By Nick Robinson | Updated Mar 24, 2022
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Thermodynamics is the branch of physics that explores how energy behaves in large systems. It connects the motion of particles (kinetic energy) and the stored energy of their positions (potential energy) to the heat and work a system can generate. Over centuries, scientists such as Isaac Newton and James Joule distilled this knowledge into the three fundamental laws that govern all energy exchanges.
The so‑named “zeroth” law establishes the concept of thermal equilibrium. It states that if two systems are each in equilibrium with a third system, they are in equilibrium with each other. In practice, this means that when you heat a pot of water, the heat spreads uniformly until every molecule in the pot reaches the same temperature, even though heat was applied only at the bottom.
Also known as the law of conservation of energy, the first law tells us that energy cannot be created or destroyed—only transformed. The total energy of a closed system (the sum of kinetic and potential energy) equals the heat added minus the work performed. This principle explains why a car must refuel: the chemical potential energy in gasoline is converted into mechanical work and waste heat as the engine runs.
The second law introduces the unavoidable concept of entropy, the measure of disorder or unusable energy in a system. It asserts that no process can convert all available energy into useful work; some fraction will always become waste heat. In combustion engines, even the most efficient designs lose energy to entropy, which also rules out perpetual‑motion machines.
