Series Circuit

Two or more electrical elements are connected in series if they have only one common node in between and no other electrical element is connected to that common node. So there are two conditions for being in series:

0             1.   The elements should have only one common terminal (node). This means that they should be connected one after another.

0             2.   No other current carrying electrical element should be connected to that common terminal (node).

To illustrate this lets have a look to the following circuit:

Series Circuit

In this circuit resistor R1 and R2 are in series as they have only b point common in between them and no other current carrying electrical element is connected at node b.

As there is no voltage source connected in between a series circuit, the voltage is shared among the resistors according to their values. So the ratio of voltage and resistor becomes constant and current does not change in the series circuit. For example see the following circuit:

Current is same throughout a series circuit

In this circuit, at R1, R2 and R3 voltage will be shared according to their resistance as there is no other voltage source in between them.  

Total sum of their resistance = (6+3+1) O = 10 O

So, according to ratio: for 1 ohm resistance voltage will be = (20 / 10) volt = 2 V

Therefore, Voltage of R1 = 6 x 2 V = 12 V,

                Voltage of R2 = 3 x 2 V = 6 V

                Voltage of R3 = 1 x 2 V = 2 V

Total voltage = (12 + 6 + 2) V = 20 V = Voltage of the source.

Now let�s find the current in all the resistors.

Current through R1 = (Voltage of R1 / Resistance of R1) = (12 V / 6 O) = 2 A

Current through R1 = (Voltage of R2 / Resistance of R2) = (6 V / 3 O) = 2 A

Current through R1 = (Voltage of R3 / Resistance of R3) = (2 V / 1 O) = 2 A

So its clear that, current through a series circuit remains same throughout the circuit. But the voltage of the circuit drops and is shared by the resistors or electrical elements of the circuit.