ELECTROSTATICS 5 



A unit positive charge is always associated with each 4 TT lines 

 leaving a surface and a unit negative charge with each 4?r lines 

 entering a surface. 



9. Field between Two Point Charges. The field between 

 two point charges q and q consists of curved lines extending 

 from the positive to the negative charge. 



The direction and intensity of the force at any point may be 



FIG. 2. Field between two point charges. 



obtained as shown in Fig. 2. If a unit positive charge is placed 

 at any point P it is repelled by q with a force /i = -^ and is at- 



tracted by q with a force /i = %. The resultant force at the 



7*2 



point is cF and it is the vector sum of f\ and /2. If D cm. is the 

 distance between the charges, the force at a point on the line 

 joining the charges distant r cm. from the charge q is 



Since the medium is air the flux density at any point is numerically 

 equal to the force cF at the point. 



10. Field between Parallel Plates. The field between two 

 parallel plates A and B, Fig. 3, charged respectively with + q 

 and q units per square centimeter consists of straight lines 



