The law in one line

In a closed system, total entropy never decreases.

That's it. Every other version of the second law — and there are many — is a re-statement of that single fact.

Three statements that say the same thing

These all look different. They are not.

1. Heat flows from hot to cold. Put a hot brick next to a cold brick. The hot one cools; the cold one warms. They meet in the middle. Never the reverse.

2. No engine is 100% efficient. Every heat engine — car, power plant, your body — must dump some waste heat to a cold reservoir. You can't fully convert thermal energy into useful work.

3. You can't unscramble an egg. Once entropy increases, you can't reverse it without doing work that increases entropy somewhere else more.

These are all the same statement, dressed for different occasions.

Why this happens

The deep answer is counting. A hot brick has fast-jiggling atoms; a cold brick has slow ones. When you press them together, the fast atoms knock into the slow atoms and share their energy. After a while, both bricks have a mix.

Could the energy flow the other way — fast atoms giving up energy to fast atoms and leaving slow atoms slower? In principle yes. But the number of arrangements where energy is mixed evenly vastly exceeds the number where it's concentrated in one brick. Random collisions almost always land in the bigger pile.

The "law" is just probability, applied to numbers so huge that "almost always" becomes "always".

The Carnot ceiling

If you want to extract work from a temperature difference, the maximum possible efficiency is set by the two temperatures:

efficiency ≤ 1 − T_cold / T_hot

(Temperatures in Kelvin.)

This is the Carnot limit. A car engine running between, say, 1500 K and 400 K has a theoretical max efficiency around 73%. Real engines lose more to friction, incomplete combustion, and so on, ending up around 25–35%.

You can't beat Carnot. The universe won't let you. Anyone selling a "free energy" device that violates this is selling fiction.

The arrow of time

The second law is the only fundamental law that distinguishes past from future. Every other law of physics — Newton's mechanics, Maxwell's electromagnetism, quantum mechanics — works the same forwards and backwards in time.

Time feels like it has a direction because entropy increases. Memories form (low entropy → high entropy as the recording substrate warms up). Plans become outcomes. Coffee cools. You age.

If you saw a movie of atoms colliding, you couldn't tell whether it was running forwards or backwards. Zoom out to a glass of water, and suddenly you can. That's the second law making itself known.

Practical consequences

  • Refrigerators and AC work by moving heat from cold to hot. They don't violate the second law; they pay for the move with electricity (which produces more entropy elsewhere than the cooling removes).
  • Life is a temporary, locally-low-entropy structure. The sun pumps low-entropy energy in, life uses it to maintain order, and dumps the leftover heat (high entropy) back out.
  • Computing has a thermodynamic floor: Landauer's principle says erasing a bit costs at least kT·ln 2 of energy as heat. Your laptop fan is the second law made audible.

The takeaway

The second law isn't a rule the universe is enforcing. It's the unavoidable consequence of having so many particles that statistics dominate. Order is rare. Disorder is generic. Time is the direction in which we notice this happening.