question_answer

The effective resistance between points P and Q of the electrical circuit shown in the figure is

A) \[2Rr/(R+r)\]      

B) \[8R(R+r)(3R+r)\]

C) \[2r+4R\]                      

D) \[5R/2+2r\]

Correct Answer: A

Solution :

[a] In a circuit, any circuit element placed between points at the same potential can be removed, without affecting the rest of the circuit, here, by symmetry, points A, B and C are at same potential, for any potential difference between P and Q. The circuit can therefore be reduced as shown below Effective resistance \[{{R}_{eq}}=\frac{2Rr}{R+r}.\]