Describe the effects of the thickness and thermal conductivity of the…

The idea

A building cools because energy is transferred by conduction through its walls from the warmer inside to the cooler outside. Two wall properties control how fast this happens. Thickness: a thicker wall slows the rate of cooling, because energy must travel through more material, so the temperature inside falls more slowly. Thermal conductivity: a material with low thermal conductivity (a good insulator, such as foam) slows the rate of cooling, while a high-conductivity material (such as metal) lets energy pass quickly and the building cools faster. Combine the two: thick walls of low-conductivity material give the slowest cooling; thin walls of high-conductivity material give the fastest. Keep the temperature difference and wall area the same when comparing, so only thickness or conductivity changes.

What earns the marks

• States that a thicker wall reduces the rate of cooling because energy takes longer to conduct through more material.
• States that a lower thermal conductivity reduces the rate of cooling because the material conducts energy more slowly.
• Identifies that a higher thermal conductivity increases the rate of cooling for the same thickness.
• Combines both factors to conclude that thick walls of low-conductivity material cool most slowly and thin walls of high-conductivity material cool fastest.

Common mistakes and how to put them right

• Confuses thermal conductivity with heat capacity.

  Put it right: Keep the focus on how quickly energy conducts through the wall.

• Claims a thicker wall makes a building cool faster.

  Put it right by explaining that more material slows conduction.

• Treats a high thermal conductivity as good insulation.

  Put it right by stating that high conductivity speeds up cooling.

• Ignores the need to hold other variables constant.

  Put it right: Keep area and temperature difference the same when comparing walls.

Exam tips

• Name the property being changed (thickness or thermal conductivity) and state the direction of its effect on the rate of cooling.
• Use the word 'rate' and link it to how quickly the inside temperature falls, not to the total energy lost.
• When a question gives two walls, compare them one property at a time and say which cools faster and why.
• Avoid quoting numbers unless the question supplies them; a qualitative answer needs a clear cause-and-effect link.