Conduction and convection

Questions:
1. Why is the outlet pipe at the top of a water tank?
2. Why is the freezing compartment of a fridge at the top of the fridge?
3. Why is it not vital to have the top of a chest freezer closed all the time?
4. What is the main difference between conduction and convection?
5. What is the main way in which heat is given out by an ordinary house radiator?
6. How does a miner's lamp prevent explosions?
7. Explain why foam filling in walls helps to stop conduction.

Answers:

1. Hot water rises.

2. Hot air rises and is then cooled and sinks back to the bottom of the fridge.

3. Cold air falls and so hot air is rises out of the top of the freezer.

4. Convection is a movement of the material as a whole as in a fluid. You cannot have convection in a solid.

5. Conduction through the radiator walls and then convection in the surrounding air.

6. The gas inside the miner's lamp needs to be hot enough to catch fire. The copper gauze surrounding it conducts the heat away from the lamp into the miner's hand or the wall of the tunnel. This means the gas stays cool, it does not catch fire and so explosions are avoided.

7. Yes - air is a better insulator than polystyrene but polystyrene foam contains a lot of air pockets. The movement of cold air in a cavity wall takes heat away by convection. The foam in the walls prevents the air from moving about, this reduces convection and so the cold air cannot circulate in the cavity. This reduces heat loss by convection.