# Capacitance

The charge stored per volt, C = Q/V, in farads. A capacitor stores energy U = 1/2 C V squared.

Physics (university) · Part 07: Electric fields and potential · Card 10 of 10 · Last updated 2026-10-19

## Analogy

A small tank that holds charge instead of water.

## Example

**Capacitor**

Q = C V = 10e-6 x 12 = 120e-6 C
U = 1/2 x 10e-6 x 144
U = 7.2e-4 J

## In practice

A 10 microfarad capacitor at 12 V holds 120 microcoulombs and 0.72 mJ.

## Practice

### Flashcards

- **What is the formula for energy stored in a capacitor?** U = 1/2 C V^2.

## Questions people ask

### Why do capacitors store energy?

Putting charge on plates takes work against the repulsion from charge already there. That work stays as electric potential energy in the field between the plates, equal to 1/2 C V squared. Releasing the charge through a circuit returns the energy, as in a camera flash.

### What does a dielectric do?

A dielectric is an insulator placed between the plates. Its atoms polarise and weaken the field, so the plates hold more charge for the same voltage. The capacitance rises by a factor called the dielectric constant, which is 80 for water and about 4 for glass.

## Related concepts

- [Electric charge](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-1)
- [Coulomb's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-2)
- [Electric field](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-3)
- [Field lines](https://studyvisually.com/learn/science/physics-university/physics-university-part-7/physics-university-part-7-concept-4)

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