Energy Transfers and Power

IGCSE Edexcel Physics
4.1–4.17 Energy Transfers, Efficiency, Work and Power
Key Concepts: Energy is transferred between stores. Use conservation of energy and efficiency. Thermal transfer occurs by conduction, convection and radiation.

Section A: Units

1. State the SI units for mass, energy, distance, speed, acceleration, force, time and power. [8]

Section B: Energy Stores and Transfers

2. Name the energy store for each scenario and the main transfer pathway. [4]
(a) A stretched spring released to move a toy
(b) A kettle heating water
3. State the principle of conservation of energy. [2]
4. Calculate GPE for a 3 kg object lifted 4 m ($g = 9.8\,N/kg$). [2]

Section C: Efficiency and Sankey Diagrams

5. A motor transfers 2000 J of energy and 600 J is useful. Calculate efficiency. [2]
6. Describe what a Sankey diagram shows. [2]

Section D: Thermal Energy Transfer

7. Describe conduction, convection and radiation with one example each. [6]
8. Explain one everyday example of convection (e.g. sea breeze or radiators). [2]
9. Explain how emission and absorption of radiation depend on surface color and temperature. [3]
10. Describe a practical to compare thermal energy transfer by conduction, convection and radiation. [4]
11. Explain how to reduce unwanted energy transfer in a house. [3]

Section E: Work and Power

12. A 50 N force moves a box 3 m. Calculate the work done. [2]
13. State that work done equals energy transferred. [1]
14. A 10 kg object moves at 4 m/s. Calculate its kinetic energy. [2]
15. A 2 kg object is lifted 3 m and then falls back. Explain how GPE, KE and work are linked by conservation of energy. [3]
16. State that power is the rate of energy transfer or the rate of doing work. [1]
17. A motor does 600 J of work in 20 s. Calculate the power. [2]
Total marks: 45

Mark Scheme

1. kg, J, m, m/s, $m/s^2$, N, s, W [8]
2. Elastic -> kinetic (mechanically); electrical -> thermal (heating) [4]
3. Energy cannot be created or destroyed, only transferred/stored [2]
4. $GPE = 3 \times 9.8 \times 4 = 117.6\,J$ [2]
5. $\text{efficiency} = 600/2000 \times 100 = 30\%$ [2]
6. Shows energy transfers with arrow widths proportional to energy [2]
7. Conduction through solids; convection in fluids; radiation as IR (any six points) [6]
8. Warm fluid rises, cool fluid sinks (any valid example) [2]
9. Dark/dull surfaces emit/absorb more; shiny/light surfaces emit/absorb less; hotter objects emit more [3]
10. Compare conduction in rods, convection in water, radiation with black/shiny cans (any correct outline) [4]
11. Insulation, double glazing, draught excluders, reflective surfaces (any three) [3]
12. $W = Fd = 50 \times 3 = 150\,J$ [2]
13. Work done equals energy transferred [1]
14. $KE = \tfrac{1}{2}mv^2 = 0.5 \times 10 \times 16 = 80\,J$ [2]
15. Energy converts between GPE and KE; total conserved; work transfers energy [3]
16. Power is energy transfer per unit time [1]
17. $P = W/t = 600/20 = 30\,W$ [2]