I SO ELEMENTS OF ELECTRICITY AND MAGNETISM. 



B q 



f~^7 (70 



in which / is the electric field intensity in volts per centimeter 

 at a point r centimeters from a concentrated charge of q 

 coulombs. 



Equation (71) applies not only to the ideal case of a concen- 

 trated charge, but also to the case in which the charge is uniformly 

 distributed over a sphere. 



100. Electrostatic attraction and repulsion of concentrated 

 charges. Consider a concentrated charge q' at a point distant 

 r centimeters from another concentrated charge q" . The elec- 

 tric field intensity at q' due to q" is given by equation (71), 

 namely, 



and according to equation (66), the force with which this field 

 acts upon the charge q' is equal to the product q'f t that is, 



F.L.* ; (72) 



47T 1* 



in which F is the form in joule-units * with which the two con- 

 centrated charges q' and q" repel each other when they are at 

 a distance of r centimeters apart, q' and q" being expressed 

 in coulombs. When q' and q" are both positive or both nega- 

 tive, the force F is a repulsion ; when one is positive and the 

 other is negative, the force F is an attraction. 



The unit of charge of the "electrostatic system." The c.g.s. units of magnetic 

 pole strength, magnetic field intensity, electric current strength, electromotive force, 

 electric charge, electrostatic capacity, etc., which have been used in the foregoing 

 chapters constitute what is called the electromagnetic system of units. This " electro- 

 magnetic system " starts out with the definition of a unit magnetic pole as a pole of 

 such strength that it will exert a force of one dyne upon an equal pole at a distance 

 of one centimeter. The so-called electrostatic system of units starts out with the defi- 

 nition of the unit charge as a charge which will exert a force of one dyne upon an equal 



* A joule-unit of force is a force which will do one joule of work in pulling a body 

 through a distance of one centimeter ; it is equal to io 7 dynes. 



