HAP. iv.] ELECTRICITY AND MAGNETISM. 



. - , w~ 



he meaning plain a little preliminary explanation is 

 ecessary. Suppose we had a charge of + electricity 



on a small insulated sphere A (See Fig. 95), placed by 

 .self and far removed from all other electrical charges 

 nd electrical conductors. If we were to bring another 



body B near it, charged also with + electricity, A would 

 epel B. But the repelling force would depend on the 

 uantity of the new charge, and on the distance at which 



it was placed. Suppose the new charge thus brought 



A P Q B" B' 



A m_ ^ _ jfe. 



-V -9- -9" -W 



Fig. 95- 



iear to be one unit of + electricity ; when B was a long 

 /ay off it would be repelled with a very slight force, and 

 ery little work need be expended in bringing it up 

 earer against the repelling forces exerted by A ; but as 

 B was brought nearer and nearer to A, the repelling 

 Dree would grow greater and greater, and more and 

 more work would have to be done against these oppos- 

 ing forces in bringing up B. Suppose that we had 

 egun at an infinite distance away, and that we pushed 

 up our little test charge B from B' to B" and then to Q, 

 and so finally moved it up to the point P, against the 

 opposing forces exerted by A, we should have had to 

 pend a certain amount of work; that work represents 

 he potential 1 at the point P due to A. For the follow- 

 ing is the definition of electrostatic potential: The 

 potential at any point is the work that must be spent 



1 In its widest meaning the term "potential" must be understood as 



<% power to do work." For if we have to do a certain quantity of work 



gainst the repelling force of a charge in bringing up a unit of electricity 



from an infinite distance, just so much work has the charge power to do, for 



will spend an exactly equal amount of work in pushing the unit of electri- 



ity back to an infinite distance. If we lift a pound five feet high against 



le force of gravity, the pound can in turn do five foot-pounds of wrrk in 



illing back to the ground. See the Lesson on Energy in Professor Bulfour 



tewart's Lessons in Elementary Physics. 



O 



