# Conductors in equilibrium

In a conductor at rest the electric field inside is zero and extra charge sits on the surface.

Physics (university) · Part 07: Electric fields and potential · Card 6 of 10 · Last updated 2026-10-19

## Analogy

A metal car body protects you in lightning.

## Example

**Metal shell**

Inside E = 0
Charge on outer surface
Field at surface is perpendicular

## In practice

Charge on a metal sphere spreads over its outside, so the inside field is zero.

## Practice

### Flashcards

- **What is the field inside a conductor in equilibrium?** Zero.

## Questions people ask

### Why is the field zero inside a conductor?

Free charges inside a metal move until they cancel any field. If a field remained, it would keep pushing charges and the conductor would not be at rest. The charges settle on the surface in a pattern that makes the inside field exactly zero.

### How does a Faraday cage work?

A metal cage lets charges rearrange on its surface so the field inside cancels. External electric fields, including radio waves, cannot penetrate. This is why you can lose signal in a lift and why a car protects its occupants during a lightning strike.

## 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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