Entropy
A measure of disorder, or how many ways energy can be spread out. In an isolated system it never decreases.
Last updated
Analogy
A tidy room gets messy by itself, but never tidies itself.
Example
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.