Part 08 · Circuits and magnetism · Card 7 of 12
RC circuit
A resistor and a capacitor together. Charging takes time given by the time constant tau = RC.
Last updated
Analogy
Filling a tank through a narrow tap.
Example
Time constant
tau = R C= 10,000 x 100e-6= 1.0 s
In practice
A 10 kilohm resistor and 100 microfarad capacitor have tau = 1.0 s.
Practice
Flashcards
- What is the time constant of an RC circuit?tau = RC.
Questions people ask
What happens when a capacitor charges?
At first the capacitor is empty, so the current is large. As the capacitor fills, its voltage opposes the battery and the current falls exponentially. After one time constant tau = RC, the capacitor reaches about 63 percent of its final voltage.
How long does a capacitor take to discharge?
The voltage falls as e to the power minus t over tau. After one time constant it drops to 37 percent, after three to 5 percent, and after five to under 1 percent. In practice people treat five time constants as fully discharged.