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
(AndréMarie 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 1.2×10^{19}
electrons per second!) 



Having measured the current I_{1}
a moment's thought gives the conclusion that the other two ammeters
are redundant... if 1.2×10^{19}
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
I_{1}= 2A,
I_{2} and
I_{3} 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
I_{1},
I_{2}
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.

