Energy transfers in a system: Students should be able to describe how the rate of cooling of a building is affected by the thickness and thermal conductivity of its walls.

The idea

A building cools because energy is transferred from its warmer interior to the cooler outside through its walls. The rate of this transfer depends on the wall's thermal conductivity and thickness. A material with high thermal conductivity, such as metal, transfers energy quickly, so the building cools rapidly. A material with low thermal conductivity, such as foam or brick, transfers energy slowly, so cooling is slower. Increasing wall thickness also slows cooling because energy must conduct through more material, reducing the rate of transfer for the same temperature difference. In practice, thick walls made from a low-conductivity material keep a building warmer for longer.

What earns the marks

• State that energy is transferred from the warmer inside of the building to the cooler outside through the walls.
• Explain that a higher thermal conductivity means a faster rate of energy transfer, so the building cools more quickly.
• Explain that a lower thermal conductivity means a slower rate of energy transfer, so the building cools more slowly.
• Explain that increasing the thickness of a wall reduces the rate of energy transfer and therefore slows cooling.
• Compare walls correctly, for example a thick low-conductivity wall cools a building more slowly than a thin high-conductivity wall.
• Link the description to the temperature difference between inside and outside as the driving factor for the transfer.

Common mistakes and how to put them right

• Saying that thick walls 'absorb cold' or 'keep cold out' rather than reducing the rate of energy transfer by conduction.

  Put it right by describing energy transfer from hot to cold and the effect of thickness on that rate.

• Confusing thermal conductivity with thickness, for example claiming a thicker wall has higher thermal conductivity.

  Put it right by treating them as separate variables that each affect the cooling rate.

• Stating that a high thermal conductivity keeps a building warm.

  Put it right by explaining that high conductivity means faster energy transfer out of the building, so it cools faster.

Exam tips

• Name both variables explicitly in your answer: thermal conductivity and thickness of the walls.
• Use comparative language such as 'faster', 'slower', 'higher' and 'lower' to make the direction of each effect clear.
• Apply the ideas to a real example, such as a cavity wall with insulating foam, to show understanding rather than repeating the statement.