Thermal energy

“ Heat is an Excretum.-The third principle of biological energetics may be therefore thus enunciated:—Vital energy in its final form becomes thermal energy. This principle teaches us that if chemical energy is the primitive generating form of vital energies, thermal energy is the form of waste, of emunctory, the degraded form as the physicists would say. ”

“ In the case of the conduction or diffusion of heat the process is not only irreversible, but it involves the irreversible diminution of that part of the whole stock of thermal energy which is capable of being converted into mechanical work. ”

“ The relation of heat energy to work as determined is a demonstration of the first law of thermo-dynamics, namely, that heat and mechanical energy are mutually convertible in the ratio of 778 foot pounds for one British thermal unit. ”

“ Extension to Thermal and Chemical Systems.—This dominant position of the principle of energy, in ordinary statical problems, has in recent times been extended to transformations involving change of physical state or chemical constitution as well as change of geometrical configuration. In this wider field we cannot assert that mechanical (or available) energy is never lost, for it may be degraded into thermal energy; but we can use the principle that on the other hand it can never spontaneously increase. ”

“ As this process continues, heat moves toward those parts of the object that have a lower temperature. This flow of thermal energy as a result of changing molecular activity is heat conduction. Heat can be conducted from higher to lower temperature regions within an object, or between objects in contact when they are at different temperatures. The transfer of heat is accomplished without appreciable movement or displacement of the substances. Whenever heat is transferred through an opaque solid substance, the transfer must be by conduction. ”

“ The heat-tendency of the universe is also revealed in the
far-reaching "law of maximum work," which defines that chemical
change, accomplished without the intervention of external energy,
tends to the production of the body, or system of bodies, which
disengage the greatest quantity of heat. [1] And, again, vast
numbers of actions going on throughout nature are attended by
dissipatory thermal effects, as those arising from the motions of
proximate molecules (friction, viscosity) , and from the fall of
electrical potential. ”

“ As a gas at high temperature must contain molecules with all amounts of internal thermal energy from nothing upwards, we should on the other hand, on the ordinary gas-theory, expect both a shift of the brightest part of a spectral line when the temperature is raised, and also a widening of its diffuse margin. ”

“ The theory points out, in the first place, that different phases of energy are not transformable into one another with equal ease and completeness. Heat is the only form into which any other can be totally converted. When electricity, mechanical motion, or any kind of energy but heat is sought, an undesired production of thermal effect is unavoidable in the most favorable conditions for efficient conversion known to science. Therefore, in the catalogue of terrestrial forces heat is continually gaining in amount at the expense of every other mode of motion. ”

“ The law which states the relation between heat and other forms of energy such as electricity, mechanical work, is called the principle of the conservation of energy, and forms the first law of thermodynamics. ”

“ It follows, in fact, from the
second law of thermodynamics, that whenever work is derived from
heat, a certain quantity of heat falls in potential without doing
work or, in short, is dissipated. On the other hand, work may be
entirely converted into heat. The result is the heat-tendency of
the universe. Heat, being an undirected form of energy, seeks, as
it were, its own level, so that the result of this heat-tendency
is continual approach to uniformity of potential. ”

“ Work is thus done by the expanding gas, and in consequence of this work done, a definite quantity of atmospheric material gains energy of position or potential energy relative to the earth's surface. At the same time, the rise of temperature of the gas will indicate an accession of heat energy to its mass. These familiar phenomena of expansion under constant pressure serve to illustrate the important fact that, when heat energy is applied to a gaseous mass, it really manifests itself therein in two aspects, namely, heat energy and work energy. ”

“ Now, something of this kind may be taking place when a body is heated, for we may very well suppose heat to consist of a vertical or circular motion, the tendency of which would be to drive the particles asunder against the force of cohesion. Part, therefore, of the energy of heat will be spent in augmenting the motion, and part in driving asunder the particles. We may, however, suppose that, in ordinary cases, the great proportion of the energy of heat goes towards increasing the molecular motion, rather than in doing work against the force of cohesion. ”

“ Just analogous is our action when we apply a match to the coal. The heat first disintegrates or disunites little atoms of the hydrocarbons which make it up, and sets them in a state of rapid vibration among themselves. This vibration brings them into contact with the atoms of oxygen, which at once unite with them, causing a fresh development of heat, and a liberation of all the dormant energy, which immediately assumes the active form. ”

“ Careful investigations have shown that a given amount of mechanical work will always produce a certain definite amount of heat, that is, 400 foot-pounds of work, if converted into heat, will always produce 1 B.T.U. of heat, which is the amount necessary to raise the temperature of 1 pound of water 1° F. Conversely, when heat is converted into work, 1 B.T.U. of heat “vanishes” every time 400 foot-pounds of work are produced. Heat then is just a special form of energy, and the development of heat by friction or collision is merely a transformation of energy from one form to another. ”

“ Heat, like water, flows from high to low level, and, consequently, just as we can let the water of the lake run down to the sea, so we are able to let heat from the earth's surface travel up into the cold region above. Heat, like water, can perform work in flowing down, and if we had any doubt as to whether we could derive energy from the medium by means of a thermopile, as before described, it would be dispelled by this analogue. But can we produce cold in a given portion of the space and cause the heat to flow in continually? ”

