When the resistors are conriected end to end, so that they form only one path for the flow of current, then resistors are said to be connected in series and such circuits are known as series circuits.

Let resistors R₁, R₂ and R₃ be connected in series as shown in and the potential difference of V volts be applied between extreme ends A and D to cause flow of current of I amperes through all the resistors R₁,R₃ and R₃.

Now according to Ohm’s law
Voltage drop across resistor R_1,   V₁ = I R₁
Voltage drop across resistor R₂,   V₂ = I R₂
Voltage drop across resistor R₃,   V₃ = I R₃

Voltage drop across whole circuit. Voltage drop across resistor R₁+ voltage drop across resistor R₂ + voltage drop across resistor R₃.

And according to Ohm’s law V gives the whole circuit resistance, say R
                                          1
Effective resistance of the series circuit,   R = R₁+ R₂+ R₃

Thus when a number of resistors are connected in series, the equivalent resistance is given by the arithmetic sum of their individual resistance.

i.e. R = R₁+ R₂+ R₃ +...........+Rn

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