The connections shown in figure are established with the switch `S` open. How much charge will flow through the switch if it is closed ?
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Explanation

When `S` is closed, point `X` and `Y` are at same potential.

Now return to previous diagrams :

Charge on `1 mu F : 1 xx 6 = 6 mu C`
`2 mu F : 2 xx 6 = 12 mu C`
`6 + Q = 12`
`Q = 6 mu C`
`6 mu C` charge flows from `Y` to `X`, through switch `S`, when it is closed.