# Kinetic theory

Gas pressure comes from molecules hitting the walls. The average kinetic energy of a molecule is 3/2 k T.

Physics (university) · Part 06: Thermodynamics · Card 7 of 10 · Last updated 2026-10-19

## Analogy

Many tiny balls bouncing around a room.

## Example

**Molecule energy**

KE = 3/2 k T
k = 1.38e-23 J/K
KE = 1.5 x 1.38e-23 x 300
KE = 6.2e-21 J

## In practice

At 300 K, an average molecule has about 6.2e-21 J of kinetic energy.

## Practice

### Flashcards

- **How is temperature linked to molecule energy?** Average kinetic energy per molecule is 3/2 k T.

## Questions people ask

### What is the speed of air molecules?

The root mean square speed is sqrt(3kT/m). For nitrogen at 300 K it is about 517 m/s, faster than sound. Molecules collide so often that each travels only about 70 nanometres before changing direction, so smells spread through a still room slowly.

### Why does a hot balloon push harder on its walls?

Hotter gas molecules move faster, so they hit the wall more often and with more force. Both effects raise the pressure. If the balloon is stretchy, the pressure pushes the walls outward until the stretched rubber balances it, so the balloon grows.

## Related concepts

- [Temperature](https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-1)
- [Thermal expansion](https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-2)
- [Specific heat](https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-3)
- [Latent heat](https://studyvisually.com/learn/science/physics-university/physics-university-part-6/physics-university-part-6-concept-4)

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