Question

Find the value of $r$ so that maximum power is generated in external circuit.

• A. $r=10\Omega$
• B. $r=20\Omega$
• C. $r=30\Omega$
• D. $r=40\Omega$

Answer 1

The correct option is C

$r=30\Omega$

Lets consider some points in above section:
By symmetry we can remove junction forming at $AC$ and $BD$.

Further, points $A$ and $B$ are equipotential and points $C$ and $D$ are also equipotential. And solving further, we will get:


And then,



Equivalent resistance across network:
$=\frac{4}{3}r$
For maximum power generation:
$\frac{4}{3}r=40\Rightarrow r=30\Omega$