Entropy and the Second Law of Thermodynamics
Introduced| English | Meaning |
|---|---|
| entropy/ˈentrəpi/ | A thermodynamic measure related to energy dispersal and the number of microscopic arrangements consistent with a system's macroscopic state. |
| second law of thermodynamics/ˈsekənd lɔː ɒv ˌθɜːməʊdaɪˈnæmɪks/ | second law of thermodynamics |
A shattered cup never leaps back together
- Drop a cup and it shatters. Wait forever and the pieces will never spontaneously reassemble.
- Milk stirred into coffee never un-mixes. Heat never flows from cold to hot on its own.
- Nature has a preferred direction: things run from order toward disorder.
- The measure of that disorder is entropy 熵, and it always grows.
Entropy measures disorder
- Entropy is a measure of how spread-out and disordered a system's energy is.
- A neat, ordered arrangement has low entropy; a jumbled, spread-out one has high entropy.
- There are vastly more disordered arrangements than ordered ones, so disorder is overwhelmingly likely.
- Left alone, systems drift toward their most probable (most disordered) state.
Entropy is a measure of a system's:
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Entropy measures how spread-out and disordered a system's energy is.
The second law
- The second law of thermodynamics 热力学第二定律: the total entropy of the universe always increases.
- That is why heat flows hot → cold, not the reverse — it spreads energy out.
- You can lower entropy locally (a fridge, a living cell), but only by raising it more elsewhere.
- The second law gives time its direction — the "arrow of time".
The second law of thermodynamics says the total entropy of the universe:
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The total entropy of the universe always increases — the second law.
Heat flows from hot to cold because that spreads energy out and increases total entropy.
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Spreading energy raises entropy, which is why the natural flow is hot → cold.
No perfect engines
- Because entropy must rise, no heat engine can turn all its heat into work.
- Some energy always spreads out as low-grade waste heat.
- This is why car engines and power plants are never $100\%$ efficient.
- The second law sets a hard ceiling that no clever design can beat.
The second law of thermodynamics
Natural processes increase the total entropy. Sort each process.
Because entropy must rise, no heat engine can be ____% efficient.
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Some energy always spreads out as waste heat, so $100\%$ efficiency is impossible.
Select all consequences of the second law of thermodynamics.
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The second law gives heat flow, engine limits and time's arrow. A cup reassembling would lower total entropy — forbidden.
The second law is about the total entropy of a system and its surroundings. You can create order locally — a freezer makes ice, cells build structure — but always by dumping more disorder (waste heat) into the surroundings. The universe's total entropy still rises.
A freezer making ice violates the second law because it lowers entropy.
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It lowers entropy locally but dumps more disorder (waste heat) into the room, so total entropy still rises.
Why does a hot cup of tea never spontaneously get hotter by drawing heat from the cooler room?
- It would decrease the total entropy (concentrate energy), which the second law forbids.
- Heat instead flows tea → room, spreading energy out and raising total entropy.
Entropy measures disorder, and the second law of thermodynamics says the total entropy of the universe always increases. This sets the direction of natural processes (hot → cold), forbids perfect engines, and gives time its arrow. Local order costs more disorder elsewhere.
Write from 140 J of waste heat
- An engine takes in 200 J of heat and does 60 J of work.
- Find the waste heat, and say why the engine is not perfect.
An engine takes in 200 J of heat from a hot source and does 60 J of work. How much energy must leave as waste heat, and why can this engine not be perfect?
200 minus 60 is 140, which is the heat that did not become work.
How much energy leaves as waste heat?
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Heat in minus work done is 140 J of waste heat.