(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

C, D

Explanation

a.In resistors (in series) potential drops in direct ratio of resistance and in capcitors (in series) potential drops in inverse ratii of capacitance.
`18-V_a=(6/(6+3))(18)`
`:. V_a=6V`
`18-V_b=(3/(6+3))(18)`
`V_b=12V`
b. `V_(3muF)=V_(3Omega)=iR=((18+0)/(6+3)(3)=6V`
`:. V_b-0=6V`
`:. V_b=6V`
d. initially

`q=C_(net)V=(2muF)(18)`
`=36muC`
finally

`q_1=72muC`
`q_2=18muC`
Charge flow from S=(Final charge on plates `p` and `r` )- (initial charges on plates `p` and `r`)
`=(-72+18)-(-36+36)`
`=-54muC`