CHAPTER III 

 ENERGY 



Energy is usually defined in physics as the capacity to do work. 

 It may be more simply expressed as that which is behind all action in 

 this universe. Every change in the state of matter, in the form or 

 position of a mass of matter, or in the chemical composition of matter 

 involves a change in energy. 



21. Forms of Energy.— Energy appears in two forms: potential, or 

 fixed; and kinetic, or free. Potential energy is energy of position. Every 

 particle of matter in this world possesses an amount of potential energy 

 varying with its size and with its distance from the center of gravity of 

 the earth. Gravitational attraction, if all restraint were removed, would 

 cause it to fall to that center; in measure as it approached the center its 

 potential energy would be changed to kinetic, and if it could be conceived 

 as having arrived at that center it would possess no potential energy at all 

 with respect to this terrestrial system. However, it would still have 

 potential energy as a part of a larger system, the solar system, of which 

 this terrestrial system is a part. It would also have potential energy as a 

 part of the largest system, which is the universe. A molecule is an 

 energy system, as is also an atom. In the free movement of a bit of 

 matter its potential energy is changed into kinetic energy, or energy of 

 motion. Any mass possesses potential energy because of the relation 

 of its particles to one another. Energy of motion is manifested not only 

 when masses change position but also when changes occur within them. 

 It is also manifested when the atoms or molecules of which matter is 

 composed cause by their movement certain characteristic phenomena such 

 as heat, fight, and the passage of an electric current. These are all forms 

 of kinetic energy. 



22. Chemical Energy.— In many cases, when elements are made to 

 combine to form a compound, the apphcation of kinetic energy is neces- 

 sary to bring them into the proper relationship to each other. Part 

 of this energy, at least, is represented by the potential energy which 

 these particles possess by virtue of this relation. This form of potential 

 energy is known as the energy of chemical union or simply as chemical 

 energy. Every substance has an amount of chemical energy proportion- 

 ate to the complexity of its structure— that is, to the number and variety 

 of atoms which make it up. A very simple substance has a smaU amount 

 of such energy; a very complex one may have a great deal. In general, 



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