Energy (physics)

“ Energy Recognized by its Effects.—Like force, energy is invisible and we are aware of the forms only by the effects produced by it.
We recognize heat by warming, by expansion, by pressure.
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We recognize light by warming, by its affecting vision.
We recognize electrical energy by its heat, light, motion, or magnetic effect. We recognize mechanical energy by the motion that it produces. We recognize chemical energy by knowing that the source of energy does not belong to any of the foregoing.
A boy or girl is able to do considerable work. They therefore possess energy. ”

“ It was proved that light and heat, and [Pg 28] the manifestations of force which we group together under the name of electricity, are various modes of undulatory motion transformable one into another; and that, in the operations of nature, energy is never annihilated, but only changed from one form into another. This generalization includes the indestructibility of matter, and thus lies at the bottom of all chemistry and physics and of all science. ”

“ Conservation of Energy.—In the study of matter we learned that it is indestructible. Energy is also believed to be indestructible. This principle stated concisely teaches that despite the innumerable changes which energy undergoes the amount in the universe is unchangeable, and while energy may leave the earth and be lost as far as we are concerned, that it exists somewhere in some form. The principle which teaches this is called the "Conservation of Energy." The form into which energy is finally transformed is believed to be heat. ”

“ Nature of the Science of Energetics. Energy, or the capacity to effect changes, is the common characteristic of the various states of matter to which the several branches of physics relate; if, then, there be general laws respecting energy, such laws must be applicable, mutatis mutandis, to every branch of physics, and must express a body of principles as to physical phenomena in general. ”

“ In examining the concept of energy, we start with purely mechanical energy. In isolated mechanical systems, in which there are only conservative forces, the sum of kinetic and potential energy is constant. The kinetic energy may be defined as ∑ 1⁄2 mv2, formed for all parts of the body. The potential energy is determined by the position of the parts of the system, and has physical significance only [Pg 108] with reference to a datum position, that is, only changes of potential energy have meaning in terms of operations. ”

“ Energy as considered in physics, apart from chemistry has been classified in various forms, viz., energy of motion (translation or rotation) , strain, vibration, beat, radiation, electrification, electricity in motion, magnetization, and gravitative separation. Those forms which are represented directly by bodies (whether extended masses or molecules) in motion or deformation, and which do not appeal to our special senses for recognition, constitute mechanical energy. ”

“ But if the phenomenon is more carefully examined, we shall discover the presence of the air, and we shall understand that every part of the volume of this air could, if it were drawn off into a recipient of equal volume, carry away with it a fraction of the energy of the system, and that consequently this energy belongs really to the air and not to the pistons, which are there solely for the purpose of enabling this energy to manifest its existence. ”

“ After a brief discussion of kinematics and dynamics, mass physics is further divided into work and energy, attraction and potential, properties of matter, energy of mass vibration, sound. Then physics of the ether has energy of ether vibration, radiant energy, energy of ether stress, electrostatics, energy of ether vortices, magnetism, energy of ether flow, electro-kinetics, electro-magnetic character of radiation. ”

“ On this assumption it is clear that no form of energy with which we have to deal in physical science would have any existence in the ether; for every one of those forms, gravitational, thermal, electric, magnetic, or any other—all are the results of the forms of energy in matter. ”

“ When the action involves the bodily movement of material mass in space, the dynamical energy thus manifested, and which may be transmitted by the movement of this material, is termed mechanical or "work" energy (§ 31) ; when the energy active in the process is manifested as heat, chemical, or electrical energy, we apply to it the term "molecular" energy. The significance of these terms is readily seen. The operation of mechanical or "work" energy on a mass of material may readily proceed without any permanent alteration in the internal arrangement or general structure of that mass. ”

“ While the factors of kinetic and of gravitational energy, velocity and momentum, on the one hand, and force and distance, on the other, are simply related to each other, the capacity factors of other forms of energy—surface, in the case of surface-energy; volume, in the case of volume-energy; entropy for heat; electric capacity when electric charges are being conveyed by means of ions; atomic weight, when chemical energy is being gained or lost—all these are simply connected with the fundamental chemical capacity, atomic weight or mass. ”

“ In the downward motion, the action is completely [Pg 65] reversed, and when the body reaches the starting-point its energy of position has again been completely transformed into energy of motion. It might be well to draw attention here to the fact, often overlooked, that this energy of position gained by the rising mass is, in reality, a form of energy, separate and distinct, brought into existence by the transformation and disappearance of the energy of the moving mass. ”

“ The first kind of energy, that of motion, is called Kinetic energy, and when once understood it appears so fundamental a fact of nature that we can hardly conceive the possibility of resolving it into anything else. The second kind of energy, that depending on position, is called Potential energy, and is due to the action of what we call forces, that is to say, tendencies towards change of relative position. ”

“ It appears, therefore, that a system containing an electric current is a seat of energy of some kind, and, since we can form no conception of an electric current except as a kinetic phenomenon, its energy must be kinetic energy, that is to say, the energy which a moving body has in virtue of its motion. ”

“ Molecules are composed of atoms, but the atoms, having no extension, are in reality nothing but the points of application of forces. Strictly speaking, not of forces but of energy, that same energy which is as much a unity and just as indestructible as matter. But matter, though one, has many different aspects, and the same is true of energy. Till recently only four forms of energy, convertible into one another, have been known to us: energies known as the dynamic, the thermal, the electric, and the chemic. ”

