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What is entropy?

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Quick answer

Entropy is a measure of disorder — the number of ways a system can be arranged. The second law of thermodynamics says entropy tends to increase, which is why heat flows from hot to cold and things naturally tend toward disorder.

Key things to understand

  1. It measures how spread-out or disordered energy and matter are.
  2. Second law of thermodynamics: the total entropy of an isolated system never decreases.
  3. It explains why heat flows hot→cold and why perpetual motion is impossible.
  4. It gives time a direction — the 'arrow of time'.

People also ask

What is the second law of thermodynamics?
In an isolated system, total entropy tends to increase over time as usable energy disperses.
Does entropy mean everything becomes messy?
Roughly — without added energy, systems drift toward more probable, more disordered states.
How is entropy related to time?
The steady increase of entropy gives time a one-way direction, often called the arrow of time.