A PREAMBLE ON ENERGY 63 



And so on, almost ad infinitum. The events that we call 

 physical changes are energy-transformations. - 



2oh. Transformers. In all such cases there is some system 

 of things by reason of which the energy-transformation occurs. 

 Such systems are : 



Heat engines ; dynamos ; glow lamps ; accumulators ; electric 

 motors ; the living cells of green plants, etc. 



21. ON THE PHASES OF ENERGY IN THE AVAILABLE 



FORMS 



Available energy may be kinetic or potential. 



Kinetic Energy is that of the motion of physical things that have 

 mass. Thus : 



The energies of the moving parts of steam-engines, electric 

 motors, vehicles, projectiles, etc. ; 



the energy of the molecules of bodies that are hot — it is because 

 of this kinetic molecular motion that we recognize heat ; 



the energy of radiation : The " waves " of light, etc. — that is, 

 the periodic changes in the intensity of an electro-magnetic field, 

 have mass ; light can cause very small particles to move (this 

 is " radiation pressure "). 



21a. Potential Energy is energy that exists and is available 

 but which is not manifest until it is released. Thus : the gravita- 

 tional energy of a heavy body which is at rest but which is free 

 to fall when released (the weights of a clock which is at rest) ; 

 the energy of stress of a coiled spring (that, for instance of a clock 

 or watch at rest) ; 



chemical energy : which is that of the positions and linkages 

 of atoms or molecules in relation to each other — thus the atoms 

 of carbon and hydrogen in coal gas, and those of oxygen in the 

 atmosphere are so arranged that they are free to react with other 

 so that CO 2 and OH 2 are formed. In this reaction the potential 

 energy of the reactants (the hydrocarbon and oxygen) becomes 

 the kinetic energy of the resultants (CO 2 and OH 2). Motions 

 of translation and oscillation are exhibited by these energized 

 molecules and oscillatory motions of their electrons also occur. 

 This kinetic energy we recognize as heat and light. 



21b. Releasing Transformations. Generally potential 

 energy passes into some other form, or into kinetic energy after 

 a releasing transformation occurs. Thus : 



