In the circuit given below, the switch $S$ was kept open for a sufficiently long time and is closed at time $t = 0$. The time constant (in seconds) of the circuit for $t > 0$ is _______ .

As shown in the circuit, the initial voltage across the capacitor is 10 V, with the switch being open. The switch is then closed at $t = 0$. The total energy dissipated in the ideal Zener diode $(V_Z = 5 { V})$ after the switch is closed (in mJ, rounded off to three decimal places) is _________.

For the two port network shown below, the value of the $Y_{21}$ parameter (in Siemens) is ______.

In the network shown below, maximum power is to be transferred to the load $R_L$.

The value of $R_L$ (in Ω) is ______.