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Overview
The laws of thermodynamics are broad principles governing heat, work, energy, temperature, entropy, and equilibrium. They apply to engines, refrigerators, living systems, stars, materials, chemical reactions, climate, and many technological systems.
Zeroth Law
The zeroth law states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This supports the concept of temperature measurement.
First Law
The first law is a statement of energy conservation. The internal energy of a system changes when heat is transferred or work is done.
Second Law
The second law describes the direction of natural processes. Heat flows spontaneously from hotter objects to colder objects, and entropy tends to increase in isolated systems.
Third Law
The third law concerns behavior as temperature approaches absolute zero. It places limits on reaching absolute zero and relates to entropy at very low temperatures.
Heat Engines
Heat engines convert some thermal energy into work, but the second law limits their efficiency. No real heat engine can convert all input heat into useful work.
Refrigerators and Heat Pumps
Refrigerators and heat pumps move thermal energy from cooler regions to warmer regions by using external work.
Entropy
Entropy is a thermodynamic quantity associated with energy dispersal and the number of microscopic arrangements compatible with a macroscopic state.
Common Mistakes
A common mistake is treating the second law as saying energy is destroyed. Energy is conserved, but useful energy becomes more dispersed and less available for work.
Why This Matters in Physics
The laws of thermodynamics connect energy, heat, work, engines, climate, chemistry, biology, astronomy, and the arrow of time.