6 



ELECTRICAL ENGINEERING 



normal to the plates except near the edges where they become 

 slightly curved. 



The dielectric flux density is uniform throughout the volume 

 between the plates, and its value is 



<D = 4 irq lines per square centimeter. (5) 



If the medium between the plates is air, the elec- 

 trostatic force at any point is 



eF = 9) = 4 nq dynes. ... (6) 



If the medium has a dielectric constant K y the 

 force is 



cr 9) 4 irq , 



= jr -^ dynes. ... (7) 



11. Potential. The difference of potential be- 

 tween two points is measured by the work done 

 in carrying a unit charge from one to the other 

 against the electrostatic force in the field; it is 

 therefore the line integral of the force between 

 the points. 



The difference of potential between the two 

 tcm >l plates in Fig. 3 is 



FIG. 3. Field be- e= TVdr, (8) 



tween parallel /o 



where t cm. is the distance between the plates. 



Difference of potential tends to cause electricity to flow from 

 one point to the other and is therefore called electromotive force. 



Unit difference of potential (electrostatic) exists between two 

 points when one erg of work is done in conveying unit charge 

 from one to the other. 



The practical unit difference of potential or electromotive 

 force is called the volt. One electrostatic unit is equal to 300 volts. 



The earth is usually taken as the arbitrary zero of potential, 

 and the potentials of other bodies are given with reference to it. 



If a difference of potential is produced between two points on 

 a conductor, electricity will flow from the point of high potential 

 until the potentials of all parts of the conductor are the same. 



When, however, the difference of the potential is maintained 

 by an electric generator a current of electricity flows continuously 

 from one point to the other. 



