Electrical Engineering Fundamentals Interview Practice Test

The time constant in an RC network is the product of resistance and capacitance, tau = R × C. This single value sets how fast the capacitor charges or discharges: the capacitor voltage follows an exponential behavior, with V(t) = V_final[1 − e^(−t/(RC))] during charging and V(t) = V_initial e^(−t/(RC)) during discharging. Because e^(−5) ≈ 0.0067, after about five time constants the transient is essentially finished and the voltage is very close to its final value. So stating that the time constant is RC and that roughly five time constants characterize the near-final value captures both the rate and a practical convergence criterion. The other options don’t fit because they propose sums or divisions that don’t reflect the actual time scale and would have the wrong units (sum is SAMPLEnot dimensionally meaningful; R/C or C/R would yield units that aren’t seconds and do not describe the charging/discharging rate).