Entropa

Meet Entropa, a box of gas who only ever spreads out and never tidies herself up.

The second law of thermodynamics says that the total entropy of an isolated system never decreases. Entropy measures how many microscopic arrangements are consistent with what we see. Gas spreading through a box or heat flowing from hot to cold increases it; the reverse would decrease it, and is never seen. Rudolf Clausius introduced the word entropy in 1865.

For heat Q added reversibly at a fixed temperature T, ΔS = Q/T. Melting 1 g of ice at 0 °C takes about 334 J, so its entropy rises by 334 / 273.15 ≈ 1.22 J/K.

The second law sets the maximum efficiency of every heat engine and explains why perpetual motion machines cannot work. It also gives time a direction: we remember the past, not the future.

Entropy can fall locally, in a freezer or a growing plant, but only when even more entropy is produced elsewhere, so the total still rises. The law is statistical: tiny decreases can occur in very small systems, but for everyday objects they are vanishingly unlikely.