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Convection

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-251

2025-11-22

428

If a part of a fluid body (liquid or gas) is heated, it expands  and becomes lighter, bulk for bulk, than the other parts. As the molecules are free to move, the cooler, heavier parts tend to sink to the bottom, and thus to push the warmer parts to the top. There is thus set up an ascending current wherever any part of the liquid or gas at the bottom is heated above the rest; and downward and lateral currents in the cooler parts. All these currents are called convection currents.

Demonstration. - Fit a rubber stopper with two holes to the mouth of a thin glass flask. Through these holes thrust two glass tubes about 8 cm. long, making an end of one nearly even with the top of the stopper, and an end of the other nearly even with the bottom. Fill the flask with colored water and heat it. Put in the stopper and sink the flask in a large glass of cold water, as in Fig. 1. Convection currents will be set up by the warm water coming out of one tube while the cold water goes into the other.