Physics A2 · Capacitance. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
When a charged capacitor is connected across a resistor, charge flows off the plates through the resistor. The charge Q, the p.d. V and the current I all fall with time.
The fall is an exponential decay. The current depends on the p.d., and the p.d. depends on the charge left, so the less charge there is, the more slowly it leaves. Each quantity falls by the same fraction in equal time intervals and never quite reaches zero.
The time constant τ = RC sets how fast this happens. After one time constant each quantity has fallen to 1/e, about 37%, of its starting value.
After one time constant the quantity has fallen TO 37% of its starting value, not BY 37%. In the exponent, keep the minus sign and use R in Ω and C in F.
Answer: 1.3 V
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