CHAPTER VII. 

 ELECTRIC CHARGE. THE CONDENSER. 



85. Electric charge. A current of water through a pipe is a 

 transfer of water along the pipe. Let q be the amount of water 

 which, during / seconds, flows past a given point in the pipe, 

 then the quotient q\t is the rate of flow of water through the 

 pipe, and this rate of flow may be spoken of as the strength / 

 of the water current. Suppose the strength / of the water cur- 

 rent to be given (rate of flow of water in units of volume per 

 second) then the amount of water flowing past a given point of 

 the pipe in t seconds is given by the equation : 



Similarly, an electric current in a wire may be looked upon as 

 a transfer of electricity along the wire, and the quantity q of 

 electricity which flows past a point on the wire during / seconds 

 may be defined as the product of the strength of the current and 



the time, that is, 



q~It (57) 



If the strength of the current is variable, then equation (57) must 



be written in the form 



A? = /-A/ (58) 



in which A^ is the small quantity of electricity which flows past 

 a given point on the wire during the short intervals of time A/. 



Quantity of electricity is usually spoken of as electric charge or 

 simply as charge. 



Quantity of water is the fundamental and easily measured thing 

 in hydraulics and water current is most conveniently defined as 

 quantity of water passing per second. In the case of electricity, 

 the fundamental and easily measured thing is electric current, and 



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