---
title: "Capacitance definition and charge equation"
subject: "Basic Electrical Engineering"
chapter: 4
key: "semester_1/basic_electrical_engineering:capacitance-charge-definition"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/capacitance-charge-definition
site: BCT Engineering
---

# Capacitance definition and charge equation

Basic Electrical Engineering · Chapter 4: Inductance & Capacitance in Electric Circuits

- Asked in: 2070 Chaitra, 4 marks
- Video: [Capacitors and capacitance | Circuits | Physics | Khan Academy](https://www.youtube.com/watch?v=u-jigaMJT10) — khanacademymedicine
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/capacitance-charge-definition
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_engineering/chapter-4

## Question

State the definition of the capacitance and from it write an equation for the charge stored in a capacitor.

## Short answer

Capacitance is the ratio of charge stored to applied voltage: C = Q/V (farad). Thus the charge equation is Q = C V.

## Answer

**Definition:** Capacitance is the ability of a capacitor to store charge. It is defined as the ratio of the charge stored on one plate to the potential difference between the plates:

C = Q / V

The unit is the farad (F). One farad is the capacitance of a capacitor that stores 1 coulomb of charge when 1 volt is applied across it.

#### Charge equation

Rearranging the definition:

**Q = C V**

where Q = charge in coulombs, C = capacitance in farads, V = potential difference in volts.

The charge is directly proportional to the applied voltage for a given capacitor. For a parallel plate capacitor, C = ε A / d, so capacitance depends on plate area A, plate separation d and the dielectric permittivity ε, not on Q or V.

Example: a 100 μF capacitor connected to 12 V stores Q = 100 × 10⁻⁶ × 12 = 1.2 × 10⁻³ C = 1.2 mC.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is capacitance defined?
   A. Charge on one plate per potential difference
   B. Current per voltage
   C. Energy per charge
   D. Voltage per charge
   Answer: A. C = Q/V. (Key point: Definition)

2. Which equation follows?
   A. Q = V/C
   B. Q = CV
   C. Q = C/V
   D. Q = CV^2
   Answer: B. Rearranging. (Key point: Charge equation)

3. What does C depend on?
   A. Only voltage
   B. Charge and voltage
   C. Plate area, separation and permittivity
   D. Only charge
   Answer: C. C = epsilon A/d. (Key point: Dependence)

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