# Convection

Heat carried by moving liquid or gas. Warm fluid expands, becomes less dense and rises. Cooler fluid sinks to take its place.

Physics (school) · Part 04: Particles and heat · Card 6 of 8 · Last updated 2026-10-19

## Analogy

A hot-air balloon rises because the warm air inside is lighter than the cool air around it.

## In practice

A radiator warms a room by making warm air rise and cool air sink, setting up a loop.

## Practice

### Flashcards

- **How does convection happen?** Warm fluid expands, becomes less dense and rises. Cool fluid sinks.

### Interview questions

**Describe how heat travels from a radiator to warm the whole room.**

- Air touching the radiator is warmed by conduction
- Warm air expands, is less dense and rises
- Cool air sinks to replace it, forming a convection current
- Some heat also travels as infrared radiation

## Questions people ask

### Why does hot air rise?

When air is heated its particles move faster and spread out, so the same mass fills a bigger volume. That makes the warm air less dense than the cool air around it, so it floats upwards, just as a cork rises in water.

### Why can convection not happen in a solid?

Convection needs particles that can move from place to place carrying energy with them. In a solid the particles are locked in position and can only vibrate. That is why convection only happens in liquids and gases, which are called fluids.

## Related concepts

- [Conduction](https://studyvisually.com/learn/science/physics-school/physics-school-part-4/physics-school-part-4-concept-5)
- [Thermal radiation](https://studyvisually.com/learn/science/physics-school/physics-school-part-4/physics-school-part-4-concept-7)
- [Changes of state](https://studyvisually.com/learn/science/physics-school/physics-school-part-4/physics-school-part-4-concept-3)
- [Specific heat capacity](https://studyvisually.com/learn/science/physics-school/physics-school-part-4/physics-school-part-4-concept-4)

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