對流的問題 (20點)

1)點解燈罩須在頂部開一個洞?
Air warmed by the glowing lamp has a lower density and tends to rise. The hole is to let the hot air to escape, such that cooler air could come in from the other side. This serves as a cooling mechanism, otherwise the temperature of the lamp may get too hot.

2)藏在棉花和羽毛裏的空氣能有效地阻止散熱,但點解被空氣包圍着的熱煎鍋散熱那麼快?
The air surrounding the hot pot is free to move and could carries away the heat. On the other hand, air insode the pores of wool and feather could not move away.

3)發泡膠盒怎樣令食物保溫?
Foam is a bad conductor of heat, thus preventing heat loss by conduction.

4)電熱水瓶的發熱線通常安裝在近瓶底的地方.試解釋點解當發熱線安裝在瓶頂時,水加熱會較慢.
Water heated by the heater has a lower density and thus rises, cold water comes in and is heated by the heater. A convection current can be set up easily if the heater is placed near the bottom of the hot pot. Should it be placed near the top, such convection current cannot be set up, as water, after being heated, always stays on the top.

5)試解釋點解對流不會在真空中發生.
In the convection process, heat transfer is due to movement of substance that is being heated. There is no substance in vacuum, thus no convection can take place.