Convection in Gases
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-253
2025-11-22
41
Air that is in contact with a heated surface becomes itself heated, and convection currents are set up in the same way as in liquids. This can be shown by lighting a piece of touch-paper and holding it over a piece of heated metal, when the smoke will rise with the current of air.
Convection currents in air are made use of in warm air systems of heating. Cold air enters the heater at the base, and after being heated, passes out through the warm air delivery tubes, which distribute it throughout the building.
NOTE. - Touch-paper is made by dipping filter paper or blotting paper into a solution of saltpeter. When dry it will burn without flame, but will give off smoke freely.
Demonstration. -Set a short piece of candle in a saucer and light it. Set over it a chimney from a student lamp. Pour water into the saucer to prevent air going into the chimney from the bottom, and the candle will soon go out. Why? Cut a piece of tin of the shape shown in (a) in Fig. 1, and put it down the chimney with the wide part resting on the top. Light the candle again, and it will keep on burning. Why? Examine the air on both sides of the tin, by the smoke from touch paper.
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