IB · B · extended response
Selecting insulation from cooling evidence
The particulate nature of matter · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- discriminating
- Marks
- 6
- Topics
- 1
- Answer
- Complete
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.
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
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(a)
Vessel B reaches 70°C after 6 min. Compare average rates of energy loss for A and B over the first 6 min.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
For A, energy loss is mcΔT = 0.50(4200)(80−55) = 52500 J over 360 s, giving about 146 W.
- 2
For B, energy loss is 0.50(4200)(80−70) = 21000 J, giving 58.3 W.
- 3
These are time averages. A's decreasing temperature drop per interval supports a cooling rate that falls as its temperature approaches the room temperature.
(b)
Explain the role of conduction, convection, evaporation and radiation, and propose a fair test separating the lid from the shiny finish.
Build a chain of claim, evidence and physics reasoning. Address more than one relevant factor, identify a limitation or assumption, and finish with a conclusion that is conditional on the evidence rather than absolute.
- 1
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.
- 2
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.
- 3
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.
Complete marking guidance
Use these worked steps alongside the part-by-part answers. They explain the method and mark allocation; they do not add extra marks.
- 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.
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