AQA Physics 6.1.2.1
Thermal conductivity and reducing unwanted energy transfers
Why a house with thick, well-insulated walls stays warm for longer. One idea explains walls, lofts, lagging, double glazing and flasks.
Combined Science · Higher tier
Start here
Two spoons, one pan of hot soup.
A metal spoon and a wooden spoon are put into the same pan of hot soup at the same moment. You keep touching the top of each handle.
If you had to guess, which handle gets hot first?
Conduction · conductors and insulators
Higher conductivity, higher rate.
The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across it.
Metals are good conductors: they have free electrons that carry energy through the metal quickly, as well as vibrating particles. Wood, plastic, wool and air are poor conductors, so they make good insulators.
Air is a very poor conductor, because its particles are far apart and only occasionally collide. Trapped in small pockets, it also can't move about, so it can't form convection currents.
A house wall · predict, then change it
Thickness and conductivity both set the rate of cooling.
Choose a change to the wall. Predict what it does to how fast the house cools, then apply it.
Now · thin stone
Reducing unwanted energy transfers
Insulation slows the transfer. It never stops it.
Loft insulation
A thick layer of fibreglass, a poor conductor full of trapped air, slows energy transfer through the roof.
Cavity wall insulation
Foam traps air in small pockets: trapped air is a very poor conductor, and it stops convection currents forming in the cavity.
Double glazing
A layer of air or gas between two panes conducts far less than glass alone.
Lubrication
Oil reduces friction between moving parts, so less energy is dissipated as thermal energy.
Think again · cold to the touch
“The metal bench feels colder than the wooden one, so it must be at a lower temperature.”
Your call
ChooseA metal bench and a wooden bench have been outside all night. Both are at 8 °C. Why does the metal one feel colder?
Sort
Higher or lower rate of energy transfer?
Compare each with the alternative in brackets.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- Say how the rate of cooling changes with thickness and with conductivity.
- Explain
- Property of the material, then the rate of energy transfer, then the result.
- Suggest
- Apply the idea to something new: a flask, a cool bag, a pipe.
Key fact
The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across it. A building cools more slowly if its walls are thicker and have a lower thermal conductivity.
Key note · AQA 6.1.2.1
Thermal conductivity
- The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across it.
- Metals are good conductors (free electrons); wood, plastic, wool and air are poor conductors.
- A building cools faster through thinner walls and walls of higher thermal conductivity.
- Trapped air is a very poor conductor and cannot form convection currents: cavity, loft and double glazing use this.
- Insulation slows energy transfer; it never stops it. Lubrication reduces energy wasted through friction.
- A metal feels colder than wood at the same temperature because it conducts energy from your hand faster.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Not sure why trapped air works so well?
The wall model compares rates only: doubling the thickness, or halving the conductivity, roughly halves the rate. No values are needed or given.