Thermal Energy Transfer

IGCSE Edexcel Physics
4.6–4.10 Conduction, convection, radiation and reducing heat loss
Key Concepts: Thermal energy transfers by conduction (solids — vibrating particles), convection (fluids — density differences) and radiation (infrared waves — no medium needed). Dull/dark surfaces absorb and emit radiation better than shiny/light surfaces.

Section A — Conduction

1. Describe how thermal energy is transferred by conduction in a metal rod when one end is heated. [3]
2. Explain why metals are better conductors than non-metals. [2]
3. Explain why air is a poor conductor of heat and how this is used in insulation. [2]

Section B — Convection

4. Describe how a convection current is set up when water in a beaker is heated from below. [4]
5. Explain why convection cannot occur in solids. [2]

Section C — Radiation

6. State what type of wave carries thermal radiation and state one property that differs from conduction and convection. [2]
7. Compare the emission of thermal radiation from a dull black surface and a shiny silver surface at the same temperature. [2]
8. A solar panel is designed to absorb as much radiation as possible. State what colour the surface should be and explain why. [2]

Section D — Reducing Heat Loss

9. For each method of reducing heat loss from a house, state which method of thermal transfer it mainly reduces: [4]

a) Loft insulation

b) Double glazing

c) Cavity wall insulation

d) Foil-backed insulation behind radiators

Total marks: 23

Mark Scheme

1. Particles at the hot end vibrate more vigorously [1]; they collide with neighbouring particles, passing on energy [1]; this process continues along the rod toward the cooler end [1] [3]
2. Metals have free electrons [1]; these move quickly through the metal and transfer energy faster than particle vibrations alone [1] [2]
3. Air particles are far apart and rarely collide [1]; trapped air layers in insulation (e.g. fibreglass, wool) prevent convection as well as conduction [1] [2]
4. Water at the bottom heats up and expands [1]; becomes less dense and rises [1]; cooler, denser water from above sinks to replace it [1]; a circular convection current is established [1] [4]
5. Particles in solids are fixed in position [1]; they cannot move freely through the material to carry energy by bulk flow [1] [2]
6. Infrared (electromagnetic) waves [1]; radiation can travel through a vacuum (no medium needed) [1] [2]
7. Dull black surface emits more radiation [1]; shiny silver surface emits less (reflects more) [1] [2]
8. Dull/dark (black) surface [1]; dark surfaces are better absorbers of radiation [1] [2]
9. a) Loft insulation — reduces conduction (and convection) [1]; b) Double glazing — reduces conduction and convection through the gap [1]; c) Cavity wall insulation — reduces convection in the air gap [1]; d) Foil-backed insulation — reduces radiation [1] [4]