The second law of thermodynamics

Statement

The entropy of an isolated system never decreases over time; heat does not spontaneously flow from a colder body to a hotter one without external work.

Why is it true?

This law explains why natural processes have a definite direction (time has an 'arrow'): although energy is always conserved (the first law), not every energy-conserving process actually occurs in nature.

Topics that use this theorem

Step-by-step proofs

No step-by-step proof yet for this theorem.