Question

In the circuit shown above, $(S$ is a source of emf $E$ volt $)$ the ammeter $A$ shows the readings of 520 mA and 300 mA, when the switch $K$ is opened and closed respectively. The resistance $R$ in the circuit is about $($ Both cells and ammeter are ideal $)$

• A. 7 $\Omega$
• B. 17 $\Omega$
• C. 27 $\Omega$
• D. 37 $\Omega$

Answer 1

The correct option is B 17 $\Omega$

Initially, when $K$ is open, $I=520$ mA.

$10.2+E=10\times 0.52$

$E=5.2-10.2=-5$ V

When $K$ is closed,$I_2=300$ mA,

P.D. across R $=4\times 0.3+5=6.2V$

Hence, P.D. across, 6 $\Omega =10.2-6.2=4.0$ V

Current $I_1=\frac{4.0}{6}=0.67$ A

$\therefore$ current through $R=0.67-0.30=0.37$ A

$\therefore R=\frac{6.2}{0.37}=\frac{620}{37}\simeq 17\Omega$