HEAT
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mal condition of any body are the following: What is its temperature? and How much heat does it contain? But, before either of these can be answered, we must distinguish carefully between heat and temperature and learn how to measure each of these quantities. ^
TEMPERATURE
If two bodies, A and B, are placed side by side in contact, and B thereby gains heat and becomes warmer, be it ever so little, A has imparted heat to B; and A is said to have a higher temperature than B. But if we place B in contact with another body, C, and find that B thereby loses heat, we say that C is at a lower temperature than B. In general, difference in temperature between two bodies is that which determines which way the heat flows. Accordingly, Maxwell defines temperature as follows: "The temperature of a body is its thermal state with reference to its power of communicating heat to other bodies." An instrument which indicates temperatures is called a thermometer. See THERMOMETER.
TRANSFER OF HEAT
Having denned temperature as that which determines the direction of the flow of heat, we next consider the various modes by which heat is transferred or diffused. Of these only two methods are known, namely: Conduction and Radiation.
CONDUCTION
The former of these processes is familiar to everyone who has used a silver spoon in a cup of hot tea or in a plate of icecream; for in the one case the spoon evidently conducts heat from the tea to the hand and in the other case from the hand to the icecream.
Very little is known about the mechanism by which the particles of a solid body hand on heat from one to another. But experiment has shown that the rate at which heat flows through a solid depends upon four things only: the material of the bar; the length, /, of the bar; the area of cross-section, 5; and the difference in temperature, 0, between the ends of the bar. We may summarize these facts by writing
Rate of Flow of Heat=
where k is a constant depending upon the material of which the bar is composed. This is the fundamental equation of heat-conduction. In the case of fluids the equalization of temperature goes on much more rapidly than it otherwise would, on account of the mixing which is always going on automatically; the cooler and hence denser portions falling to the bottom of the containing vessel. This process of transferring heat from one part of a fluid to another is called convection. It is illustrated in the heat which is carried up the chimney. The mix-
ing is a mechanical process; the transfer of heat takes place by conduction.
RADIATION
One of the earliest observations made after the discovery of the airpump was that both light and heat pass through a vacuum with the utmost ease. Even the perfect vacuum which exists between us and the sun is traversed every day by enormous amounts of heat. The process by which this transfer is made is called radiation. Numerous experiments show that radiant heat is reflected, refracted and polarized according to the same laws that light obeys and that light and heat are merely different kinds of radiant energy — alike except in wave-length. See LIGHT.
The fact that a body, when heated, emits exactly those kinds (wave-lengths) of radiant energy which the same body would absorb, if cool, is the foundation of the modern science of spectroscopy.
EFFECTS OF HEAT
Among the numerous effects of heat the most important probably are the following three: Cnange of dimension or change of stress; change of molecular state; and change of temperature.
Nearly all bodies increase in size when heated. Stretched rubber is an exception to this rule; and it is well known that water, when heated from o°C to 4°C, diminishes in volume. The fraction of its length by which a solid changes its length when heated one degree Centigrade is known as its coefficient of linear expansion.
The following table will show how the principal metals behave when heated:
COEFFICIENTS OF LINEAR EXPANSION
Glass........................ 0.0000085
Platinum.................... 0.0000085
Steel........................ 0.000012
Brass........................ 0.000018
Copper...................... 0.000019
Aluminium.................. 0.000023
Zinc........................ 0.000029
It may be easily shown that the rate at which the volume of a solid changes with temperature is very exactly three times its coefficient of linear expansion. Liquids can change, of course, only in volume. It is the volume-expansion of mercury that is employed in our ordinary thermometers. For the laws of expansion of gases see Charles' law and Boyle's law.
CHANGE OF MOLECULAR STATE
If a piece of cold ice have heat applied to it, the temperature of the ice will keep on increasing until a certain point is reached where it begins to melt. Then it is found to be impossible to change the temperature of the ice by applying heat to it; for all the heat applied goes to melt the ice into water at the same temperature* After the ice is all
