F.3 phy questions~20pts

1. The reasons are:
(i) black surface is a good absorber of heat than silvery surface; and
(ii) heat loss by convection and radiation of an object can be regarded as approximately equal to the difference in temperature between the object and the surroundings (this is known as Newton's Law of Cooling).

Based on the above two facts, it is not difficult to see why the naswer is option B.

2. Air is a bad conductor of heat. Hence, conduction is not a main process for heat transfer by air.

But the air above the hot filament is heated up by the filament and becomes less dense, the air thus rises, carrying the heat away. This is the process of convection.

Any object with temperature above the surroundings loses heat by the process of radiation.

Therefore, the heat-loss processes involved are convection and radiation.

3. A hot air balloon obtains its lifting force by the hot air (which is less dense than the surrounding air) filled in the balloon. Once the hot air cools down, the balloon loses its lifting force and descends. Thereofre, for a balloon to travel very long distance, it is essential not to have the hot air in the balloon to cool down.

By using a light-colour on the balloon surface helps to reduce heat loss by radiation, as light or silvery colour is a poor absorber of heat. The temperature of the hot air thus could be maintained with the burning of less fuel.