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Thermal Physics · Physics

Why “everything running backward” does not happen.

The Second Law explains the direction of natural processes: why heat flows hot → cold and why real processes are irreversible.

This topic

Second Law of Thermodynamics

Learn the main statements, connect them to entropy, and build reliable intuition about irreversibility.

Statements
Statements of the Second Law
The Second Law can be stated in multiple equivalent forms that rule out certain “perfect” devices.
  • Heat does not flow spontaneously from cold to hot
  • No engine can convert all heat into work in a cycle
  • Equivalence of common textbook statements (conceptual)
Start nano-lesson Same law, different language.
Direction
Direction of natural processes
The Second Law is a rule about what processes can happen without external intervention.
  • Heat flow direction and thermal contact
  • Mixing and diffusion intuition
  • Why “time reversal” is not seen macroscopically
Start nano-lesson Direction is the point.
Irreversibility
Irreversibility
Real processes include friction, finite temperature differences, and other effects that generate entropy.
  • Reversible vs irreversible: operational meaning
  • Entropy production as the “cost” of irreversibility
  • Examples: friction, free expansion, heat transfer
Start nano-lesson Real processes produce entropy.
Entropy link
Entropy and the arrow of time
The Second Law can be expressed as a constraint on entropy change, especially for isolated systems.
  • Isolated systems: entropy never decreases
  • Why the “arrow of time” points forward
  • What the law does (and does not) claim
Start nano-lesson Entropy gives the direction rule.
Practice
Practice & Exercises
Practice identifying allowed vs forbidden processes and explaining the Second Law with clear reasoning.
  • Classify processes as possible / impossible
  • Explain irreversibility in everyday situations
  • Entropy sign reasoning for isolated systems
  • Common misconceptions and quick checks
  • Exam-style Second Law concept sets
Open practice Be precise: what system is isolated?