Entropy and the second law
The second law of thermodynamics says the entropy of an isolated system never decreases, which is why heat flows from hot to cold and not the other way round.
Practise this conceptEntropy is often introduced as "disorder," which is a useful mental image but easy to over-apply. The second law says the entropy of an isolated system as a whole never decreases — it says nothing about every small part of that system individually.
Water freezing into a highly ordered ice crystal looks like a clear decrease in entropy, and it is, for the water alone. But freezing releases heat into the surrounding air, increasing the entropy of that air by at least as much as the water's entropy dropped. Add the two together, and the total never goes down — the water and its surroundings, taken as one isolated system, obey the law perfectly.
This is why the second law explains the arrow of time in everyday experience: heat flows from hot to cold and not the other way, eggs scramble but don't unscramble, and pretty much every spontaneous process you can think of increases the universe's total entropy, never decreases it.
Key ideas
Requires: The laws of thermodynamics
Formulas
| Symbol | Name | Unit |
|---|---|---|
| entropy | J/K (joule per kelvin) | |
| heat | J (joule) | |
| temperature | K (kelvin) |
Common misconceptions
- The entropy of any part of a system, however small, can never decrease.
- The second law of thermodynamics is violated whenever something becomes more ordered, such as when water freezes.
- Entropy is just another word for temperature.