CHAPTER II. 



THE ELECTROSTATIC FIELD OF FORCE. 



CONCEPTIONS USED IN THE SURVEY OF A FIELD OF FORCE. 



I. The Intensity at a point. 



30. THE space in the neighbourhood of charges of electricity, considered 

 with reference to the electric phenomena occurring in this space, is spoken 

 of as the electric field. 



A new charge of electricity, placed at any point in an electric field, 

 will experience attractions or repulsions from all the charges in the 

 field. The introduction of a new charge will in general disturb the arrange- 

 ment of the charges on all the conductors in the field by a process of 

 induction. If, however, the new charge is supposed to be infinitesimal, the 

 effects of induction will be negligible, so that the forces acting on the new 

 charge may be supposed to arise from the charges of the original field. 



Let us suppose that we introduce an infinitesimal charge e on an infin- 

 itely small conductor. Any charge ^ in the field at a distance r x from the 

 point will repel the charge with a force e^/Ty 5 . The charge e will experience 

 a similar repulsion from every charge in the field, so that each repulsion will 

 be proportional to e. 



The resultant of these forces, obtained by the usual rules for the com- 

 position of forces, will be a force proportional to e say a force Re in some 

 direction OP. We define the electric intensity at to be a force of which 

 the magnitude is R, and the direction is OP. Thus 



The electric intensity at any point is given, in magnitude and direction, by 

 the force per unit charge which would act on a charged particle placed at this 

 point, the charge on the particle being supposed so small that the distribution 

 of electricity on the conductors in the field is not affected by its presence. 



The electric intensity at 0, defined in this way, depends only on the 

 permanent field of force, and has nothing to do with the charge, or the size, 

 or even the existence of the small conductor which has been used to explain 



