IGCSE Physics Energy Notes
Energy is conserved — it can only be transferred between stores. Learn the stores, the equations and how to calculate efficiency.
Key concepts
- Energy stores
- Kinetic, gravitational potential, elastic, chemical, thermal, nuclear, electrostatic and magnetic.
- Conservation of energy
- Energy cannot be created or destroyed, only transferred; total energy in a closed system stays constant.
- Efficiency
- The fraction of input energy transferred usefully; the rest is dissipated, usually as heat.
- Work done = energy transferred, measured in joules.
- Power is the rate of energy transfer, measured in watts.
- No real device is 100% efficient — some energy is always wasted.
Key formulas
Kinetic energy
Eₖ = ½ × m × v²
Gravitational PE
Eₚ = m × g × h
Efficiency
efficiency = useful energy out ÷ total energy in
× 100 for a percentage.
Required practicals
- Investigate the efficiency of an electric motor lifting a load.
- Investigate energy transfers using a pendulum or falling mass.
Worked example
A motor takes in 500 J and does 300 J of useful work. Find its efficiency.
- 1Write the equation: efficiency = useful ÷ total.
- 2Substitute: efficiency = 300 ÷ 500.
- 3Calculate: 0.6.
- 4Convert to a percentage: 60%.
Answer: Efficiency = 60%
Common mistakes
✗ Saying energy is 'lost'.
✓ Energy is transferred or dissipated to the surroundings, never destroyed.
✗ Giving efficiency greater than 100%.
✓ Useful output can never exceed total input — check your values.
✗ Forgetting to square v in kinetic energy.
✓ Eₖ = ½mv² — square the speed first.
Quick check
Try these, then reveal the answers.
State the principle of conservation of energy.Reveal answer
Energy cannot be created or destroyed, only transferred between stores.
A device wastes 40% of its energy. What is its efficiency?Reveal answer
60%
What is the unit of power?Reveal answer
The watt (W).
Exam tips
- Describe transfers as 'from … store to … store by … pathway'.
- Express efficiency as a fraction or percentage, never above 100%.
- Show the equation and substitution to earn method marks.
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Frequently asked questions
What does efficiency measure?+
The proportion of input energy that is transferred usefully rather than wasted.
Why can't a device be 100% efficient?+
Some energy is always dissipated to the surroundings, usually as heat, through friction and resistance.
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