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The Relation Between Current, Charge and Time
It is sometimes found helpful to make an analogy between electric currents flowing in wires, resistors etc and water (currents) flowing through pipes (of different diameters and lengths), taps etc  
   
To find the relation between electric current, I electric charge, Q and time, t we will use this analogy.  
   
Consider a pipe through which water is flowing.
If the rate of flow of water through the pipe is, for example, 25litresmin-1, then in 15minutes, the total quantity of water which has moved through the pipe is obviously 215 = 375litres
 
   
Apply the same simple logic to the case of electric current:  
A current of I Amps means that I Coulombs of charge flow per second.  
Therefore in time t the total charge which has passed any point through which the current flows will (again, obviously!) be I×t  
Therefore, the relation we are looking for is  
 
and this equation can now be used to define the Coulomb  
 
1C is the quantity of charge which flows past any point in a circuit in which a current of 1A flows for for 1s  
 
This, of course, assumes we have already defined precisely what we mean by a current of 1A (see here for the definition of the Ampère)  
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