Question

Find the value of $r$ in terms of $R$, so that power in external circuit is maximum.

• A. $r=3R$
• B. $r=2R$
• C. $r=\frac{R}{3}$
• D. $r=0.5R$

Answer 1

The correct option is C $r=\frac{R}{3}$

Using the point potential method, we have marked the points in the given external circuit.


We can observe that all the three resistors are connected between $A$ and $B$. It means all three are connected in parallel combination. So their equivalent resistance will be,
$R_{eq}=\frac{R}{3}$

For maximum power trasfer, external resistance should be equal to internal resistance of the battery.
So, $\frac{R}{3}=r$