POTENTIAL ENERGY 7 



distortion. The hackneyed example of energy storage is, of 

 course, our coal supply. 



Energy in the potential state, as long as it remains potential, is 

 useless. It cannot be transformed into any other form of energy 

 without altering its state, and its state cannot be altered without 

 the employment of kinetic energy. This point is biologically 

 important. 



1. A weight hanging from a string has potential energy accord- 

 ing to its mass and its distance from the centre of the earth, etc. 



2. An explosive has potential energy depending on its chemical 

 composition and physical state. 



3. Petrol, coal, or any other fuel has similar energy bound in it. 



4. A sleeping man may be said to have potential energy. 

 Now, in order to get work out of these quiescent bodies, all 

 that is necessary is the application of a suitable stimulus, i.e. 

 a small quantity of free kinetic energy. E.g. 



(1) The resistance that prevents the weight from falling must 

 be overcome, i.e. the string severed. 



(2) The explosive must be fired or detonated. 



(3) The fuel must reach ignition temperature. 



(4) The sleeper must be awaked. 



In 1882 Helmholtz introduced the terms " free " and " bound " 

 to denote respectively that part of the kinetic energy of a system 

 free or available for conversion to work, and that part not free 

 or not available for this purpose. Later writers, realising that 

 potential energy as such was not available for work, widened the 

 connotation of " bound " to include this dormant energy. As 

 a matter of logical definition, only potential energy is really 

 bound. That which is bound can be freed by cutting the bonds, 

 i.e. by doing an amount of work which bears no relation to the 

 amount of energy bound, but depends on the nature of the bonds. 

 Dissipated or degraded energy is not in any true sense bound. 

 It is not free to do work, i.e. it is not available. To render it 

 available an amount of energy would have to be expended on 

 its environment at least equal to the amount of energy rendered 

 available. 



A weight resting on the ground may be considered as representing a 

 body having degraded energy. Its energy potential is the same as its 

 environment. No work could be got from it without the previous ex- 

 penditure of work on it. If a certain amount of work were done in raising 

 it from the ground, then the same amount of work would be recovered on 

 letting it fall to the ground (taking into account the mechanical equivalent 

 of the degraded heat). On the other hand, a weight resting on a ledge 



