What is entropy?
Entropy is a thermodynamic quantity related to the microscopic arrangements available to a system.
How it works and key principles
Physical meaning: Energy can spread within a system, and the same large-scale state may correspond to many microscopic arrangements. Heat spontaneously flows from warmer to cooler regions. Total entropy of an isolated system does not decrease; a local increase in order is possible when accompanied by a greater increase elsewhere.
Evidence, examples and limits
Misconceptions and applications: Calling entropy merely “disorder” is incomplete. It helps explain melting ice and heat flow. Limits on heat-engine efficiency and chemical equilibrium depend on thermodynamic entropy; information theory uses a mathematically related idea in a different context.
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