Warm-up

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# Warm-up - PowerPoint PPT Presentation

##### Warm-up

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1. Warm-up In Chinese herbal tea shops, bowls of herbal tea are usually covered by glass plates. Why do the shop owners need to do this?

2. Warm-up Some freezers at supermarkets do not have doors. This is just opposite to what herbal tea shops do. What could be the reason?

3. Introduction Have you noticed... Air-conditioner is usually installed high on the wall immersion heater is fixed near the bottom

4. Introduction These have to do with the transfer of heatby convection(對流).

5. Convection in water (video) Put a purple crystal into a beaker of water near the side. note its movement

6. Convection in air (video) Light the candle & hold a piece of smoking paper on theother side of the cardboard. note smoke's movement

7. Convection current When the water is heated at the bottom, water expands, rises & is replaced by cold water from above.

8. Convection current The flow of water is called convection current. This method of heat transfer is called convection.

9. Convection current air expands, rises & Similarly, when air is heated, is replaced by cold air from above. convection current

10. Convection current Why air-conditioners are usually set up high on the wall? An air-conditioner gives out cold airwhich falls. Warm air is taken in by the air-conditioner&cooled. This pushes the warm airup. This results ina convection current.

11. Why crawl on ground during a fire?

12. Why crawl on ground during a fire? In a fire, why should you crawl close tothe floor in a smoke-filled room? Smoke is warmer than the surrounding air. It rises & its places would be replaced by thesurrounding cooler air. Smoke is toxic. We should crawl close to the floor to prevent smoke inhalation.

13. Convection current Heat can be transferred in a fluid (liquid/gas) by convection, in which the heated fluid expands, rises and is replaced by the surrounding cooler fluid. Such movement of fluid is called convection current.

14. 2 Convection and particle motion In kinetic theory, particles in a liquid are close together. But they are not in fixed positions.

15. 2 Convection and particle motion When a fluid is warmed, the fluid particles move faster  warm fluid expands  less dense than surrounding cold fluid  warm fluid rises  cool fluid from the top moves down to replace warm fluid  convection current

16. Convection and particle motion (simulation)

17. Examples of convection Wind and sea breezes Land and sea breezes over the coast in the summer are caused by convection current.

18. 3 Examples of convection Wind and sea breezes warm air expands Warm air rises warm air cools wind During the day, the Sun heats up the land more quickly than the sea. cool air blows in from the sea to replace it

19. Examples of convection Shape of candle flame hot air rises Fire or flames always point upwards... since convection current flows upwards

20. Matters of which of the following states CANNOT have convection? A Solids B Liquids C Gases

21. When liquid/gas is warmed… When liquid/gas is warmed, the warmer liquid/gas expands and becomes _______ dense than the surrounding liquid/gas. Hence, it rises. less Thesurrounding cooler liquid/gas moves to replacethewarm liquid/gas. Such liquid/gas movement forms a ____________ current. convection