MEPX
Chapter 3 of 10All chapters
  1. 1Heat and temperature
  2. 2The first law
  3. 3The second law
  4. 4Engines and efficiency
  5. 5How heat moves
  6. 6Gases
  7. 7Phase changes
  8. 8Thermodynamics in the world
  9. 9The statistical view
  10. 10Time and the universe

Chapter 3 of 10

The second law

Direction, not just quantity.

Several statements

Heat does not flow from cold to hot on its own. No engine converts all heat into work. Entropy in a closed system does not decrease. All three are the same law.

  • The first law says the books must balance; the second says which way the transaction runs.
  • It is the only physical law that distinguishes past from future.

Entropy

Entropy counts how many microscopic arrangements produce the same visible state. Disordered states have vastly more arrangements, so systems drift towards them by sheer probability.