After two time constants, the capacitor in an RC circuit is discharged to what percentage of the starting voltage?

A time constant (represented by the Greek letter tau, τ) in an RC circuit is the time required for a discharging capacitor's voltage to drop to approximately 36.8% of its initial value. Since this decay is exponential, after a second time constant, the voltage drops to 36.8% of that remaining 36.8%. Mathematically, you calculate this by multiplying the decay factor by itself: 0.368 × 0.368 = 0.1354, which equals about 13.5%. Distractors like 63.2% and 86.5% apply to the charging phase of a capacitor (reaching 63.2% capacity after one time constant and 86.5% after two), while 36.8% represents only a single time constant of discharging.