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Heat Is a Form of Energy

The statement that heat is a form of energy raises the question: What form of energy is heat? In mechanics we classify energy as either kinetic or potential. To which of these classes does heat belong?

Let us consider some of the ways in which heat is produced from mechanical energy. The molecules forming a nail are, we believe, in motion. When the head of the nail is struck by a hammer, the top layers of molecules receive the first impulse and move with greater speed, that is, their kinetic energy is increased; but, owing to the frequency of collisions between molecules, the motions are entirely at random, and the particles dash around in all possible directions. Part of the increase of kinetic energy is handed on by impact to the next layers, and so on. Much the same thing happens when the body is rubbed by another. This suggests that heat is kinetic energy of the particles of a body.

But we have seen that there are forces between the particles of a body, as shown by the properties of cohesion, elasticity, etc., and, when work is done against forces of attraction or repulsion, potential energy is produced. Hence changes in the potential energy of the particles of a body must accompany changes in their kinetic energy. Potential might not, it is true, affect the sense of touch in the same way as kinetic energy would, but, since we cannot separate it from the kinetic energy of the particles, we must let it go into the same account and consider it as part of what we call heat. Gases consist of particles which exert such feeble forces on one another, except at impact, that the potential energy is usually negligible. For this reason we shall see that gases play a particularly important part in the study of heat.

Let us now consider what we mean by particles. We know all bodies to consist of molecules, and these of atoms, and further that the atoms contain electrons, and that these are also free electrons temporarily separated from atoms and molecules. Moreover, in many bodies molecules are probably united in aggregates forming larger particles. All these various particles receive kinetic and potential energy when a body is heated, and, without distinguishing between them in any way, we may say that their total increase of kinetic and potential energy constitutes the increase of heat.

We must, however, distinguish the motions of a body as a whole, or its mass-motions from the random motions of the particles. In the former all the particles move in the same way (as in translation) or in some regular way (as in rotation), and, as we have seen, the energy of this motion can be calculated. It is the energy of the motion of the particles in random directions that constitutes heat. A gas or liquid flowing at a high speed in a pipe does not possess more heat because of its mass-motion, though by impact of the particles on the surface of the pipe part of this motion may be turned into the random motion that constitutes heat. Similarly a current of electricity in a wire consists of a stream of electrons in the wire, but this also does not constitute heat, although it may give rise to heat. For the same reason, wave motions of the particles in a body do not constitute heat.

With these explanations we may regard heat as the kinetic energy of the irregular motions of the particles of a body and the potential energy associated with it.

Note. It should be mentioned that the author of mechanical theory of heat is a great Russian scientist M.V. Lomonosov.

Before M.V. Lomonosovs time there existed a so called caloric theory which is in its essence a reactionary theory of heat. Even more than a century after M.V. Lomonosovs death the conception of heat in the form of caloric theory was being spread everywhere.

Only in the last quarter of the 19th century Lomonosovs point of view considering heat as the result of the motion of molecules was revived.

Temperature. - The most familiar temperature is that of the human body. Objects are said to be warm, hot, cool, or cold compared with this temperature. This is not a very reliable or accurate standard. Therefore it is necessary to look for some more accurate method of estimating temperatures. For this purpose at ordinary temperatures it is customary to use a thermometer, which depends for its operation on the fact that a liquid like mercury expands when its temperature is increased.

Mercury Thermometer. - The most common type of thermometer for ordinary use is the mercury-in-glass thermometer. It consists of a small glass bulb to which is sealed a glass tube with a very small bore. The bulb and part of the tube are filled with mercury. The residual air above the mercury in the tube is carefully removed so that the space above the mercury is empty. The glass tube is then sealed off. In order to use this bulb with its fine tube for a thermometer, it is necessary to have on the stem a scale divided into equal divisions called degrees.

Fixed Points. - The two fixed points that are ordinarily chosen for a thermometer are the melting point of ice and the boiling point of water under atmospheric pressure.

To determine the first of these fixed points, the bulb of the thermometer is surrounded with finely divided ice or snow. This melting ice or snow keeps the same temperature while melting. After the mercury in the bulb has reached the same temperature as the ice or snow, the height of the mercury in the stem of the thermometer does not change. The point at which the mercury stands is now taken and used as one of the fixed points on the thermometer. On the centigrade scale, this point is called 0, while on the ordinary Fahrenheit scale it is arbitrarily called 32. The bulb and as much as possible of the stem of the thermometer are now placed in steam rising from water boiling at standard atmospheric pressure. The mercury expands and assumes a new position in the stem. This position, which does not change after the temperature of the thermometer has reached the temperature of the steam, is marked on the scale and used as a second fixed point for the thermometer. On the centigrade scale this point is called 100, and on the Fahrenheit scale it is called 212.

Comparison of Centigrade and Fahrenheit Thermometers. - Consider the thermometer. On one side is a centigrade thermometer and on the other is a Fahrenheit thermometer. The freezing point on the centigrade scale is 0 C and that on the Fahrenheit is taken as 32 F. The boiling point on the centigrade scale is 100 C and that on the Fahrenheit is 212 F. Hence, 100 divisions, or degrees, on the centigrade scale correspond to 180 on the Fahrenheit scale, and 1 F on the Fahrenheit scale equals five ninths of 1 C on the centigrade.

To change from the Fahrenheit to the centigrade scale, first find how many degrees above or below the freezing point of water the temperature is on the Fahrenheit scale, and then take five ninths of this; the result will be the reading on the centigrade scale. In other words, subtract 32 from the reading on the Fahrenheit scale and take five ninths of the remainder.

 

5

Reading on centigrade = (reading on Fahrenheit) 32 F.

9

 

9

Reading on Fahrenheit = (reading on centigrade) + 32 C.

5

 

Unit of Heat.- Although heat is a form of energy and may be measured in the units in which energy is measured, it is convenient to use a unit that is based on the effect of heat in raising the temperature of a substance. In looking for a substance to use as a standard, it is natural to choose water because of the ease with which it is obtained. Since it always takes the same amount of energy to raise the temperature of 1 g of water 15 C to 16 C, it is possible to define an arbitrary unit in which to measure other quantities of heat. Here the choice of the unit is largely a matter of convenience. In this respect, however, it does not differ from the unit of length or the unit of mass which is also chosen arbitrarily.

Calorie.- The unit of heat in the cgs system is called calorie. It is defined as the quantity of heat or energy which is necessary to raise the temperature of 1 g of water from 15 C to 16 C on the centigrade scale. Because of the fact that the heat required to raise the temperature of 1 g of water 1C is not the same at all temperatures, it is necessary to state the temperature at which the calorie is defined.

The British Thermal Unit. - In the English system of units, the unit of heat is known as the British thermal unit (Btu). It is defined as the quantity of heat required to raise the temperature of 1 lb of water from 59 F to 60 F on the Fahrenheit scale (Figure 21). This is a much larger unit of heat than the calorie. Here, as in the case of the calorie it is necessary to state the temperature, for the amount of heat required to raise 1 lb of water 1 F varies with the temperature.

 

59 to 60F

 

 




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Texts Heat | Figure 32 - One British thermal unit is the

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