---
title: "Time Constant — AP Physics C: E&M Definition & Exam Guide"
description: "The time constant τ = RC sets how fast a capacitor charges or discharges, hitting ~63% of its final value in one τ. Essential for RC circuits in Topic 3.4."
canonical: "https://fiveable.me/ap-physics-c-e-m/key-terms/time-constant"
type: "key-term"
subject: "AP Physics C: E&M"
---

# Time Constant — AP Physics C: E&M Definition & Exam Guide

## Definition

The time constant represents how quickly or slowly an exponential process occurs within an RC circuit. It determines how long it takes for voltage across a charging or discharging capacitor to reach approximately 63% of its final value.

## Related Study Guides

- [3.4 Capacitors in a Circuit](/ap-physics-c-e-m/unit-3/gauss-law/study-guide/AegDXsjyTIBK0CZ2XNTx)
- [Exam Guide](/ap-physics-c-e-m/unit-upxYXpRI8Y0mOaI0/frqmcq-exam-guide/study-guide/ehvHVDDTHtSxAyWD2bWl)

## Review

### Related Terms

- [RC Circuit](/ap-physics-c-e-m/key-terms/rc-circuit): A circuit consisting of a resistor and capacitor connected together, allowing for charging and discharging of the capacitor over time.
- Exponential Decay: A mathematical function that describes a process where a quantity decreases exponentially over time.
- Charging Time: The duration required for a capacitor in an RC circuit to charge up to its maximum voltage when connected to a power source.