1 78 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



area at right angles to an electric field of intensity /. When f 

 is expressed in volts per centimeter and a in square centimeters, 

 the flux 4> is expressed in terms of a unit which may be called 

 the volt-centimeter. Electric flux is in many respects similar to 

 magnetic flux, but the two must not be confused, although the 

 same letter 3> is here used for both. 



98 Amount of electric flux which emanates from an electric 

 charge. It was shown in Art. 39 that ^jrm units of magnetic 

 flux emanate from a magnet pole of which the strength is m, 

 that is, the strength of a magnet pole may be expressed in terms 

 of the magnetic flux which emanates from it. There is also a 

 simple proportional relationship between the amount of charge on 

 a body and the amount of electric flux which emanates from the 

 body, and the amount of charge on a body may be expressed in 

 terms of the electric flux which emanates from it. The relation- 

 ship between electric charge and flux will be established for the 

 simplest case, namely, the case in which a charge is spread uni- 

 formly over a flat surface, as on the flat metal plate of a 

 condenser. 



Consider the two parallel metal plates A A and BB, Fig. 

 107, the area of each plate being a square centimeters, their dis- 

 tance apart being x centimeters, and the electromotive force be- 

 tween them being E volts. The lines of force of the electric 

 field in the region between the plates are indicated by the fine 

 arrows, and the intensity of this field is equal to /x, according 

 to equation (67), so that the total electric flux 4> emanating from 

 the plate AA is a x E\x. The capacity of the condenser AA 

 BB in farads is 884 x icr 16 x ajx, according to equation (63), 

 so that the charge on one plate is equal to 884 x icr 16 x aEfx 

 (q = CE] which is equal to 884 x icr 16 x <I>, because aEjx is 

 equal to 4> ; therefore 



q= 884 x icr 16 x 4> 

 or 



3> = Bq (69) 



in which 



