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Text 16. Transfer of Heat
Convection. — The simplest way in which heat may be transferred from one place to another is by the motion of the heated substance. Such a transfer Is known as convection. It is caused by the change in density that takes place when the substance is heated. For example, when a gas or a liquid is heated, it expands and becomes lighter than the cold gas or liquid. When water is heated in a vessel on a stove, the liquid in the bottom of the vessel is hotter than that on the top. The density at the bottom is less than that near the top. The cool liquid sinks down and forces the warmer liquid to a higher level. The currents of water thus set up in the liquid are known as convection currents.
Conduction. — When a metal rod is held in the fire, the heat travels along the rod and after a time the rod becomes tod hot to hold." In this process the vibrations of the molecules are handed on from molecule to-molecule. The layer of molecules in contact with the fire m heated first and thus made to vibrate more rapidly. This layer hands the motion on to the adjacent layer, because each layer is bound to the adjacent layer by certain cohesive forces. It is thus impossible for the molecules in one layer to vibrate without setting the molecules in the neighboring layers in vibration. As this process goes-on, the entire medium is heated after a time. When heat, as in this case, is transferred from one part of the body to another without any progressive motion of the parts of the substance, the heat is said to be transferred by conduction. The conductivity differs widely according to the nature of the substance. Steam pipes are covered to reduce heat losses.
Measurement of thermal conductivity. — In order to study more carefully the law applying to conduction of heat, consider the case in which heat flows along a uniform bar whose ends are kept at uniform temperatures so that the fall of temperature in the bar remains always the same. The quantity of heat that will be conducted down this bar depends on the following relationships:
1. The quantity of heat passing through the conductor in a given time is in direct proportion to the area of the conductor. In this case, just as in the case of a water pipe the larger the area through which the flow takes place, the greater the quantity transferred per unit time.
2. The quantity of heat is also directly proportional to the differ-' ence in temperature between the ends of the bar. This difference'iff temperature may be compared with the difference in pressure that forces the water through the pipe. No flow of water-occurs unless there is a difference in pressure, and the greater the difference in pressure the greater the flow. Likewise, in conduction there is no flow of heat unless there is a difference in temperature between the ends of the bar; and the greater this difference, the greater is the flow of heat-
■9. The quantity of heat decreases as the length of the path or the distance between the ends of the bar is increased. This is also analogous to the flow of water in a pipe in which it is found that the quantity of water flowing' is inversely proportional to the length of the pipe under a given set of conditions.
4. Tlje quantity ctf heat la directly proportions! to the time of flow.
Effeot of temperature on thermal conductivity. —- The thermal OOnduotlyUy of carbon increases with (.lie temperature. On tho other hand, the thermal conductivity of metals decreases with an Increase of
the temperature. The thermal conductivity changes abruptly When a substance like sulfur changes from one crystalline state to another.
Transfer of hoat by radiation. — A pereon'sitting in front of a stove receives heat from the stove, or an electric heater although the air in the room la cold. In like manner, sunlight falling on a body will warm it above the temperature of the,surroundlng air. This method of transfer of heat Is distinguished from convection and oonduotlon by the fnot that the medium through whioh the transfer occurs is not heated* Thus the earth receives great quantities of heat from the sun, although the space that separates the sun from the earth is very Gold. The fact that the earth receives such quantities of heat from the aun shows that this heat can pass through the empty space between the sun and the atmosphere that surrounds the earth. When this energy reaches the earth, it causes the molecules of the body on which It falls to vibrate more rapidly, and the body is heated, This "Movement of heat from one place to another Is known as transfer of heat by radiation. It Is a twofold process: the conversion of the heat energy of the hot body into a transverse wave motion, and a Reconversion of the wave motion Into heat by the body on which it falls.
