The Second Law of Thermodynamics states that in every natural process, the entropy (a measure of disorder or randomness) of an isolated system always tends to increase, and energy transformations are never 100% efficient. While the First Law tells us that energy is conserved, the Second Law explains the direction in which energy flows and why some processes are irreversible.
Key points:
- Heat flows naturally from a hot object to a cold one, not the other way around, unless external work is done.
- No heat engine can convert all the heat energy into work; some energy is always lost as waste heat.
- Entropy increases with every spontaneous process, meaning systems tend toward greater disorder over time.
Examples:
- In a car engine, fuel energy is partly converted to mechanical work, but some energy is lost as heat to the surroundings.
- An ice cube melts at room temperature because the entropy of water molecules in the liquid state is higher than in the solid state.
- Over time, energy in the universe becomes more spread out and less useful for doing work.