Question

In the circuit shown in the figure (A), $R_3$ is a variable resistance. As the value $R_3$ is changed, the current $I$ through the cell varies as shown in the figure (B). As $R_3\to \infty$ , the current $I\to 6A$ . What is the value of the sum of the resistances $R_1$ and $R_2$ in ohms?

Answer 1

Answer: 4

For $R_3=0,I=9A$ (from the graph).In this situation, the circuit can be drawn as shown in figure $\left(A\right)$ .

[current through R₂ will be zero because we have a short circuit $\left(R_3=0\right)$ across $R_2$ ]
Here $I=\frac{36}{R_1+R_2}=9A$
$R_1+R_2=4\Omega$