ELECTRIC CHARGE. THE CONDENSER 



179 



B= 1.131 x io 13 (70) 



(see Art. 91), that is, B lines of electric flux emanate from one 

 coulomb of positive charge or converge towards one coulomb 

 of negative charge. // is a great help towards a clear under- 

 standing of electrostatics to think of electric charge, positive or 

 negative, as the beginning or ending of lines of electric force. 



99. Electric field due to a concentrated charge. Consider a 

 concentrated positive charge q from which lines of electric force 

 emanate in all directions, as shown in Fig. 1 1 1, and let it be re- 



sphere^ 



Fig. 111. 



quired to find the intensity / of the electric field at a point p 

 distant r centimeters from the center of q. Describe a sphere 

 of radius r with its center at the center of q. ^T{ie area of the 

 surface of this sphere is ^.irr 2 , and the electric field f is every- 

 where at right angles to this surface and everywhere the same in 

 value at the surface. Therefore the electric flux across the sur- 

 face of the sphere is equal to ^irr 2 x /, and this must be equal to 

 Bq according to equation (69), where B has the value given in 

 equation (70). Therefore we have 



Bq = 

 or 