“ The Conservation of Energy and the inter-convertibility of forces—light, heat, electricity,—taking place constantly everywhere, often on a stupendous scale, require bewildering calculations by an ever-present God. No energy, not even potential energy, can be lost in converting one force into another. ”

“ This principle was not easy to state in a simple way, because of the different forms of energy; but the essential point was that energy is never created or destroyed, though it can be transformed from one kind into another. ”

“ You may wonder why, having said that the sun contains 2,000 quadrillion tons of energy at the most, I now assume that it contains just this amount. It is really only a verbal point depending on the scientific definition of energy. All mass is mass of something, and we now call that something ‘energy’ whether it is one of the familiar forms of energy or not. You will see in the next sentence that we do not assume that the energy is convertible into known forms, so that it is a terminology which commits us to nothing. ”

“ Further measurements on the transformations of other forms of energy, chemical energy and electrical energy, have shown that Joule's law of equivalents is general, and that the quantity of mechanical work represented by any form of energy remains undiminished after transformation, whatever the nature of that transformation.
Energy presents itself to us under two forms, potential and actual. Potential energy is slumbering energy, energy localized or locked up in the body. ”

“ Now, since the masses thus moving are assumed to be a material and inherent portion of the spherical system, the source of the energy which is thus alternately supplied to and returned by them is the original energy of motion of the system; this original energy being assumed strictly limited in amount, the increase of the energy of each mass as it moves towards the equator will, therefore, be productive of a retardative effect on the revolution of the system as a whole. ”

“ This increase is the evidence of a definite energy process by which certain particles or portions of the mass have in distortion gained energy of position—energy of separation—or potential energy relative to the centre of the sphere. In fact, if the mass were allowed to cool back to its normal condition, this energy might by a suitable arrangement be made available for some form of external work. ”

“ It is impossible to create a molecule out of nothing or to reduce a single particle of energy to nothing. Energy, like matter, can be changed from one form to another or from one place to another, but all matter is one matter in its elemental form, and all energy is from one original energy, which I hold to be electricity. All matter is the aggregate manifestations of the invisible atom, and all energy is the varied manifestations of the one ultimate and only force in nature—electricity. ”

“ There are three great laws of energetics:
1. The sum total of the energy of the universe is invariable.
2. The several forms of energy are interconvertible, and possess an exact quantitative equivalence.
3. The tendency of all forms of kinetic energy is continually toward reduction to forms of molecular motion, and to their final dissipation uniformly throughout space. ”

“ Therefore, the energy principle generally requires that the change of total energy contained in a certain space is like a surface integral, namely, the algebraic sum of the entire incoming energy through the surface of that space. The flux can be submitted by radiation, such as the Poynting vector, by conduction as with pressure or with pushes and with heat conduction, and by convection as with the admission of ponderable atoms or electrons through the considered surface. ”

“ For example, the potential energy of our stone may be taken to be the kinetic energy it would acquire in falling to the surface of the earth, or what it would acquire in falling down a well to the center of the earth, or any assigned lesser distance. It does not matter which we take, so long as we stick to our decision. ”

“ The illustrative value of our experimental devices, however, will be very distinctly improved if it be borne in mind that the operations of mechanical energy are not restricted to solids only, but that the various processes of transformation and transmission here illustrated by the motions of solid bodies may, in other circumstances, be carried out in a precisely similar fashion by the movements of liquids or even of gases. ”

“ But as the mass is thus raised, it is gaining energy of position (§ 20) , and as it has absolutely no communication with any external source of energy in its ascent, the energy of position thus gained can only be obtained at the expense of its initial inherent heat and work energy. The operation is, in fact, a simple transformation of this inherent energy into energy of position, a transformation in which gravity is the incepting agency. The external evidence of transformation will be a fall in temperature of the material. ”

“ The first two states are available as sources of mechanical energy, but in the third state the energy is scattered into an infinite number of infinitely small energies, as it were, and, in the case of small masses at least, is lost forever as far as doing work is concerned; and so much energy is let down from a high to a low class, and the whole energy of the universe is rendered less available by a corresponding amount. ”

“ Before leaving the question of energy transmission, there are still one or two interesting features to be considered. Although energy, as already pointed out, is ever found associated with matter, this association does not, in itself, always furnish phenomena sufficient to distinguish the precise phase in which the energy may be manifested. ”

“ The energy of this moving mass depends upon both its weight and its velocity, and when its motion is arrested by the spile that energy of motion is largely converted into heat energy, from which both the spile and the spile driver get hot. ”

“ If light appears, its antecedent has been impact or friction, condensation or chemical action, and if electricity appears the same sort of antecedents arc present. Whether the one or the other of these forms of energy is developed, depends upon what kind of a structure the antecedent energy has acted upon. If radiant energy, so-called, falls upon a mass of matter, what is absorbed is at once transformed into heat or into electric or magnetic effects ”

“ It must not be forgotten that energy is not a simple factor, but is always a product of two factors—a mass with a velocity, a mass with a temperature, a quantity of electricity into a pressure, and so on. One may sometimes meet the statement that matter and energy are the two realities; both are spoken of as entities. It is much more philosophical to speak of matter and motion, for in the absence of motion there is no energy, and the [Pg 19] energy varies with the amount of motion; and furthermore, to understand any manifestation of energy one must inquire what kind of motion is involved. ”