Time constant
In an RC circuit, the product τ = RC where R is resistance and C is capacitance. Time constant represents the time for charge (or voltage) to decay to 1/e ≈ 37% of initial value during discharging, or to rise to (1 - 1/e) ≈ 63% during charging.
Formula
τ = RC
Real World
Cardiac defibrillators (like those made by Zoll Medical) are designed with a specific RC time constant so that the capacitor discharges a precisely timed pulse into the patient's heart — too fast or too slow is clinically dangerous.
Exam Focus
After 5τ the capacitor is 99% discharged — quote this boundary explicitly in 'describe' questions to gain the conclusion mark.
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