Key Concepts: Energy stores: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, electrostatic, magnetic. Energy is conserved — it cannot be created or destroyed. Efficiency = useful output energy ÷ total input energy (× 100%). Sankey diagrams show energy transfers with arrow widths proportional to energy.
Section A — Energy Stores and Conservation
1. Name the main energy store for each: [4]
a) A compressed spring
b) A moving car
c) A hot cup of tea
d) Food
2. State the principle of conservation of energy. [2]
3. Describe the energy transfers when a ball is dropped from a height and bounces. Include at least three stages. [3]
Section B — Efficiency
4. Write the equation for efficiency. Give two forms: one as a fraction and one as a percentage. [2]
5. A motor takes in 500 J of electrical energy and produces 150 J of useful kinetic energy. Calculate the efficiency as a percentage. [2]
6. A light bulb is 8% efficient. It uses 200 J of electrical energy. Calculate: [4]
a) The useful light energy output
b) The energy wasted as heat
7. A heater is 70% efficient and the input energy is 800 J. Calculate the useful energy output. [2]
Section C — Sankey Diagrams
8. Describe what a Sankey diagram shows and state what the width of each arrow represents. [2]
9. A car engine takes in 1000 J of chemical energy. It produces 300 J of kinetic energy, 100 J of sound energy and wastes the rest as heat. Describe the Sankey diagram you would draw for this engine and calculate its efficiency. [4]
Total marks: 25
Mark Scheme
1. a) Elastic (potential) energy [1]; b) Kinetic energy [1]; c) Thermal (internal) energy [1]; d) Chemical energy [1] [4]
2. Energy cannot be created or destroyed [1]; it can only be transferred from one store to another or from one form to another [1] [2]
3. At height: gravitational potential energy store [1]; as it falls: GPE transfers to kinetic energy [1]; on impact: KE transfers to elastic PE and thermal energy; on bounce: elastic PE back to KE [1] [3]
4. Efficiency = useful output energy / total input energy [1]; efficiency (%) = (useful output / total input) × 100 [1] [2]
5. Efficiency = 150/500 × 100 = 30% [2]
6. a) Useful output = 8% × 200 = 16 J [2]; b) Wasted = 200 − 16 = 184 J [2] [4]
7. Useful output = 0.70 × 800 = 560 J [2]
8. A Sankey diagram shows the energy transfers through a device [1]; the width of each arrow is proportional to the amount of energy [1] [2]
9. Input arrow: 1000 J (chemical); useful output: 300 J (kinetic) + 100 J (sound); wasted: 600 J (thermal) [2]; efficiency = (300 + 100)/1000 × 100 = 40% [2] [4]