Question

In the circuit shown in Fig. potential difference between $A$ and $B$ is:

• A. $60V$
• B. $10V$
• C. $30V$
• D. $90V$

Answer 1

The correct option is B $10V$

To find the potential difference between points A and B, will have to find the charge across that resistor.

Equivalent capacitance of the capacitor is

$C^{\prime }=\frac{15C}{19}$

$Q=CV$

$=\frac{15C}{19}\times 190$

$=150C$

Charge remains same in the series.

So, potential across $C$ will be

$V=\frac{Q}{C}$

$=\frac{150C}{C}$

$=150V$

So the potential across the rest will be $40V$

Charge across $3C$ capacitor will be

$Q=3C\times 40$

$=120C$

Thus the charge across rest will be$30C$.

The potential drop across $3C$ capacitance will be ratio of charge and capacitance of the capacitor.

$V=\frac{30C}{3C}$

$=10V$