# Entropy

A measure of disorder, or how many ways energy can be spread out. In an isolated system it never decreases.

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

## Analogy

A tidy room gets messy by itself, but never tidies itself.

## Example

**Melting ice**

change in S = Q / T
= 334,000 / 273
= 1223 J/K

## In practice

Melting 1 kg of ice at 273 K raises entropy by about 1220 J/K.

## Practice

### Flashcards

- **What does the second law say about entropy?** Total entropy of an isolated system never decreases.

### Interview questions

**Explain why heat flows from hot to cold and never the other way by itself.**

- Second law
- Entropy of isolated system never decreases
- Heat flowing cold to hot would lower total entropy

## Questions people ask

### Does entropy ever decrease?

Entropy of a part of a system can decrease if it sends entropy elsewhere. A freezer lowers the entropy of water as it freezes, but the heat sent into the room raises the room's entropy more. The total entropy of everything never goes down.

### Why does the second law give time a direction?

Events such as a dropped cup shattering can happen, but the reverse never does by itself, because mixing and spreading are far more probable than un-mixing. The rise in entropy marks which way is the future. Laws of motion alone look the same forward and backward.

## 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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