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Electric Current
An electric current is a flow of charged particles.  
A current in a metal is due to the movement of electrons. In a conducting solution, the current is due to the movement of ions.  
Current is measured using an ammeter.  
   
An ammeter measures the rate of flow of charge.  
   
For simplicity, an ammeter can be thought of as a "counter of electrons": it gives a reading which is proportional to the number of electrons which pass through it per second.  
   
The unit of current is the Ampere, A (André-Marie Ampère)
A current of 1A is equivalent to a rate of flow of 1Coulomb of charge per second, 1Cs-1
 
(See here for the complete definition of one Amp)  
   
An ammeter is always connected in series with other components. The resistance of an ammeter must be low compared with other components in the circuit being investigated.  
   
If this is not the case, then the inclusion of the ammeter in the circuit will (significantly) change the current it is being used to measure... not a very desirable outcome!  
   
Measuring Electric Current in Series Circuits  
 
The current has been measured at the point shown and found to be 2A. A current of 2A corresponds to a certain number of electrons passing per second (approximately 1.1019 electrons per second!)  
   
Having measured the current I1 a moment's thought gives the conclusion that the other two ammeters are redundant... if 1.1019 electrons pass through that part of the circuit in one second, the same number must pass through any other point in the circuit because, being a series circuit, there is nowhere else for the electrons to go!

So if I1= 2A, I2 and I3 must also be 2A.
 
   
Currents in Parallel Circuits  
Consider the following circuit which includes two resistors connected in parallel with each other.  
 
If the three current I1, I2 and I are measured it is found that  
 
Again, this conclusion is inevitable if we remember that current readings correspond to numbers of electrons passing per second.  
   
This result is called Kirchhoff’s current rule, stated as follows.  
The total current flowing towards a junction in a circuit is equal to the total current flowing away from that junction.  
   
As an analogy, consider vehicles at a road junction.  
   
 
The number of vehicles passing point 1, per minute, must be equal to the number of vehicles passing point 2 per minute plus the number of vehicles passing point 3 per minute.  
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