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QUESTIONS AND PROBLEMS
1. With the aid of Fig. 197, which represents a fireless cooker, explain the principle on which fireless cooking is done.
2. Why do firemen wear flannel shirts in summer to keep cool and in winter to keep warm?
3. If a package of ice cream is put inside a paper bag, it will not melt so fast on a hot day. Explain.
4. If the ice in a refrigerator is wrapped up in blankets, what is the effect on the ice? on the refrigerator?
5. If a piece of paper is wrapped tightly around a metal rod and held for an instant in a Bunsen flame, it will not be scorched. If held in a flame when wrapped around a wooden rod, it will be scorched at once. Explain.
6. If one touches the pan containing a loaf of bread in a hot oven, he receives a much more severe burn than if he touches the bread itself, although the two are at the same temperature. Explain.
7. Why are plants often covered with paper on a night when frost is expected?
8. Why will a moistened finger or the tongue freeze instantly to a piece of iron on a cold winter's day, but not to a piece of wood?
9. Does clothing ever afford us heat in winter? How, then, does it keep us warm?
10. Why is the outer pail of an ice-cream freezer made of thick wood and the inner can of thin metal?
255. Convection in liquids. Although the conducting power of liquids is so small, as was shown in the experiment of § 251, they are yet able, under certain circumstances, to transmit heat much more effectively than solids. Thus, if the ice in the experiment of Fig. 194 had been placed at the top and the flame at the bottom, the ice would have been melted very quickly. This shows that heat is transferred very much
more readily from the bottom of the tube toward the top than from the top toward the bottom. The mechanism of this heat transference will be evident from the following experiment:
Let a round-bottomed flask (Fig. 198) be half filled with water and a few crystals of magenta dropped into it. Then let the bottom of the flask be heated with a Bunsen burner. The magenta
will reveal the fact that the heat sets up currents the direction of which is upward in the region immediately above the flame but downward at the sides of the vessel. It will not be long before the whole of the water is uniformly colored. This shows how thorough is the mixing accomplished by the heating.
The explanation of the phenomenon is as follows: The water nearest the flame became heated and expanded. It was thus rendered less dense than the surrounding water, and was accordingly forced to the top by the pressure transmitted from the colder and therefore denser water at the sides which then came in to take its place.
FIG. 198. Convection currents
It is obvious that this method of heat transfer is applicable only to fluids. The essential difference between it and conduction is that the heat is not transferred from molecule to molecule throughout the whole mass, but is rather transferred by the bodily movement of comparatively large masses of the heated liquid from one point to another. This method of heat transference is known as convection.
256. Winds and ocean currents. Winds are convection currents in the atmosphere caused by unequal heating of the earth by the sun. Let us consider, for example, the land and sea breezes so familiar to all dwellers near the coasts of large bodies of water. During the daytime the land is heated more rapidly than the sea, because the specific heat of water is much greater than that of earth. Hence the hot air over the
land expands and is forced up by the colder and denser air over the sea which moves in to take its place. This constitutes the sea breeze, which blows during the daytime, usually reaching its maximum strength in the late afternoon. At night the earth cools more rapidly than the sea and hence the direction of the wind is reversed. The effect of these breezes is seldom felt more than twenty-five miles from shore.
Ocean currents are caused partly by the unequal heating of the sea and partly by the direction of the prevailing winds. In general, both winds and currents are so modified by the configuration of the continents that, it is only over broad expanses of the. ocean that the direction of either can be predicted from simple considerations.
257. A third method of heat transference. There are certain phenomena in connection with the transfer of heat for which conduction and convection are wholly unable to account. For example, if one sits in front of a hot grate fire, the heat which he feels cannot come from the fire by convection, because the currents of air are moving toward the fire rather than away from it. It cannot be due to conduction, because the conductivity of air is extremely small and the colder currents of air moving toward the fire would more than neutralize any transfer outward due to conduction. There must therefore be some way in which heat travels across the intervening space other than by conduction or convection.
It is still more evident that there must be a third method of heat transfer when we consider the heat which comes to us from the sun. Conduction and convection take place only through the agency of matter; but we know that the space between the earth and the sun is not filled with ordinary matter, or else the earth would be retarded in its motion through space. Radiation is the name given to this third
method by which heat travels from one place to another, and which is illustrated in the passing of heat from a grate fire to a body in front of it, or from the sun to the earth.
258. The nature of radiation. The nature of radiation will be discussed more fully in Chapter XXI. It will be sufficient here to call attention to the following differences between conduction, convection, and radiation.
First, while conduction and convection are comparatively slow processes, the transfer of heat by radiation takes place with the enormous speed with which light travels, namely 186,000 miles per second. That the two speeds are the same is evident from the fact that at the time of an eclipse of the sun the shutting off of heat from the earth is observed to take place at the same time as the shutting off of light.
Second, radiant heat travels in straight lines, while conducted or convected heat may follow the most circuitous routes. The proof of this statement is found in the familiar fact that radiation may be cut off by means of a screen placed directly between a source and the body to be protected.
Third, radiant heat may pass through a medium without heating it. This is shown by the fact that the upper regions of the atmosphere are very cold, even in the hottest days in summer, or that a hothouse may be much warmer than the glass through which the sun's rays enter it.
259. The Dewar flask and the thermos bottle. For the retention of extremely cold liquids, such, for example, as liquefied air, whose boiling point is -190° C. (=— 310° F.), Dewar invented a double-walled vessel. The space between the walls is a vacuum, and the inner surface of the outer vessel and the outer surface of the inner vessel are silvered. There are three ways in which heat may pass inward through the double wall- conduction, convection, and radiation. The vacuum prevents almost entirely the first two, while the silvering eliminates passage of heat by radiation. The well-known
glass part of the thermos bottle (Fig. 199) is simply a cylindrical Dewar flask for keeping liquids either hot or cold, since it is as difficult for heat to pass outward through the walls as to pass inward. The glass flask is provided with a cork stopper, and a strong outside metal
case for its protection. Hot liquids, as well as those that are cold, may be kept for several hours in a thermos bottle with only a few degrees change in temperature.
THE HEATING AND VENTILATING OF
260. The principle of ventilation. The heating and ventilating of buildings are accomplished chiefly through the agency of convection.
FIG. 199. The inner glass flask of
a thermos bottle
To illustrate the principle of ventilation let a candle be lighted and placed in a vessel containing a layer of water (Fig. 200). When a lamp chimney is placed over the candle so that the bottom of the chimney is under the water, the flame will slowly die down and will finally be extinguished. This is because the oxygen, which is essential to combustion, is gradually used up and no fresh supply is possible with the arrangement described. If the chimney is raised even a very little above the water, the dying flame will at once brighten. Why? If a metal or cardboard partition is inserted in the chimney, as in Fig. 200, the flame will burn continuously, even when the bottom of the chimney is under water. The reason will be clear if a piece of burning touch paper (blotting paper soaked in a solution of potassium nitrate and dried) is held over the chimney. The smoke will show the direction of the air currents. If the chimney is a large one, in order that the first part of the above experiment may succeed, may be necessary to use two candles; for too small
FIG. 200. Convection currents
a heated area permits the formation of downward currents at the sides.