ENERGY 105 



In the explosive mixture the separations are even smaller 

 in size and concern the electrons and the nuclei of 

 the atoms, which come together into new combinations 

 when explosion occurs. 



Potential energy, then, is the ability of a system to do 

 work as the result of the displacement of its parts, 

 while kinetic energy is its ability as a result of their 

 motions. If the motions are those of whole atoms 

 or molecules the energy is molecular kinetic energy. 

 When we realize that every moving electron, atom, 

 or molecule in the universe possesses kinetic energy, 

 and that wherever there are separations which tend to 

 change there is potential energy, we recognize what 

 an enormous store of energy there is in the universe 

 in which we live. The questions this realization sug- 

 gests are : How much of this energy is available ? and 

 How may the available energy be utilized? 



To obtain the idea which is involved in the word 

 " available" let us consider the case of the attraction 

 of the earth for parts of itself. If we lift a shovelful 

 of earth from the surface we do so against the attrac- 

 tion of the rest of the earth. In this separation work 

 is done and potential energy is given to the system 

 composed of the earth and the shovelful, which we are 

 holding apart. As another example note that each 

 particle of a mountain forms with the rest of the earth 

 a system with potential energy, for it is only necessary 

 to start a stone to have it roll crashing down the moun- 

 tain side. But suppose that we stand on relatively 

 flat country beside a deep well or a mine shaft. If 

 we start a stone it will fall down the shaft, for in this 

 case also the system possesses potential energy. In 



