Conduction · Grade 9
In Grade 9 you learn: insulation, payback and U-values.
Conduction is how energy passes through a solid. Particles at the hot end vibrate more and pass energy to their neighbors, and in metals free electrons carry it quickly too, so metals are good conductors. Materials that trap air, such as wool, foam and double glazing, are good insulators and slow the energy down.
Key facts
- Rate of heat loss through a wall or window = U × area × temperature difference.
Worked examples
Example 1
Why does a thick woollen sweater keep you warm on a cold day?
Answer: wool traps lots of air, which is a poor conductor; trapped air cannot move, so it stops convection too, and less energy is transferred from your body
Example 2
A wall has area 20 m² and U-value 0.30 W/m²°C. It is 21°C inside and 1°C outside. How much energy passes through the wall each second?
Answer: 120 W (120 J each second)
Example 3
A double-glazed window has area 2.5 m² and U-value 2.8 W/(m² °C). Inside is 21°C, outside 3°C. Find the rate of heat loss.
Answer: 126 W
Example 4
A single-glazed window of area 2.0 m² has a U-value of 5.0 W/m²°C. It is replaced by double glazing with a U-value of 2.0 W/m²°C. It is 20°C inside and 5°C outside. How much less power is lost through the window?
Answer: 90 W