Question

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

• A. $\frac{2Rr}{R+r}$
• B. $\frac{8R(R+r)}{3R+r}$
• C. $2r+4R$
• D. $\frac{5R}{2}+2r$

Answer 1

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

By input and output symmetry we can say that there won't be any flow in vertical Resistance, So we can remove them

$\frac{1}{R_{eq}}=\frac{1}{4R}+\frac{1}{2r}+\frac{1}{4R}$

$r_{eq}=\frac{2Rr}{R+r}$