202] Stresses in Dielectric Media 177 



We now proceed to consider the action of the second system of forces 

 the system of negative hydrostatic pressures. There are pressures per unit 

 area of amounts 



acting respectively on the two sides of the boundary. There is accordingly a 

 resultant tension of amount 



per unit area, tending to drag the boundary surface from region 1 to region 2. 

 Thus the total tension per unit area, dragging the surface into region 1, is 



In 139, in considering a parallel plate condenser with a movable 

 dielectric slab, we discovered the existence of a mechanical force tending to> 

 drag the dielectric in between the plates. This force is identical with the 

 mechanical force just discussed. But we have now arrived at a mechanical 

 interpretation of this force, for we can regard the pull on the dielectric as the 

 resultant of the pulls of the tubes of force at the different parts of the surface 

 of the dielectric. 



Let us attempt to assign physical interpretations to the terms of ex- 

 pression (120) by considering their significance in this particular instance. 

 Consider first a region in the condenser so far removed from the edges of the 

 condenser and of the slab of dielectric, that the field may be treated as 



absolutely uniform (cf. fig. 44, p. 124). We put K a =l, X, = 0, E^ 

 in expression (120) and obtain 



as the force per unit area on either face of the dielectric, acting normally 

 outwards. 



The forces will of course act in such a direction that they tend to 

 decrease the electrostatic energy of the field. Now this energy is made up 



of contributions 2?rA 2 per unit volume from air, and -j^- per unit volume 



-&i 



from the dielectric. From the conditions of the problem h must remain 

 unaltered. Thus the total energy can be decreased in either of two ways 

 by increasing the volume occupied by dielectric and decreasing that occupied 

 by air, or by increasing the value of K in the dielectric. There will therefore 

 be a tendency for the boundary of the dielectric to move in such a direction 

 j. 12 



