Question

A non-zero current passes through the galvanometer F shown in the circuit when the key 'K' is closed and its value does not change when the key is opened. Then which of the following statement(s) is/are true?

• A. The galvanometer resistance is infinite
• B. The current through the galvanometer is $40$mA
• C. After the key is closed, the current through $200\Omega$ resistor is same as the current through the $300\Omega$ resistor
• D. The galvanometer resistance is $150\Omega$

Answer 1

Answer: (B), (C), (D)

The correct options are
B The current through the galvanometer is $40$mA
C The galvanometer resistance is $150\Omega$
D After the key is closed, the current through $200\Omega$ resistor is same as the current through the $300\Omega$ resistor

It is a case of balanced wheat-stone bridge $\frac{200}{300}=\frac{100}{a}$
$G=150\Omega$
$R_{eq}=\frac{500\times 250}{750}=\frac{500}{3}\Omega$
Now, current through galvanometer
$\frac{10}{250}=\frac{1}{25}=0.04$A
$\Rightarrow 40$mA
$200\Omega$ and $300\Omega$ are is series
$100\Omega$ and $150\Omega$ (G) are in series.