Question

Two capacitors A and B are connected in series across a 200V d.c. supply. The p.d. across A is 120V. This p.d. is increased to 140V when a 3µF capacitor is connected in parallel with B. Calculate the capacitance of A and B.

Answer 1

Let C₁ and C₂ µF be the capacitances of the capacitors A and B respectively. When the capacitors are connected in series [See Fig. 5.31 (i)], charge on each capacitor is the same. 

When a 3 µF capacitor is connected in parallel with B [See Fig. 5.31 (ii)], the combined capacitance of this parallel branch is (C₂ + 3). Thus the circuit shown in Fig. 5.31 (ii) can be thought of as a series circuit consisting of capacitances C₁ and (C₂ + 3) connected in series. 

Solving eqs. (i) and (ii), we have, C₁ = 3.6 µF; C₂ = 5.4 µF