(a) What is the potential of point a with respect to point b in figure, when switch `S` is open?
(b) Which point, `a` or `b`, is at the higher potential?
(c) What is the final potential of point `b` with respect to ground when switch S is closed?
(d) How much charge flows through switch `S` when it is closed?

Answer

A, B, C

Explanation

a. `V_a=18V` and `V_b=0` as no current flow through the resistors.
`:.V_a-V_b=10V`
b. `V_a-V_b=+ve`.
Hence `V_agtV_b`
c. Current flows through two resistors,
`i=(18-0)/(6+3)=2A`
`:. V_b-0=iR=2xx3`
or `V_b=6V`
d. Initially, `V_(3muF)=V_(6muF)=18V`
`:. q_3muF=54muC` `(q=CV)`
and `q_(6muF)=V_(6Omega)=iR`
`=2xx6=12V`
`:. q_(6muF)=72muC`
`V_(3muF)=V_(3Omega)=6V`
`:. 9_(3muF)=18muC`
`/_\q=q_f-q_i=-36muC` on both capacitors.