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The idea of energy predates, is implicit in, and outlives Newtonian mechanics. — Leon Cooper Energy Energy - a concept which we commonly use is surprisingly abstract [2]. All we know from our experience is that the total energy is conserved. The law of conservation states that there is a quantity, called energy that doesn't change with all the changes occurring around us. But, what is energy? We will try to become familiar with this important concept. Kelvin and Rankine defined the concept of energy in order to understand the physical principles underlying the heat engines [3]. Engines were simplified as devices that raised weights by a certain height, given some amount of coal to burn [4]. In 1840s, there were very significant discoveries and publications by Joule and Mayer where they proved that heat is simply a mode of motion. The definition by Kelvin and Rankine was inspired by this.With slightly different terminology, Helmholtz proved that the ability to perform work is conserved in a wide range of processes. When we strike a matchstick to light it, the energy provided by us and the chemical reaction produces heat and light. In a moment, several transformations of energy take place. We can also ask - is there a macroscopic quantity that has changed when motion ceases “without having caused another motion”? Answer could be that objects get warmer. In his remarkable essay, the French engineer Sadi Carnot (1824) asked: Is the motive power of heat invariable in quantity, or does it vary with the agent which one uses to obtain it? After a number of subsequent developments, Helmholtz (1870 ca) concluded that energy has many forms and that the total energy should be taken as a sum of all possible forms of energy. We can appreciate it as we realize [1] that 4.18 Joules (J) is associated with all the following - (i) work done in pushing with a force 4.18 N through a distance of 1 m; (ii) can produce 1 calorie of heat; (iii) can move 2.6 x 10 19 electrons through a potential difference of 1 volt; (iv) can raise 100 g by 418 cm. From the work of Carnot and others, we were led to the concept of efficiency of engines. This led to a technological revolution where one was attempting to invent the most efficient engine.

620.] E very electromagnetic quantity may be defined with reference to the fundamental units of Length, Mass, and Time. If we begin with the definition of the unit of electricity, as given in Art. 65, we may obtain definitions of the units of every other electromagnetic quantity, in virtue of the equations into which they enter along with quantities of electricity. The system of units thus obtained is called the Electrostatic System. If, on the other hand, we begin with the definition of the unit magnetic pole, as given in Art. 374, we obtain a different system of units of the same set of quantities. This system of units is not consistent with the former system, and is called the Electromagnetic System. We shall begin by stating those relations between the different units which are common to both systems, and we shall then form a table of the dimensions of the units according to each system. 621.] We shall arrange the primary quantities which we have to consider in pairs. In the first three pairs, the product of the two quantities in each pair is a quantity of energy or work. In the second three pairs, the product of each pair is a quantity of energy referred to unit of volume.

442.] W e have seen that Poisson supposes the magnetization of iron to consist in a separation of the magnetic fluids within each magnetic molecule. If we wish to avoid the assumption of the existence of magnetic fluids, we may state the same theory in another form, by saying that each molecule of the iron, when the magnetizing force acts on it, becomes a magnet. Weber's theory differs from this in assuming that the molecules of the iron are always magnets, even before the application of the magnetizing force, but that in ordinary iron the magnetic axes of the molecules are turned indifferently in every direction, so that the iron as a whole exhibits no magnetic properties. When a magnetic force acts on the iron it tends to turn the axes of the molecules all in one direction, and so to cause the iron, as a whole, to become a magnet. If the axes of all the molecules were set parallel to each other, the iron would exhibit the greatest intensity of magnetization of which it is capable. Hence Weber's theory implies the existence of a limiting intensity of magnetization, and the experimental evidence that such a limit exists is therefore necessary to the theory. Experiments shewing an approach to a limiting value of magnetization have been made by Joule and by J. Muller.