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 25×15
= 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) 
