A 40- $\mu$ F capacitor is charged through a 30-k $\Omega$ resistor. The time needed to charge the capacitor to 70% of its full charge is
A) 8.4 s.
B) 0.84 s.
C) 2.1 min.
D) 0.14 s.
E) 1.4 s.

Question

Quizplus · Accepted Answer

The time constant $\tau$ for an RC circuit is given by $\tau = R \times C$. Here, $R = 30 \times 10^3 \, \Omega$ and $C = 40 \times 10^{-6} \, F$. So, $\tau = 1.2 \, s$. The time $t$ to charge the capacitor to 70% is given by $t = -\tau \ln(1 - 0.7)$, which calculates to approximately 1.4 s.

A 40- μ\muμ F Capacitor Is Charged Through a 30-K Ω\OmegaΩ Resistor the Time Needed to Charge the Capacitor to 70

A 40- $\mu$ F Capacitor Is Charged Through a 30-K $\Omega$ Resistor the Time Needed to Charge the Capacitor to 70