A car travelling at 30 m/s |
| Gravitational potential | Energy stored due to position in a gravitational field | Water at the top of a waterfall |
| Chemical | Energy stored in chemical bonds | Petrol, food, batteries |
| Elastic (strain) | Energy stored in stretched or compressed materials | A compressed spring or stretched rubber band |
| Nuclear | Energy stored in atomic nuclei | Uranium in a nuclear reactor |
| Electrostatic | Energy stored due to electric charge separation | Separated positive and negative charges |
| Internal (thermal) | Energy of random motion of particles; related to temperature | Hot water, friction heating |
Understanding which store is relevant in a given scenario is essential for applying conservation of energy correctly.
Examiners expect you to identify the correct energy stores in multi-stage processes. Always list which stores are involved at the start and end of any energy transfer.
Section 2
How is energy transferred between stores?
Energy is not created or destroyed; it is transferred from one store to another through four main mechanisms:
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By forces (mechanical work): A force moving an object through a distance transfers energy. Examples include lifting an object (work against gravity), pushing a box across a floor (work against friction), or compressing a spring.
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By electrical currents: Energy is transferred through circuits. A battery transfers chemical energy to electrical energy, which can then be converted to heat, light, or kinetic energy in an appliance.
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By heating: Thermal energy flows from hotter regions to cooler regions. This occurs through conduction, convection, and radiation, transferring internal energy between stores.
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By electromagnetic, sound and other waves: Energy travels via:
- Electromagnetic waves (light, radio waves, microwaves) – energy transferred from the sun to Earth, or from a microwave oven to food
- Sound waves – energy transferred from a loudspeaker to the air and ears
- Other waves – seismic waves, water waves
In every energy transfer, you should be able to identify the starting store, the mechanism of transfer, and the finishing store.
Think of energy stores as bank accounts and transfer mechanisms as payment methods – the money (energy) is the same, but it moves via different routes (cheque, online transfer, cash).
Students often forget to identify the transfer mechanism. Simply saying 'energy is transferred' is incomplete; you must state whether it is by force, electrical current, heating, or waves.
Section 3
What is the principle of conservation of energy?
The principle of conservation of energy states that:
Energy cannot be created or destroyed; the total energy of a closed system remains constant. Energy is only transferred from one store to another.
In a closed system with no external inputs or outputs, the sum of all energy stores must remain the same before and after any process.
Formula (general form):
Total energy before = Total energy after
Or: ΣE_initial = ΣE_final
Applying conservation of energy to simple examples:
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A ball thrown vertically upward: Kinetic energy is converted to gravitational potential energy as it rises. At the highest point, all kinetic energy has been converted to potential energy. As it falls, potential energy converts back to kinetic energy.
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A car braking: The kinetic energy of the car is transferred to internal (thermal) energy in the brakes and tyres due to friction.
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A battery-powered torch: Chemical energy in the battery is transferred to electrical energy in the circuit, then to light and heat energy in the bulb.
Flow diagrams show energy transfers as boxes (stores) connected by arrows (mechanisms and amounts). The total energy entering must equal the total energy leaving any component.
When asked to 'show that energy is conserved,' calculate the total energy at the start and the total energy at the end, then state they are equal. This demonstrates understanding of the principle.
A 2 kg ball is dropped from a height of 5 m. At the point of release, all energy is gravitational potential energy: E_p = mgh = 2 × 10 × 5 = 100 J. Just before hitting the ground, all this has converted to kinetic energy: E_k = 100 J (assuming no air resistance). Total energy is conserved at 100 J throughout.
Section 4
How do you calculate kinetic and gravitational potential energy?
Two fundamental equations allow you to calculate energy in these stores:
Kinetic Energy:
E_k = ½mv²
Where:
- E_k = kinetic energy (joules, J)
- m = mass (kilograms, kg)
- v = velocity (metres per second, m/s)
Key points:
- Kinetic energy depends on mass and the square of velocity, so doubling velocity quadruples the kinetic energy.
- Any moving object possesses kinetic energy.
Gravitational Potential Energy:
ΔE_p = mgΔh
Where:
- ΔE_p = change in gravitational potential energy (joules, J)
- m = mass (kilograms, kg)
- g = gravitational field strength (9.8 m/s² or 10 m/s² on Earth)
- Δh = change in height (metres, m)
Key points:
- This equation calculates the change in potential energy relative to a reference point (usually the ground, where E_p = 0).
