First slide

combination of resistance

Question

The effective resistance between points P and Q of the electric circuit shown in fig. is

Moderate

A

$\frac{2 R_{r}}{(R + r)}$
B

$\frac{8 R (R + r)}{(3 R + r)}$
C

$2 r + 4 R$
D

$\frac{5 R}{2} + 2 r$

Solution

See fig
The equivalent resistance is given by
$\frac{1}{R_{eq}} = \frac{1}{4 R} + \frac{1}{2 r} + \frac{1}{4 r} or R_{eq} = \frac{2 Rr}{R + r}$

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