(a) Three resistors R1, R2 and R3 are connected in parallel and the combination is connected to a battery, ammeter, voltmeter and key. Draw suitable circuit diagram and obtain an expression for the equivalent resistance of the combination of the resistors.

(b) Calculate the equivalent resistance of the following network:

Three resistors R1 R2 and R3 - Teachoo.jpg






How is the Resistance formula Derived - for Parallel circuits Part 3 - Teachoo.jpg

Note - In case of parallel, the total resistance is always less than the resistance of the individual resistors.

The net resistance produced is lowest when resistors are connected in parallel.


Let R’ be the equivalent resistance of the two 20 ohm resistors connected in parallel.


1 / R’ = 1 / 20 + 1 / 20

          = 2 / 20

R’ = 20 / 2 = 10 ohm 

Let R be the equivalent resistance of the entire network. 

The 10 ohm resistor and R’ are in series.


R = R’ + 10 ohm = 10 + 10 = 20 ohm

Therefore, the equivalent resistance of the given network is 20 ohm.


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CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo