ENERGETICS (Gk. iverynruc6s, energi. tikos, active, from Ivep-yeiv, encrycin, to be ac tive, from Iv, en, in + fpyov, ergot!, work). The theory of energy: a theory which states the conditions and laws under which the phenomena of energy are manifested. Energy, pending a more technical definition, may lie understood broadly to he a condition or at tribute by virtue of which matter can effect changes in other matter.
The modern doctrine of energy, dating from 1840, takes the phenomena of physical sci ence from the position of speculative reason ing or of disconnected and unrelated occur rences, and unites them in one general scheme of exact quantitative relationship. :Many of the phenomena, particularly in electricity and in chemistry, have only become known in re cent years, and the laws of energeties have therefore been formulated in that. period. In mechanics, however, energy has been recog nized since the time of Newton, Leibnitz, Descartes, and Huygens. These philosophers, as well as others, were engaged in contro versy over the proper mode of estimating the effect of force in terms of the motion estimating its application produced in a given mass; and although the methods of Newton prevailed in the development of mechanics, those of Leib nitz and Huygens led most directly to pres ent methods of interpreting and correlating phe nomena not only oZ mechanics, hut of heat, sound, light, magnetism, and electricity, as well as of chemistry, geology, and other branches of sci ence.
A force applied to a body so as to move it is said to do work, and the measure of the work is the component of the force in the direc tion of motion, multiplied by the distance through which its point of application moves, or calling the work W, the force F, and the space s, = Fs. We may interpret this relation from either point of view: if we recognize the force and the distance traversed, we derive the work front their product; if we appreciate the work in the first place, we recognize the force as the relation of the work to the space in which it uats performed, since F = Experiment shows that a constant force applied to a body free to move varies as the mass, in, of the body.
and the rate of change, , in its velocity, and by a suitable choice of units, the relation may be stated ' if e is the velocity pro duced in time t by the constant force, F; furthermore, the space s, traversed in time t, while its velocity is increasing front 0 to v, is t'. Combining these
two equations we find That is. a holy of mass in, free to move by the application of a force. I', would require a velocity r„ when it had been moved over a distance s, such that Fs -_tor'. The expression Fs is the measure of the action viewed with regard to the agent. that with regard to the body; the former is called work, the latter energy. In this particu lar ease, however, the only change that has been produced in the body is a change in its motion. but the body is now possessed of more ability to change other bodies than it possessed before, and, in being brought hack to its original state of rest or motion it is found to be able either to do mechanical work equal to Fs, or to confer on other bodies a motion such that the total value of Vol/ r' for them is exactly equal to that for the given body. This is a simple form of conversion or transference of energy. Energy due to motion only is called kinetic energy. The same amount of work might have been done as before, resulting in a static condition of the body upon which work has been dime, as. e.g. in compressing or stretching an elastic body, but the holy would now he in condition to do work on its own :le mma, i.e. it would possess energy, though not itself in motion. a strain has been pro dueed by changing the position. by some or all parts of the body. Similarly a body might have conferred upon it power to do work by changing it from One place to another, as when a body is lifted to a point above the earth. It should not be thought, however, that the change in position is the essential feature of the phenomenon; it is the visibie portion of it only. Undoubtedly, the real seat of the energy is ermni.eled with the hidden meehanism which renders it neeessa•y to use a force in order to secure the strain or change in position; e.g. the mechanism of gravi intim]. Energy due to strain or to position is called slot Or 1,eM,tifi en( rgy. (The term energy is due to Thomas Young, potential energy to Bankine, static energy to Kelvin.) This affords a more precise definition of energy as the ,apabilita to do ironk, and the amount of energy \\ a ISIdy 111),,esse • is its for till.. too, may now be regarded as a t ronsferi nee of energy from one body to an other. coneeke of a body in a of strain.