1. GCSE ENERGY; THERMAL ENERGY TRANSFER 2
By the end of this presentation you should be able to:
Describe how the vacuum flask works to reduce heat transfer.
Discuss ways in which heat transfer is reduced in and around the home.

2. The structure of a vacuum flask
Screw on cover (plastic and cork are good insulators)
Vacuum. Only infra red radiation can pass through a vacuum
The inside walls of the vacuum is silver. This reflects any radiant heat that enters.

3. How heat transfer is stopped.
The screw cover prevents heat loss by convection as the air above the hot liquid gets warm.
Making the screw cover out of materials which are poor conductors reduces heat loss by conduction.
The inside wall of the vacuum may become hot as heat is conducted through the glass layer, but …..

4. How heat transfer is stopped.
Heat cannot be lost in this region by conduction or convection since it is a vacuum!
When the glass walls of the vacuum gets warm, it will radiate only a small amount of heat since shiny surfaces are poor emitters of radiation.
Hot liquid on this side
Any heat that is radiated cannot escape however as it will be reflected by the second silvery surface
Cooler air surrounding the thermos

5. Preventing heat transfer at home.
30% of
total
heat loss occurs through the walls.
25% of
heat loss occurs through the windows.
20% of
heat loss occurs through the roof.
15% of total heat loss occurs through draughts under doors and other openings (like chimneys).
10% of
heat loss occurs through the floor.

6. Preventing heat transfer at home.
Heat energy is always transferred from areas where the temperature is high to areas where the temperature is lower.
Look carefully at the thermal image of the lower half of a door. Do you think it was taken in winter or summer?
This image was taken in winter. The dark colours at the bottom of the door show lower temperatures where a draught is allowing heat to escape to the cold outside.

7. Preventing heat transfer at home.
Draught excluders cut down significantly on heat swept away by these convection currents. Look for them below doors and around windows.

8. Preventing heat transfer at home.
Double glazed windows consist of two panes of glass with a layer of air trapped in between. This reduces conduction losses since there are two panes, so increasing the thickness of the window. Also, the air trapped in between is a very good insulator. This reduces conduction losses further

9. Preventing heat transfer at home.
Insulating your loft is one of the cheaper ways of insulating your home and goes a long way to reducing heat lost through the roof by conduction.
It also prevents energy being wasted in heating the loft (with heat that is conducted through the ceiling of the rooms below!)

10. Preventing heat transfer at home.
Cavity walls (double layers of bricks) provide a thicker barrier to the outside so reduces conduction losses.
It also traps a layer of air between the walls so insulates the house further.
Installing a layer of “cavity wall insulation” is even better than just air. The material used is a very god insulator, but also stops heat from being lost by convection since the air cannot move about in the space freely.