32 ELECTRICAL ENGINEERING 



(2) In Fig. 25 a sheet of glass of thickness 0.9 t and dielectric 

 constant K = 6 is introduced into the field as shown and the 

 difference of potential is the same as before. 



The dielectric flux density 2) is constant throughout the field; 

 the electrostatic force in the air is 



the drop of potential across the air portion of the field is 



Ej. = Q.ltX& A = 0.1 X 2); 

 the electrostatic force in the glass is 



cr a> 9. 



0= K = Q' 

 the drop of potential across the glass is 



E 2 = 0.9 1 X &G = 0.9 1 X = 0.15 1 X 3); 



b 



the difference of potential between the plates is 

 E = E! + E 2 



-.0.1*XS) + 0.5$X3 



= 0.25 #), 

 and the dielectric flux density is 



thus the stress in the air is 



and is four times as great as it was in the first case. 



(3) Fig. 26 shows the same pair of plates with three sheets of 

 dielectric introduced between them, of thickness h, tz and U and 

 dielectric constants KI, K z and K$ respectively. 



3) = dielectric flux, which is constant throughout the field, 



eFi = -= = stress in layer (1), 

 AI 



g\ 



cF 2 = -Tr ~ stress in layer (2), 

 K z 



cF 3 = VF- = stress in layer (3), 

 As 



9) 

 ^ = ^ = Lti = drop of potential across (1), 



