OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe how the rate of cooling of a building is affected by the…
Full specification wording
Power and efficiency: describe how the rate of cooling of a building is affected by the thickness and thermal conductivity of its walls (qualitative only)
A warm building cools because energy is transferred through its walls to the cooler outside.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 8 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A warm building cools because energy is transferred through its walls to the cooler outside. The rate of cooling depends on the walls' thickness and thermal conductivity, treated qualitatively here. A thicker wall slows the rate of energy transfer, so the building cools more slowly. A wall made from a material of lower thermal conductivity, such as one containing trapped air or insulating foam, also slows the transfer and so reduces the cooling rate. Combining greater thickness with lower thermal conductivity gives the slowest cooling. A thin wall of a good conductor lets energy pass quickly, so the building cools fastest. The comparison is about rate, not total energy, and energy is conserved throughout.
What earns the marks
- States that increasing the thickness of a wall reduces the rate of energy transfer through it, so the building cools more slowly.
- States that using a wall material of lower thermal conductivity reduces the rate of energy transfer, so the building cools more slowly.
- Explains that a thin wall made of a good conductor allows rapid energy transfer, so the building cools fastest.
- Explains that combining greater thickness with lower thermal conductivity gives the slowest rate of cooling.
- Recognises that the comparison concerns the rate of cooling rather than the total energy transferred, which is conserved.
Common mistakes and how to put them right
Claims that a thicker wall makes a building cool faster.
Put it right: Explain instead that greater thickness slows the rate of energy transfer.
Confuses thermal conductivity with thickness.
Put it right: Treat them as separate factors, where lower conductivity and greater thickness each slow cooling.
States that insulation stops cooling completely.
Put it right: Explain instead that it reduces the rate of cooling.
Exam tips
- Name both variables explicitly in the answer: thickness and thermal conductivity.
- Use comparative language such as 'slower rate of cooling' to match the qualitative demand of the statement.
- Keep the focus on rate of energy transfer rather than on total energy, which stays conserved.
Try the lesson
The start of the Power and efficiency: describe how the rate of cooling of a building is affected by the thickness and thermal conductivity of its walls (qualitative only) lesson, made from checked teaching notes for this specification point.
Have a go first
A house is heated on a cold day. Energy passes from the warm rooms through the walls to the colder air outside. What happens to the rate of this energy transfer if the walls are made thicker?
Listen
A warm building cools because energy is transferred through its walls to the cooler outside. The rate of cooling depends on the walls' thickness and thermal conductivity, treated qualitatively here. A thicker wall slows the rate of energy transfer, so the building cools more slowly. A wall made from a material of lower thermal conductivity, such as one containing trapped air or insulating foam, also slows the transfer and so reduces the cooling rate. Combining greater thickness with lower thermal conductivity gives the slowest cooling. A thin wall of a good conductor lets energy pass quickly, so the building cools fastest. The comparison is about rate, not total energy, and energy is conserved throughout. A strong answer earns marks for these points. States that increasing the thickness of a wall reduces the rate of energy transfer through it, so the building cools more slowly. States that using a wall material of lower thermal conductivity reduces the rate of energy transfer, so the building cools more slowly. Explains that a thin wall made of a good conductor allows rapid energy transfer, so the building cools fastest. Explains that combining greater thickness with lower thermal conductivity gives the slowest rate of cooling. Exam tip: Name both variables explicitly in the answer: thickness and thermal conductivity.
Key words
Say what each word means before you turn the card.
Rate of energy transfer
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Thermal conductivity
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Thickness of walls
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Cooling of buildings
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See every definition
- Rate of energy transfer
- How quickly energy passes through a wall to the cooler outside; a thicker wall slows it, so the building cools more slowly.
- Thermal conductivity
- A property of the wall material; a material of lower thermal conductivity slows the transfer and so reduces the cooling rate.
- Thickness of walls
- Greater thickness slows the rate of energy transfer, so the building cools more slowly.
- Cooling of buildings
- A warm building cools because energy is transferred through its walls to the cooler outside; the comparison concerns the rate, not the total energy, which is conserved.
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