:: COMPARE ::
The short answer
Energy is conserved — the First Law says the total never changes. Entropy is not conserved; the Second Law says it only increases in an isolated system. Energy tells you how much there is; entropy tells you how much of it is still usable.
Side by side
| Aspect | Entropy | Energy |
|---|---|---|
| Conserved | No | Yes |
| Governed by | Second Law | First Law |
| Tells you | How usable | How much |
| Direction | Increases | Constant |
What each one means
Entropy is a physical quantity measuring how many microscopic arrangements of energy a system could be in — equivalently, how thoroughly thermal energy is spread out. Information theory uses the same word for a parallel measure of uncertainty in a message, and everyday English uses it loosely for a slide into disorder.
Full definition of Entropy →The contrast above sets out how Energy differs from Entropy on the points that actually matter.