ENERGETICS. In physics, mechanics and chemistry, Energetics is the science that treats' of energy and its transformations —"energy* being defined as that attribute of a body, or of a material system, by virtue of which the body or system can do mechanical work; and "works being simultaneously defined as the overcoming of resistance through distance. Any such body or system, that can do mechan ical work by changing its shape, position or configuration, or its physical or chemical state, is said to possess "energy") — that is, power to do work.
The mechanical work is often performed directly and immediately by the body possess ing the energy, without the intervention of any other agency. This is by no means essential, however, because the conception of energy his, beep extended so as to include all systems and processes, however complicated or indirect the development of the work may be. Thus a hot body is said to possess energy, because its heat can be used to actuate a heat engine; and a galvanic battery is similarly said to possess en ergy, because it can generate electricity and thereby operate an electric motor. ' We even speak of fqod as possessing energy, because, when eaten, digested, assimilated and oxidized in the muscles it enables human beings or ani, mals to perform mechanical work, .
In view of the varied kinds of 'bodies and systems that exist, and the varied ways in which' they may perform work, we speak of "heat " " energy,electrical energy," "chemical en ergy* and energy of other types; but in using i such expressions we merely indicate, in, a' rough way, the kind of source that we are deal, ing with and the general nature of the proc-, esses to which we may have to resort, if we, attempt to utilize the energy. The energy is the same thing in every case= namely, it is the capacity 9f the body or system under con sideration to do mechanical work. In many cases, in fact, it is hard to say in what condi tion the energy exists in a body. Fqr example,, a mass of hot, compressed gas certainly pos sesses energy, but in view of the fact that we can obtain work from it either by direct adia batic expansion, or by keeping the volume con stant and using the heat (for example) to op erate a thermoelectric battery and a motor, it is scarcely logical to say that the energy of the original mass existed either in the form of heat or in the form of mechanical ,compres sion. If we try t44 solve this diffictilty by re plying that it existed in 6oth forms, we.,aza quickly made, of the of the answer, if we attempt to determine how much is present as heat, and how much is pres ent as mechanical compression. When we in
crease the energy of a body or system, we say that we °add energy° to it; and when we de crease its energy, we say that we °subtract energy) from it. We can always tell what form the energy has that we add or subtract, but it is often impossible to tell what form it has, while it actually resides within the body or vstem with which it is associated.
For purposes of measurement and compu tation it is necessary to have a satisfactory unit, in terms of which we can make definite quantitative statetnents with regard to energy; and in view of the definition of energy, it is evident that this unit must necessarily be either the same as the one that is used for measuring work or else a mere multiple or submultiple of it The unit that is adopted in the measure ment of work depends upon the nature of the problem that is under consideration. In mod ern scientific investigations the unit of work is commonly the erg, which is defined as the work done in overcoming a resistance of one dyne, through a distance of one centimeter. In engineering operations, the unit of work com monly employed (at least in the United States and in England) is the foot-pound, which is de fined as the amount of work done in overcom ing, through a distance of one foot, a resist ance equal to the weight of one pound of mat ter. (In countries using the metric system, the unit of work in engineering operations is the kilogram-meter). The foot-pound is not as precise and definite a unit as the erg, because the attraction that the earth exerts upon a pound of matter varies with the latitude and with the elevation above the sea and hence the foot-pound varies in the same manner. The variation is not great enough, however, to de stroy the usefulness of the foot-pound as a unit of work or energy for engineering pur poses, and hence this familiar unit is not likely to be superseded, for ordinary, rough purposes. To avoid the indefiniteness of the foot-pound, we might adopt the far more scientific (but exceedingly uncommon) unit known as the °foot-poundal,° which is defined as the quan tity of work that must be done in order to overcome a resistance of one °poundal* through a distance of one foot —a °poundal) being the force which, when applied for one second to a body having a mass of one pound, subject to no other forces and initially stationary, will pro duce in that body a velocity of one foot per second.