- Gravitational potential energy increases with height and mass.
Worked calculation example: A 60 kg student runs at 5 m/s and then climbs 3 m up a ladder.
- Kinetic energy: E_k = ½ × 60 × 5² = ½ × 60 × 25 = 750 J
- Change in potential energy: ΔE_p = 60 × 10 × 3 = 1800 J
- Total mechanical energy = 750 + 1800 = 2550 J
Remember that E_k = ½mv² uses velocity, not speed in the everyday sense. Always ensure velocity is in m/s, and that you square it before multiplying.
A common error is forgetting the '½' in the kinetic energy equation, or using height instead of change in height for potential energy. Always check that you are calculating change (Δh) from the reference point.
A 0.5 kg ball is thrown upward at 20 m/s from ground level. At launch: E_k = ½ × 0.5 × 20² = 100 J; E_p = 0 J. At maximum height (where v = 0): E_k = 0 J; E_p = 100 J. Conservation of energy shows total is always 100 J.
Section 5
How do you apply conservation of energy to complex, multi-stage processes?
Complex energy problems often involve multiple stages, energy dissipation, and several stores. Solving them requires systematic application of conservation principles.
Strategy for multi-stage problems:
- Identify all energy stores at the start and end of the process.
- Identify energy transfers and mechanisms (force, heat, electrical, waves).
- Calculate energy in each store using the appropriate equations (E_k = ½mv², ΔE_p = mgΔh, etc.).
- Apply conservation of energy to each stage separately, then to the whole process.
- Check: Total initial energy = Total final energy + Energy dissipated (e.g. to heat/sound).
Sankey diagrams represent energy transfers and dissipation visually:
- Width of each arrow is proportional to the amount of energy.
- Energy entering a component must equal energy leaving (conservation).
- Arrows show the flow from one store to another.
- Wasted energy (dissipated to heat or sound) is shown explicitly.
Example scenario – a falling object hitting the ground:
A 2 kg ball falls from 10 m and comes to rest on impact, with 10% lost to sound.
- Initial E_p = 2 × 10 × 10 = 200 J; E_k = 0 J → Total = 200 J
- Just before impact: E_p = 0 J; E_k = 200 J → Total = 200 J (conservation check)
- After impact: all energy is dissipated. Sound energy = 0.1 × 200 = 20 J; Heat/deformation = 180 J.
- Total dissipated = 20 + 180 = 200 J ✓ (energy conserved)
Interpreting Sankey diagrams: Width of arrows shows energy quantity. All energy in equals all energy out. Wider outgoing arrows indicate major energy transfers; thin arrows show small or 'wasted' energy.
In Sankey diagrams, the total width of arrows entering must equal the total width leaving. Use this to check your work and to calculate unknown energy quantities if values are not given.
An electric motor (efficiency 80%) converts 1000 J of electrical energy. Useful output (kinetic) = 0.8 × 1000 = 800 J; wasted (heat) = 0.2 × 1000 = 200 J. A Sankey diagram would show a thick arrow (1000 J in), split into a thicker arrow (800 J to kinetic) and thinner arrow (200 J to heat).
Must Know
- Seven energy stores: kinetic, gravitational potential, chemical, elastic, nuclear, electrostatic, and internal (thermal). Be able to identify which applies in any scenario.
- Energy transfer mechanisms: by forces (mechanical work), electrical currents, heating, and by electromagnetic/sound/other waves. Always identify the mechanism, not just 'energy is transferred'.
- Conservation of energy principle: Total energy in a closed system is constant. Energy is transferred between stores but never created or destroyed. Total initial energy = Total final energy + Energy dissipated.
- Kinetic energy formula: E_k = ½mv² (not mv² or mv). Velocity must be squared; doubling velocity quadruples kinetic energy.
- Gravitational potential energy formula: ΔE_p = mgΔh. This calculates change in height from a reference point (usually ground). Use g = 10 m/s² unless told otherwise.
- Complex problems and Sankey diagrams: Identify all stores at each stage, calculate energy in each, apply conservation at each step, and show that all energy in equals all energy out. In Sankey diagrams, arrow width is proportional to energy; dissipated energy must be shown explicitly.
That's the notes covered.
Carry on to the next subtopic.