ENERGY

is the capacity of a body or system to do work. For the purpose of measuring the quantity of work the system is capable of performing, the unit used is either a foot-pound (the work done in moving a pound weight through a vertical distance of one foot) or an erg (a gramme moved through a centimetre). The energy of a body may be due either (a) to its position, as in the case of a raised weight or a deformed spring, or (2) to the momentum it po e es when in motion. In the first the energy is potential; in the second, kinetic. Energy may take different forms, and any one form may change into another. For example, the radiant energy from the sun stored in coal is changed into chemical energy in burning, which in turn is transformed into heat energy; this can be converted into mechanical energy, from which may be_ obtained electrical energy, and soon. During all these changes, however, no energy is destroyed, although a great deal is wasted. Energy, like matter, ts wndestructible. ‘This principle is known as the Conservation of Energy. All the energy in the world is originally obtained from the sun. Another law is “Whenever mechanical energy is converted into heat, or vice vers a, the ratio of the mechanical energy to the heat is constant.’ This ratio was first investigated by Joule, and is called Joule’s Mechanical Equivalent of Heat. It is equal to 42,000,000 ergs. Energetics. Quite early in the 19th cent. experimental investigation pointed to a direct connection between heat and mechanical energy. This resulted in the rejection of the caloric theory of heat, the formulation of the first law of thermodynamics, and the development of the study of energetics, the principles of which have helped to co-ordinate and explain pnehomena and causes previously considered far removed from each other. Energy cannot be destroyed; it may undergo many changes, but the total energy in the universe must always remain the same. This summarizes the principles of Conservation and Transformation of Energy, applications of which, by mathematicians, to statical ‘and dynamical problems have yielded important results, n 1824 Carnot conceived the notion of a theoretically perfect heat engine in which a gas undergoes a reversible cycle of operations. A reversible cycle is one in which a substance after undergoing a series of operations is brought back to its initial state as regards volume and temperature, and is such that if the operations were reversed the results would be reversed. (Where friction enters as a factor in any operation, the cycle cannot be a reversible one.) Carnot’s cycle consists of four operations: (1) Work is done on the substance, and its temperature is thus raised (substance undergoes an adiabatic compre ion) (2) Work is done by the substance but its temperature is not permitted to alter {isothermal expansion), heat being supplied by some source at the temperature to which the substance has been raised in the first operation. (3) Work is done by the substance, accompanied by a fall of temperature (adiabatic expansion). (4) Work is done on the substance until it regains its initial state, but its temperature is not permitted to alter (isothermal compre ion), heat being given out to some sink, called a condenser. If the heat absorbed at the higher temperature during operation (2) is greater then that given out at the lower temperature during operation (4), the exce can be utilized to do external work. Consideration of Carnot’s cycle, and deductions therefrom, form the basis of energetics and thermodynamics, and _ have resulted in the evolution of the modern heat engine. Kelvin’s Principle of Di ipation of Energy. Although energy cannot be destroyed, yet in every transformation a certain quantity appears as heat which is diffused and becomes unavailable for use. All natural phenomena are of such a kind as to tend towards this degradation of energy, and permanent equilibrium of any system will only be attained when the limit of minimum available energy has been reached. This principle has been very usefully applied to the problems of solution, fusion, solidification, osmosis, etc.; it provides an explanation of the phenomenon of chemical combination in definite proportions— combination only taking place if, as a result, the available energy is diminished —and it is the basis of the prediction thas ultimately no energy will be available and the universe become a uniformly hot, inert ma . PER (1796-1864), Fr. social reformer; became follower of Saint-Simon; advocated communism, ‘woman sufirage,’ and other innovations; with others sent to prison for a year 1832 for offence against ‘morality’; became railway director 1845; also a journalist and author. E.), one Tunisia, N. Africa; produces cereals.