144 



ADVANCED ELECTRICITY AND MAGNETISM. 



Electrical Stress and Mechanical Stress. 



The stretching force per unit 

 of sectional area of a rod is 

 called the stress on the rod, and 

 the elongation of unit length 

 of the rod is called the strain; 

 and the strain is proportional 

 to the stress. That is : 



strain = j X stress 



where j is a constant. 



This constant is the recip- 

 rocal of what is usually called 

 the modulus of elasticity, and 

 it is large in value for a sub- 

 stance like rubber which is 

 greatly stretched by a moder- 

 ate stress. 



The intensity of an electric 

 field in volts per centimeter (or 

 the volts per centimeter in a 

 layer of dielectric between 

 metal plates) is frequently 

 called electrical stress, and the 

 electric flux density in the di- 

 electric is frequently called the 

 electrical strain. Therefore, 

 from equation (i) we have: 



electrical strain = k X electrical stress 



The inductivity of a dielec- 

 tric is analogous to the elastic 

 constant of a substance (recip- 

 rocal of what is called modulus 

 of elasticity). 



88. Continuity of electric flux or of electric flux density. 

 Electric stresses in plane layers of different dielectrics. Figure 

 92 shows a layer of oil (inductivity ko) and a layer of glass 

 (inductivity = kg) between two flat metal plates A and B. 

 The electromotive force between the plates is E volts, the 

 charges on the plates are + q and q coulombs, and the 

 thicknesses of glass and oil layers are x centimeters and y 

 centimeters respectively as shown; and it is required to find the 

 electrical stress in the glass and in the oil. 



The electric flux Bq which goes out from + q and comes into 

 q is independent of the change of dielectric, that is, the same 

 amount of electric flux crosses the oil and the glass, and therefore 

 the electric flux density F is the same in both. 



Let e be the electrical stress in volts per centimeter in the 

 oil, and let e g be the electrical stress in the glass. Then k e 

 is the flux density in the oil and k g e a is the flux density in the 



