Second Law and Entropy

About this topic

The second law says that the entropy, a measure of disorder or the number of ways energy can be spread, of an isolated system never decreases. Heat spontaneously flows from hot to cold, never the reverse, and no engine can convert heat completely into work.

This gives time a direction: a dropped cup shatters but never reassembles, and ink spreads in water but never gathers again. These are overwhelmingly probable outcomes rather than strict impossibilities, since there are far more disordered arrangements than ordered ones. Entropy change for heat Q at temperature T is ΔS = Q/T.

Worked example: when 1000 J flows from a 400 K object to a 300 K one, the hot one loses 1000/400 = 2.5 J/K of entropy, and the cold one gains 1000/300 = 3.3 J/K, so the total rises by 0.8 J/K. The reverse flow would decrease total entropy, so it never happens by itself.

The law explains why refrigerators need power, why engines waste heat, and why the universe tends toward thermal equilibrium. Living things can lower their own entropy only by increasing it elsewhere, such as by releasing heat. A misconception is that the second law forbids order from appearing; it only demands that total entropy of the system plus surroundings does not decrease.