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AQA GCSE Physics

Energy stores, transfers and efficiency — AQA GCSE Physics revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 4 minutes to read.

Stores and transfers

  • Energy is stored kinetically, thermally, chemically, elastically, gravitationally, magnetically and nuclearly.
  • Energy is transferred mechanically, electrically, by heating, or by radiation — it is never created or destroyed.
  • Wasted energy usually ends up heating the surroundings and becomes less useful (dissipated).

Equations you must know

  • Kinetic energy = ½ × mass × speed².
  • Gravitational potential energy = mass × gravitational field strength × height.
  • Efficiency = useful output energy ÷ total input energy (× 100 for a %).
  • Power = energy transferred ÷ time.

Key terms

Dissipated
Spread out to the surroundings and no longer useful.
Efficiency
The fraction of input energy that is transferred usefully.
Power
Energy transferred per second, measured in watts.
Sankey diagram
A diagram showing energy transfers where arrow width represents the amount of energy.
Specific heat capacity
The energy needed to raise the temperature of 1 kg of a substance by 1°C.
Insulator (thermal)
A material that reduces the rate of thermal energy transfer.

Common exam traps

  • Forgetting to square the speed in the kinetic energy equation.
  • Saying energy is 'lost' — it is dissipated, not destroyed.
  • Mixing up power (W) and energy (J).

Practice questions with answers

  1. 1. Calculate the kinetic energy of a 2 kg ball moving at 5 m/s.

    Answer: 25 J

    ½ × 2 × 5² = 25 J.

  2. 2. A device takes in 200 J and usefully transfers 50 J. What is its efficiency?

    • • 4%
    • • 25%
    • • 40%
    • • 75%

    Answer: 25%

    50 ÷ 200 = 0.25 = 25%.

  3. 3. State one way to reduce unwanted energy transfer in a machine.

    Answer: Lubricate moving parts to reduce friction.

    Insulation and streamlining are also valid.

  4. 4. Which unit is power measured in?

    • • Joule
    • • Newton
    • • Watt
    • • Pascal

    Answer: Watt

    1 watt = 1 joule per second.

  5. 5. Which energy store increases when an elastic band is stretched?

    • • Kinetic
    • • Elastic potential
    • • Chemical
    • • Thermal

    Answer: Elastic potential

    Stretching or compressing a stretchy object stores energy elastically.

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