Spec links: 4.1.1.1 Energy stores and systems; 4.1.1.4 Power; 4.1.2.1 Energy transfers in a system (thermal conductivity and insulation); 4.1.2.2 Efficiency · Reading time: about 4 minutes
The house that glowed
On a frosty winter night, a group of engineers walk down a street with a special camera. It is a thermal imaging camera, and it shows warm things as bright orange and cold things as dark blue. Most houses look dull and blue. But one house glows. Its roof is bright orange, and so are its windows. That house is wasting money, because the energy its owners paid for is escaping into the night air.
Where does the energy go?
When you turn on the heating, energy is transferred to the thermal energy store of the air and walls inside your home. The inside becomes warmer than the outside. Energy then transfers from the warm inside to the cold outside. It keeps doing this as long as there is a temperature difference.
This energy is not destroyed. Energy can be transferred usefully, stored or dissipated, but it cannot be created or destroyed. The energy that escapes ends up spread out in the surroundings, warming them very slightly. We say it has been dissipated, or “wasted”, because we can no longer use it.
In a typical house without insulation, about a quarter of the energy is lost through the roof and about a third through the walls. More escapes through windows, doors and gaps.
Slowing it down
Energy passes through solid walls by conduction. Some materials let energy through quickly. We say they have a high thermal conductivity. Metals such as copper and aluminium are good examples. Other materials, such as wool, foam and still air, have a low thermal conductivity. They are good thermal insulators.
The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across it. The rate also depends on thickness. A thicker wall transfers energy more slowly than a thinner wall of the same material.
That is why homes use:
- Loft insulation: a thick layer of fibre that traps air.
- Cavity wall insulation: foam or fibre that fills the gap between two layers of brick.
- Double glazing: two panes of glass with a gap of air or gas between them.
- Draught excluders: strips that block gaps around doors.
Getting more for your money
Heating systems also waste some energy themselves. A gas boiler transfers energy from the chemical store of the gas. Most of it heats the water, but some escapes up the flue. We describe how good a device is using its efficiency:
efficiency = useful output energy transfer ÷ total input energy transfer
Suppose a boiler takes in 50 000 J of energy from gas, and 45 000 J of it usefully heats the water. Its efficiency is 45 000 ÷ 50 000 = 0.9, or 90%. The other 5000 J is wasted.
We can also use power. A heater with a power of 2000 W transfers 2000 J of energy every second. Using E = P t, in 10 minutes (600 s) it transfers 2000 × 600 = 1 200 000 J.
Back to the glowing house
The engineers knock on the door. They suggest a thick layer of loft insulation and double glazing. Insulating does not make the heating any more powerful. Instead, it lowers the rate of energy transfer out of the house. The rooms stay warm for longer, so the heating can be on for less time. The family pays less, and less fuel is burned.
Next winter, the camera shows a much duller orange.
Key words
- Thermal energy store: the energy an object has because of its temperature.
- Dissipated: energy that has spread out into the surroundings in a way that is no longer useful (“wasted”).
- Thermal conductivity: a measure of how quickly energy is transferred through a material by conduction.
- Thermal insulator: a material with a low thermal conductivity.
- Efficiency: useful output energy transfer divided by total input energy transfer.
- Power: the rate at which energy is transferred or the rate at which work is done.
Questions
- What does a thermal imaging camera show as bright orange? [1 mark]
- Name two materials from the article that have a low thermal conductivity. [2 marks]
- Give two features of a house that reduce the rate of energy transfer out of it. [2 marks]
- Explain why a thick layer of loft insulation reduces energy losses through the roof. [3 marks]
- A student says, “The energy lost from a house is destroyed.” Explain why the student is wrong. [2 marks]
- An old boiler takes in 80 000 J of energy from gas. 60 000 J usefully heats the water. Calculate the efficiency of the boiler. Give your answer as a percentage. [3 marks]
- An electric heater has a power of 1500 W. Calculate the energy it transfers in 20 minutes. Give your answer in joules. [3 marks]
- Describe how a family could reduce the cost of heating their home. Use ideas about energy transfer, thermal conductivity and efficiency in your answer. [6 marks]
Show answers
- Warm things / areas at a higher temperature (1)
- Any two from: wool; foam; still air / air; fibre (1) (1)
- Any two from: loft insulation; cavity wall insulation; double glazing; draught excluders; thicker walls (1) (1)
- Loft insulation has a low thermal conductivity / traps air, which is a poor conductor (1); a thicker layer means a lower rate of energy transfer by conduction (1); so energy is transferred out of the house more slowly / less energy is lost in the same time (1)
- Energy cannot be created or destroyed (1); it is transferred to / dissipated in the surroundings (and warms them slightly) (1)
- efficiency = 60 000 ÷ 80 000 (1); efficiency = 0.75 (1); efficiency = 75% (1)
- t = 20 × 60 = 1200 s (1); E = 1500 × 1200 (1); E = 1 800 000 J (1)
- Indicative content:
– energy is transferred from the warm inside to the colder outside
– the rate of energy transfer can be reduced using materials with a low thermal conductivity
– loft insulation traps air and reduces energy transfer through the roof
– cavity wall insulation reduces energy transfer through the walls
– double glazing reduces energy transfer through windows
– draught excluders reduce energy lost through gaps
– thicker insulation reduces the rate of energy transfer further
– replacing an old boiler with a more efficient one means more of the input energy is transferred usefully
– less energy is wasted, so less fuel is needed / heating can be on for less time, so cost is lower
Level 3 (5–6 marks): a detailed and coherent description of several methods, each linked to reduced energy transfer, using ideas of thermal conductivity and efficiency, and linked to lower cost; Level 2 (3–4 marks): some methods described with some links to reduced energy transfer or efficiency; Level 1 (1–2 marks): one or more methods named with little or no explanation
More reading: See all 30 GCSE Physics comprehension articles