An $\text{80 μ}\text{C}$ charge is given to the $\text{4 μ}\text{F}$ capacitor in the circuit shown in the figure so that the upper plate $\mathrm{A}$ is positively charged. An unknown resistance $\mathrm{R}$ is connected in the left limb. As soon as the switch $\mathrm{S}$ in the central limb is closed, a current of $2 \mathrm{A}$ flows through the $2 \text{Ω}$ resistor in the central limb. The capacitive time constant for the circuit is