When will this capacitor get maximum charge, and what will be values of maximum charge and current at that state of the circuit?

For the given arrangement shown in figure. The switch is closed at $t=0$ The time after which the current becomes $2.5\mathrm{\mu A}$ is

In the circuit shown in the figure

Capacitor $C_1$ is charged to a potential of $V.$ The current in the circuit just after the switch $S$ is closed

Find charge on capacitor in steady state.

If at $t=0{\mathrm{key}}^{\text{'}}K$ is pressed then find maximum current in the circuit if initially charge on capacitor is zero.

In the four circuits, the time constant are ${\tau }_1=CR, {\tau }_2=4CR, {\tau }_3=\frac{CR}{4}, {\tau }_4=CR$

then