GCSE Physics Energy Notes
Energy is never created or destroyed — it is only transferred between stores. Nail the stores, the transfers and the equations and this topic becomes reliable marks.
Key concepts
- Energy store
- Where energy is held: kinetic, gravitational potential, elastic, chemical, thermal, nuclear, electrostatic and magnetic.
- Energy transfer
- Energy moves between stores by four pathways: mechanically (forces), electrically, by heating, or by radiation (light/sound).
- Conservation of energy
- The total energy of a closed system stays constant — energy is transferred, not lost. 'Wasted' energy is usually dissipated to the thermal store of the surroundings.
- Energy is measured in joules (J); power (rate of transfer) is measured in watts (W).
- Useful energy out is always less than total energy in — the difference is dissipated, usually as heat.
- Reducing unwanted energy transfers (lubrication, insulation) improves efficiency.
Key formulas
Kinetic energy
Eₖ = ½ × m × v²
mass in kg, speed in m/s.
Gravitational PE
Eₚ = m × g × h
g = 9.8 N/kg (use 10 if told).
Power
P = E ÷ t
power (W) = energy (J) ÷ time (s).
Required practicals
- Investigate the specific heat capacity of a material using an immersion heater.
- Investigate the effectiveness of different thermal insulators.
Worked example
A 2 kg ball is lifted 5 m. How much gravitational potential energy does it gain? (g = 9.8 N/kg)
- 1Write the equation: Eₚ = m × g × h.
- 2Substitute the values: Eₚ = 2 × 9.8 × 5.
- 3Multiply: Eₚ = 98.
- 4Add the unit: Eₚ = 98 J.
Answer: Eₚ = 98 J
Common mistakes
✗ Saying energy is 'lost' or 'used up'.
✓ Energy is transferred or dissipated to the surroundings — never destroyed.
✗ Forgetting to square the speed in the kinetic energy equation.
✓ Eₖ = ½mv² — square v before multiplying.
✗ Mixing up energy (J) and power (W).
✓ Energy is the total transferred; power is the rate of transfer per second.
Quick check
Try these, then reveal the answers.
What is the unit of energy?Reveal answer
The joule (J).
A 1500 W kettle runs for 60 s. How much energy is transferred? (E = P × t)Reveal answer
E = 90 000 J (90 kJ)
Name the four energy transfer pathways.Reveal answer
Mechanical, electrical, heating and radiation.
Exam tips
- Describe transfers as 'from … store to … store by … pathway' to hit the mark scheme.
- Always rearrange the equation before substituting — it earns a method mark even if the arithmetic slips.
- Quote the correct unit: joules for energy, watts for power.
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Frequently asked questions
What is the difference between energy and power?+
Energy is the total amount transferred, measured in joules; power is how fast it is transferred, measured in watts (joules per second).
Can energy be destroyed?+
No. Energy is always conserved — it is transferred between stores, and any 'wasted' energy is dissipated to the thermal store of the surroundings.
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