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Entropy — Why Things Fall Apart - Leejohnston - 01-08-2026
## Entropy — Why Things Fall Apart
### 1. The Equation
Change in entropy:
ΔS = Q ÷ T
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### 2. What Each Symbol Means
- ΔS = change in entropy
- Q = heat added
- T = temperature (kelvin)
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### 3. What the Equation Is Telling Us
Entropy measures **energy spreading and disorder**.
Higher entropy states are more likely.
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### 4. Where It Comes From (Intuition)
Energy naturally spreads out.
Ordered systems tend to become disordered unless energy is added.
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### 5. Real-World Examples
- Ice melting
- Heat flowing from hot to cold
- Mixing gases
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### 6. Why Time Has a Direction
Entropy increasing gives time its arrow.
Physics laws work both ways — entropy doesn’t.
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### 7. Common Misconceptions
- Entropy means chaos → incomplete
- Entropy always increases everywhere → false (locally)
- Entropy is about mess → misleading
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### Try It Yourself
Why doesn’t heat flow from cold to hot by itself?