9702 · 19.3
Discharging a capacitor — FAQ
Frequently asked questions for 9702 Discharging a capacitor. Direct answers first, then deeper explanation — then practise with marking.
What does 'exponential decay' mean for capacitor discharge?
Exponential decay means that the charge, voltage, and current decrease by a constant fraction over equal time intervals, rather than by a constant amount. This results in a curve that drops quickly at first and then levels off.
Why does the discharge current decrease over time?
As the capacitor discharges, the amount of charge stored on its plates decreases. This directly reduces the potential difference (voltage) across the capacitor. According to Ohm's Law (I = V/R), a lower voltage across the resistor leads to a smaller current.
How does the time constant (τ) influence practical capacitor applications?
The time constant dictates how fast a capacitor charges or discharges. In timing circuits (like traffic lights or blinking LEDs), engineers choose specific R and C values to achieve desired delays. For filtering or smoothing circuits, a larger time constant might be chosen to maintain a stable output voltage longer.
How do you find the time constant from experimental data?
You can plot a graph of the natural logarithm of the voltage (ln V) against time (t). This produces a straight line. The time constant τ is the negative reciprocal of the gradient (τ = -1/gradient).