“ In tropical regions the heat is not wanted. In the steam-engine, chemical energy is first changed into heat, and heat into mechanical energy; in the body the change is, probably, much of it direct, and not through the intermediation of heat. ”

“ But the energy, though spent, is not lost, for when the liquid again freezes, or when the vapour again condenses, this energy is once more transformed into the shape of sensible heat, just as when a stone is dropped from the top of a house, its energy of position is transformed once more into actual energy. ”

“ The doctrine of heat as due to vibration explains how heat is transformed force, so that the steam-hammer worked by the heat used in the furnace can be set to beat cold iron till it is white-hot; thus part of the force which came from heat has gone back into heat, and with the heat re-appears the other form of radiant energy, light. Lastly, the history of electricity comes from the time when the ancients wondered to see amber when rubbed pick up morsels of straw, and the loadstone draw bits of iron. ”

“ On the one hand it was held by the best-known physicists that heat is a substance which pervades all bodies in greater or less amount, and that heating and cooling are simply the absorption and the rejection of this “imponderable substance” by the body affected; while, on the other hand, it was asserted by a small but increasing number that heat is a “mode of motion,” a form of energy, not only imponderable, but actually immaterial; a quality of bodies, not a substance, and that it is identical, in its nature, with other forms of recognizable energy, as, for example, mechanical energy. ”

“ But, while the theory of thermodynamics asserts that the combustion of a pound of carbon yields an amount of heat equivalent to 11,116,800 foot-pounds of energy, it also asserts just as clearly and just as firmly that, under the conditions which exist upon this planet, heat can not be transformed into mechanical effect without wasting a considerable portion of the energy due to it. ”

“ This apparatus, by continually transforming heat into mechanical work, tended to become colder and colder, and if it only were practicable to reach a very low temperature in this manner, then a sink for the heat could be produced, and energy could be derived from the medium. ”

“ Heat may be devoted to the expansion of a solid against its cohesive forces. It may be expended against [Pg 26] the elastic forces of a gas, or it may be worked down against chemical or electrical forces. In every case a transformation of energy will result, varying in nature according to the peculiar conditions under which it is carried out. In this or a similar fashion each primary incepting influence may give rise to a series of secondary actions more or less complex in nature. ”

“ Ultimately, it is transmitted by the air to other bodies, and by means of their internal viscosity is sooner or later converted into heat. Thus we see that heat is the form of energy, into which all visible terrestrial motion, whether it be rectilinear, or oscillatory, or vibratory, is ultimately changed. ”

“ Now, we may suppose that its descent is accomplished in two stages; that, first of all, it falls upon a platform from the height of 424 metres, and gets heated in consequence, and that then the heated mass is allowed to fall other 424 metres. It is clear that the water will now be doubly heated; or, in other words, the heating effect in such a case will be proportional to the height through which the body falls—that is to say, it will be proportional to the actual energy which the body possesses before the blow has changed this into heat. ”

“ The effect of heat in producing combination may be easily explained according to these ideas; it not only gives more freedom of motion to the particles, but in a number of cases it seems to exalt the electrical energies of bodies:—glass, the tourmaline, sulphur, and some others, afford familiar instances of this latter species of energy. ”

“ The specific heat of any substance is “the number of units of heat required to raise one pound of such substance one degree.” We can explain this farther. When heat is communicated to a body it has two or three functions to perform. Some of it has to overcome the resistance of the air in expanding the body, more of it expands, and the remainder increases the temperature of the body. So some heat disappears as heat, and is turned into energy,—“molecular potential energy,”—as it is called, and the rest remains. ”

“ The total energy of the molecule (which we call "heat") includes also another factor—namely, potential energy, or energy of position, due to the work that has been done on expanding, in overcoming external pressure, and internal attraction between the molecules themselves. This potential energy (which will be recovered when the gas contracts) is the "latent heat" of Black, which so long puzzled the philosophers. It is latent in the same sense that the energy of a ball thrown into the air is latent at the moment when the ball poises at its greatest height before beginning to fall. ”

“ Let us assume, as is commonly the case, that the intrinsic energy of the initial system is greater than that of the final system. When the first system then is transformed into the second, the excess of energy which the former possesses must appear in the shape of heat, light, electrical energy, mechanical energy, &c. It is for the most part a simple matter to obtain the excess of energy entirely in the form of heat, the amount of which is easily susceptible of measurement, and thus the existence of thermochemistry as a practical science is rendered possible. ”

“ Let us begin first with the energy implied in fuel. We can, of course, burn fuel, or cause it to combine with the oxygen of the air; and we are thereby provided with large quantities of heat of high temperature, by means of which we may not only warm ourselves and cook our food, but also drive our heat-engines, using it, in fact, as a source of mechanical power. ”

“ If the air when raised in temperature, and at the same time increased in volume, contained less internal energy than when simply heated without alteration of volume, the energy evolved would be available to aid the performance of the work done against external forces, and less heat would be required, or, in the contrary case, more heat would be required, than would be necessary if the internal energy remained unaltered. ”

“ This is the great principle of the conservation of energy which is applicable equally to all branches of science.
Another principle, almost equally general in its applicability, is that of the dissipation of energy, for which we are indebted in the first instance to Sir William Thomson. All forms of energy may be converted into heat, and heat tends so to diffuse itself throughout all bodies as to bring them to one uniform temperature. ”