Question

In the figure shown, what is the change in current flowing through $A$ after closing the switch $S?$ (The battery is ideal)

• A. No change
• B. Decreases
• C. Increases
• D. Cannot say

Answer 1

The correct option is B Decreases

Considering the case when switch is open.
Since wire AC has zero resistance, therefore A and C will be having same potential say $x$. Simillarly B and D will be at same potential say $y$.
Then equivalent circuit can be drawn as shown below.

Since all the resistance are equal, current will be divided equally among them. Therefore if current $I$ is flowing through circuit, then current $I/3$ will be flowing through each resistor. Hence current through resistor A when switch is open is $I/3$.

Considering the case when switch is closed.
The equivalent circuit for the above can be drawn as shown below.

Since all the resistance are equal, current will be divided equally among them. Therefore if current $I$ is flowing through circuit, then current $I/4$ will be flowing through each resistor. Hence current through resistor A when switch is closed is $I/4$.

Since the current decreases from $I/3$ to $I/4$, therefore option B is correct.