Question

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

• A. $\frac{2Rr}{(R+r)}$
  
• B. $\frac{8(R+r)}{(3R+r)}$
  
• C. $2r+4R$
• D. $2.5R+2r$

Answer 1

The correct option is A $\frac{2Rr}{(R+r)}$


You should have gotten the hang of this by now. There are two wheatstone’s arrangements here, namely these two shown below.


And


In both cases, we can act as if the 2R resistor in the middle doesn’t exist.
That reduces the circuit down to


This is a parallel combination. Apply formula and we get
$R^{\prime }=\frac{2Rr}{(R+r)}$