140 ADVANCED ELECTRICITY AND MAGNETISM. 



(e) The work Eq done by the battery during one movement of 

 the ball is equal to the mechanical work Fx done on the ball, 

 therefore we have Fx = Eq, or : 



Any region in which a charged body is acted upon by a force 

 which depends upon the charge on the body and which does not exist 

 when the body is not charged, is called an electric field. Thus the 

 region between A and B in Fig. 91 is an electric field because 

 the charged ball b is acted upon by the force F. 



The force F with which an electric field pulls on a charged 

 body placed at a given point in the field is proportional to the 

 charge g on the body so that we may write : 



F = eq (2) 



in which e is the proportionality factor, and it is called the 

 intensity of the electric field at the given point. 

 From equations (i) and (2) we get: 



-f a 



That is to say, the intensity of the electric field between the 

 plates in Fig. 91 is equal to the electromotive force between the 

 plates divided by the distance between the plates. 



Direction of electric field at a point. The direction of an 

 electric field at a point is the direction in which the field would 

 pull on a positively charged body placed at that point. 



Tension of the lines of force in an electric field. Two op- 

 positely charged metal plates attract each other as stated in Art. 

 75. Thus the oppositely charged plates in Fig. 90 attract each 

 other. This attraction may be thought of as due to a tension of 

 the lines of force; that is, the lines of force may be thought of as 

 if they were filaments of rubber stretching from plate to plate 

 and pulling the plates towards each other. 



