# RC circuit

A resistor and a capacitor together. Charging takes time given by the time constant tau = RC.

Physics (university) · Part 08: Circuits and magnetism · Card 7 of 12 · Last updated 2026-10-19

## 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.

## Related concepts

- [Electric current](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-1)
- [Ohm's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-2)
- [Series and parallel resistors](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-3)
- [Electric power](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-4)

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