Cambridge IGCSE Physics · topic 2.3
Transfer of thermal energy
Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.
Extra challenging questions
Work through the linked stages and explain what each result means.
Selecting insulation from cooling evidence
Optional enrichment: this challenge may extend beyond the usual syllabus depth and assumes confidence with algebra and graph interpretation. Two identical vessels each contain 0.50 kg of water initially at 80°C in a 20°C room. Vessel A has a matte black exposed surface; B has a shiny jacket and an insulating lid. The measured cooling data are supplied. Use water specific heat capacity 4200 J kg⁻¹ K⁻¹; neglect vessel heat capacity. Supplied data (Time / min; Water temperature / °C): (0, 80), (2, 70), (4, 62), (6, 55). (a) Vessel B reaches 70°C after 6 min. Compare average rates of energy loss for A and B over the first 6 min. [3 marks] (b) Explain the role of conduction, convection, evaporation and radiation, and propose a fair test separating the lid from the shiny finish. [3 marks]
Water cooling in vessel A
Supplied question data. Read the labelled axes and use the observations to test the physical model.
Vertical axis (y): Water temperature / °C
Horizontal axis (x): Time / min
Read the exact data as a table
| Time / min | Water temperature / °C |
|---|---|
| 0 | 80 |
| 2 | 70 |
| 4 | 62 |
| 6 | 55 |
These are the supplied values, not a worked answer. Lines join the supplied values; they are not a fitted model.
Show a hint
Begin with part (a): Vessel B reaches 70°C after 6 min. Compare average rates of energy loss for A and B over the first 6 min.
Reveal the worked solution
- For A, energy loss is mcΔT = 0.50(4200)(80−55) = 52500 J over 360 s, giving about 146 W.
- For B, energy loss is 0.50(4200)(80−70) = 21000 J, giving 58.3 W.
- These are time averages. A's decreasing temperature drop per interval supports a cooling rate that falls as its temperature approaches the room temperature.
- Insulation reduces conduction through the walls; a lid restricts warm-air convection and evaporation. A shiny surface lowers thermal radiation exchange compared with a similar matte surface.
- Use a factorial comparison: identical vessels with/without a lid and with shiny/matte surfaces, holding water mass, starting temperature, geometry, room and probe depth constant.
- Repeat each setup and log the room temperature. Comparing A and B alone shows the combined improvement but cannot apportion the effect between lid and finish.
Answer (a) A loses about 146 W on average; B loses about 58.3 W. (b) Change one feature at a time with equal initial conditions; a combined design comparison cannot identify each feature's individual effect.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.