Convection current

convection currents in water，你係咪指海洋個 d？如果係的話，it is like this :

1. when the sea water passes through equator, it will become warm and rises.

2.when it flows to the poles ( north pole and south pole), as it has travelled for a long distance, it will have used up most of its energy and becomes cool down. ( heat is released when using up energy. thats why our breath is warm when we breathe out cos respiration uses up energy)

3. as we all know, cold water sinks and it will flow back to the equator to get heat up again.

4. it becomes a heat transfer belt with hot water rising and stays on top, moving towards the poles and cold water sinking and stays at the bottom, moving towards the equator.


呢個係係 wikipedia 搵到 gei explanation，不過可能有 d 難明:
http://en.wikipedia.org/wiki/Convective_heat_transfer


如果你問 gei water convection current 係煲水個 d，個 explanation :

1. when water gets heated up, it rises.

2. after travelling from the heating point for some distance, the water cools down and then starts to sink until it hits the heating point and get heated up again.

3. it will form something like a circle ( the convection current) and it will keep repeating itself until the source of heating up the water is turned off.


air convection current:

1. when the ground temperature is high, it will heat up the air.

2. the hot air expands and rises.

3. when it rises to certain level, it starts to cool down as it has used up its energy and also because of the cold temperature ( high levels in the atmosphere are very cold. i dont know if you have noticed that it is very cold outside the plane. sometimes it can get to -50 degrees C)

4. the cool air contracts and sinks back to the ground level and will be heated up again.

5. the convection current of air is formed ( like a circle of air rising and gets cooled down and sinks again and then heat up again)


呢 d 係我自己學個 d，如果你想要個 pro d gei explanation，我係 wikipedia 搵左個，不過可能有 d 難明。

http://en.wikipedia.org/wiki/Convection_current

Convection

Convection is the internal movement of currents within fluids (i.e. liquids and gases). It cannot occur in solids due to the atoms not being able to flow freely. Convection may cause a related phenomenon called advection, in which mass or heat is transported by the currents or motion in the fluid. In the case where the advected substance is heat, the heat itself may cause fluid motion, so the problem of heat transport (and transport of other substances in the fluid due to it) may become quite complicated.

Differential heating of fluids may itself cause convection in a gravity field, due to variations in density due to a transfer of heat and subsequent fluid expansion, combined with differential buoyancy forces on the different parts of fluid which have differing densities. This type of purely heat-driven convection in gravity fields is sometimes referred to as "natural heat convection," in order to distinguish it from various types of forced heat convection (i.e., heat advection not due to buoyancy of heating). In forced heat convection, transfer of heat is due to movement in the fluid from forces other than heat, such as pumps or natural and artificially-driven mechanically-pushed fluid flows (i.e., flow from external motive forces, such as occurs in rivers or pumps). In addition, buoyancy forces in gravity fields which result from sources of density variations in fluids other than those produced by heat, such as variable composition (for example, salinity), are frequent convection causes.

Current movement during convection may be invisibly slow, or it may be as fast as a tornado or twister. Convection occurs in atmospheres, oceans, and planetary mantles.


我知好多字，睇得好辛苦，又唔知我解得夠唔夠清楚。不過我幫你係我自己 gei 解釋上面分左點，希望容易 d 睇﹑清楚 d啦，希望幫到你。