ELECTROSTATICS 17 



The potential to which a body is raised varies directly as the 

 quantity of electricity stored in it and inversely as its capacity 

 and may be expressed by the formula 



#=, ........ (3D 



where Q is the quantity of electricity, or charge, 

 C is the capacity of the body, 

 E is the potential. 

 The capacity therefore is 



C=| ........ (32) 



and is equal to the charge divided by the potential, or it is equal 

 to the charge per unit potential. 



A body has unit capacity (electrostatic) when one unit of 

 electricity is required to raise its potential by unity. 



A sphere with a radius of one centimeter has unit capacity, 

 because if a charge of one unit be given to it it will act as though 

 it were concentrated at the centre and will produce at the surface 

 unit potential. The capacity of a sphere of radius R cm. is R 

 electrostatic units. 



The practical unit of capacity is the farad. A conductor has 

 a capacity of one farad when one coulomb of electricity is re- 

 quired to raise its potential by one volt. 



1 coulomb 3 X 10 9 , . ., /OON 



1 farad = - - = - - = 9 X 10 11 electrostatic units. (33) 



1 volt 



The foregoing explanation of capacity is apt to be misleading. 

 A conductor has capacity only with respect to surrounding 

 objects, since the electrostatic energy is not stored on or in the 

 conductor itself but in the field between the conductor and sur- 

 rounding conductors. 



In Fig. 13 A is a conductor placed near a large conducting 

 plane B. Assume a potential difference E to exist between A 

 and B, then E measures the work in ergs that must be done in 

 carrying a unit charge from B to A against the electrostatic 

 forces in the field. These forces produce a dielectric flux passing 

 from A to B. 



The total dielectric flux ^ is proportional to the difference of 

 potential E and to the permeance of the path % 



