Definition
The Laws of Thermodynamics describe the fundamental rules governing the transfer of internal energy and heat within physical systems.
Why It Matters
You can’t win, you can’t break even, and you can’t quit. These laws are the absolute “speed limits” of the universe, dictating that energy is conserved but its quality (order) always decays—a reality that governs everything from engines to life itself.
Core Concepts
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First Law (Conservation): Energy cannot be created or destroyed.
- How to read: “The heat added equals the increase in internal energy plus the external work done.”
- Meaning: Energy bookkeeping—heat in splits between raising internal energy and doing work on surroundings.
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Second Law (Directionality): Heat never spontaneously flows from cold to hot. In any natural process, order tends toward disorder (Entropy increases).
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Third Law (Absolute Zero): No system can reach absolute zero (0 K).
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Phase Changes and Internal Energy:
- Evaporation: Change from liquid to gas at the surface. It is a cooling process because the fastest (hottest) molecules leave.
- Condensation: Change from gas to liquid. It is a warming process as molecules give up energy to the liquid.
- Boiling: Rapid evaporation within a liquid. Also a cooling process.
- Latent Heat:
- Heat of Fusion: Energy required to change solid to liquid (334 J/g for water).
- Heat of Vaporization: Energy required to change liquid to gas (2256 J/g for water).