Читать книгу Force and Energy; A Theory of Dynamics - Allen Grant - Страница 9
ОглавлениеTHE MODES OF ENERGY.
Energy has two Modes, ordinarily known as the Potential and the Kinetic: but the terms Statical and Dynamical are much preferable. Nevertheless, in order not to disturb unnecessarily the received terminology, the former expressions will be generally preserved in this treatise.
The two Modes of Energy are interchangeable with one another: the Potential can pass into the Kinetic, and the Kinetic into the Potential. Each species of Energy, Molar, Molecular, Atomic, and Electrical, is represented in both modes.
Potential Energy (a very bad name) is equivalent to actual or statical separation. Any mass, molecule, atom, or electrical unit, in state of separation from other masses, molecules, atoms, or electrical units, possesses Potential Energy. The subject may conveniently be considered under the four heads hence arising.
Molar Potential Energy is equivalent to the statical separation of Masses. The moon possesses this Energy relatively to the earth, and the planets to the sun.[6] The cannon ball, shot vertically, has Molar Potential Energy at the instantaneous neutral point when it has reached its greatest height and has not yet begun to fall. A stone on a mountain top or a head of water on its side has also the same Energy. In short, Molar Potential Energy is possessed by all discrete Masses in virtue of their separation. It is commonly known as Visible Energy of Position.
Molecular Potential Energy is equivalent to the statical separation of Molecules. Two planed surfaces of iron possess this Energy, until by apposition they are made to unite. The molecules of water, dispersed as steam, similarly possess it, in the form commonly known as ‘latent heat.’ When steam condenses or water freezes, the Energy is yielded up in the Kinetic form.
Atomic Potential Energy is equivalent to the statical separation of Atoms. It is possessed by every free Atom of an element, and by every compound Atom whose affinities are not fully saturated. Thus an Atom of carbon has Potential Energy in relation to two separate Atoms of oxygen, with which it may unite to form carbonic anhydride. Similarly, chlorine has Potential Energy relatively to sodium, with which it may unite to form common salt. Such cases, however, must be carefully distinguished from those of preferential attraction where a body leaves its union with one element to combine with another for which it has stronger affinities: as when the Cl of HCl leaves the H to unite with Na in NaCl. This last instance is really analogous to that of the cannon ball which breaks the rope that ties it because the Force of Gravitation has outbalanced that of Cohesion.
Electrical Potential Energy is equivalent to the statical separation of Electrical Units. In a Leyden jar, the opposite electricities of the inner and outer coats exhibit this relation. In a thunder cloud and the earth beneath it we have a substantially similar division of the Positive and Negative Units. The statement of these facts must be accepted with the usual caution as to the purely symbolical nature of our electrical conceptions.
From the potential we pass on to the Kinetic Mode. It will not be immediately apparent in what sense Kinesis is an Energy in accordance with our definition: but, here again, the reader must courteously waive his objections for the present, and accept the statement provisionally, so far as he finds possible. Many difficulties of this sort necessarily beset the explanation of every new point of view, especially where previous misconceptions have clouded and embarrassed the mental vision.
Kinetic Energy is equivalent to motion. Any mass, molecule, atom, or electrical unit, in a state of motion, possesses Kinetic Energy. The subject may be conveniently considered under the four heads hence arising. But, just as before, when dealing with Energy generally, we found that we could not divide it into species so definite in their likeness as those of Force, because Energy was manifested in two Modes, the Potential and the Kinetic: so, here, when we are dealing with Kinetic Energy specially, we shall find that it cannot be divided into species so definite as those of the Potential Mode, because Kinesis itself is divisible into several Kinds, whose nature will form the subject-matter of the succeeding chapter.
Molar Kinetic Energy is equivalent to the relative motion of Masses. It is seen in the fall of an unsupported weight or a spent cannon ball to the earth. It is also seen in the rising of the ball, the flying of a bird, or the walk of a man. Again, it is seen in the orbital motion of the planets, and in the spinning of a top. These various Kinds of Kinesis will be fully discussed in the next chapter.
Molecular Kinetic Energy is equivalent to the relative motion of Molecules. It is found in the falling together of Molecules of steam into water. It also occurs in the disruption of a cohering mass. And it is more conspicuous in the phenomenon of heat.
Atomic Kinetic Energy is equivalent to the relative motion of Atoms. It is seen in that rushing together of Atoms which results in chemical combination. It also occurs in the severing of Atoms from the combined state. But it is not known to have any continuous form analogous to the orbital motion of a planet, the spinning of a top, or the regular vibration of heat.
Electrical Kinetic Energy is equivalent to the relative motion of Electrical Units. It is seen in the lightning, in the discharge of a Leyden jar, and in the galvanic current.
It will doubtless seem strange to the reader to find the motion of masses, molecules, and atoms towards one another spoken of as a manifestation of Energy: but this seeming inconsistency will be explained in the succeeding chapter.
A table will clearly exhibit the relations here described, one example only of each species being cited.
| Modes | Species | |||||
| Potential. | Molar Potential Energy. Visible Energy of Position.) | Molecular Potential Energy. (Condensing Steam.) | Atomic Potential Energy. (Chemical Energy of Free Elements.) | Electrical Potential Energy. (Tension.) | ||
| Kinetic. | Molar Kinetic Energy. (Orbital Motion.) | Molecular Kinetic Energy. (Heat.) | Atomic Kinetic Energy. (Chemical Energy in Act of Combining.) | Electrical Kinetic Energy (Galvanic Current.) | ||
