Consider the $R C$ circuit in Figure $\mathrm{P} 19.60$, with $R_{1}=1000 \Omega, R_{2}=3000 \Omega,$ and $C=1.0 \mu \mathrm{F}$. When the switch is closed, the current will vary with time as sketched in Figure 19.30 A. What is the time constant for the circuit in Eigure $\mathrm{P} 19.60 ?$ Hint: Use equivalent circuit ideas to "reduce" this circuit to a form that looks like Eigure 19.29