20 ELECTRICAL ENGINEERING 



The intensity of the electrostatic force is 



cr *J TT^J /rtf\ 



= K = AK < 36 > 



The difference of potential between the plates is the work 

 done in carrying a unit charge from one plate to the other against 

 the force eF; it is therefore 



E = * = -^K 

 since eF is constant; and the capacity of the condenser is 



C = Q 



- electrostatic units (37) 



4irQt 4irf 

 ~AK 



The capacity therefore varies directly with the area of the 

 plates and with the dielectric constant of the material separating 

 them, and inversely as the distance between them. 



When the plates are separated by air the capacity is 



C = A- (38) 



In order to increase the capacity of such a condenser a large 



FIG. 15. Electric condenser. 



number of plates are used joined in multiple and separated by 

 very thin sheets of dielectric as shown in Fig. 15. 



(b) Determine the capacity of a condenser formed of two 

 concentric cylinders, Fig. 16, of radii a and 6. 



