4PH1

Energy Stores & Transfers

Energy Resources & Energy Transfers

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

stable
Medium
Stable10%

GPE, KE and efficiency calculations are core calculation questions in every series.

The principle

  • The states:

Energy cannot be created or destroyed; it can only be transferred from one store to another.

  • For any closed system, the total energy across all the stores stays constant
  • Energy is never truly lost, but it can be (spread out) into the surroundings by heating or radiation; once dissipated, it is wasted because the original task cannot reclaim it

Useful and wasted energy

  • Every real transfer produces a mixture of:
    • Useful energy in the store the designer wanted (the kinetic store of the moving fan, the thermal store of the food)
    • Wasted energy spread around the surroundings (heating from friction, sound carried away into the air, infrared radiation off hot casings)
  • The total of useful + wasted output always equals the input, which is conservation of energy in practice
Exam tip

Conservation of energy

Stating the principle of conservation of energy is a 1-marker, so you need to know energy cannot be created or destroyed, only transferred from one store to another. Both halves — "created" and "destroyed" — are needed; "energy cannot be lost" alone doesn't score.

Worked example

Calculating kinetic energy

A 4 kg ball rolls along a surface at 6 m/s. Calculate its kinetic energy.

Solution:

  • Write the formula: KE = ½ × mass × speed²
  • Substitute: KE = ½ × 4 × 6²
  • Square the speed first: 6² = 36
  • KE = ½ × 4 × 36 = 2 × 36 = 72 J