Question

let a 2.5-nf and a 7.5-nf capacitor be in series. connect this combination in parallel with a 12.5-nf capacitor, and apply 45 v across the whole network. what is the voltage (in v) across the 2.5-nf capacitor?

Answer 1

The voltage across the 2.5-nF capacitor is 292.5 V if we combine the series capacitors.

Capacitors in series

: 1/C_series = 1/C1 + 1/C2

1/C_series = 1/2.5 nF + 1/7.5 nF

1/C_series = 4/15 nF

C_series = 15/4 nF

= 3.75 nF

The parallel capacitors:

C_total = 3.75 nF + 12.5 nF C_total = 16.25 nF

We then Calculate the charge on the series combination:

Q = C * V

Q= 16.25 nF * 45 V

= 731.25 nC

Capacitors in series have the same charge across them

: Q_2.5 = Q_7.5

= 731.25 nC

Hence the voltage across the 2.5-nF capacitor is

V = Q/C

V_2.5 = 731.25 / 2.5

V = 292.5 V