ELECTROSTATICS 3 



5. Dielectric Constants. The following table gives the 

 dielectric constants or specific inductive capacities of some of 

 the most common dielectrics. 



Material Dielectric 



Constant 



A^ 1.0 



Vacuum . 9994 



Transformer oil 2.1 



Shellac 2. 75 



Paraffin wax 2.3 



Rubber 2 . 35 



Gutta percha 3 . 0-5 . 



Ebonite 2.8 



Glass 5.0-10.0 



Mica 5 . 0-7 . 



Conductors infinity 



Dielectric constants generally decrease slightly with increasing 

 temperature and in some cases they depend on the intensity of 

 the electrostatic field. 



6. Electrostatic Field. Any space in which electrostatic 

 forces act is called an electrostatic field. The direction of the 

 force* at any point in the field is the direction in which a unit 

 positive charge placed at the point tends to move and its in- 

 tensity is the force in dynes exerted on the unit charge. 



The electrostatic field is conveniently represented by lines of 

 electrostatic induction or dielectric flux drawn in the direction of 

 the force. In air the number of lines per square centimeter a 

 equal to the force in dynes at the point and in a medium of 

 dielectric constant K the number of lines per square centimeter 

 is equal to K times the force. This may be stated in another 

 way: Unit electrostatic force produces one line of dielectric flux 

 per square centimeter in air and K lines per square centimeter in 

 a medium of dielectric constant K. 



The electrostatic force at a point is expressed in dynes and is 

 represented by cF; the dielectric flux density at a point is expressed 

 in lines per square centimeter and is represented by 2). 



7. Field Surrounding a Point Charge. At a distance r cm. 

 in air from an isolated charge q, a unit charge is acted on by a 

 force 



gF = -2 dynes; (2) 



and the dielectric flux density produced at the point is 



2) = cF = -^ lines per square centimeter ... (3) 



