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The three laws of thermodynamics govern the transfer and behavior of matter and energy. The first law states that energy in an isolated system remains constant, while the second law states that systems tend towards disorder and entropy. The second law overrides the first, and cosmologists use it to create theories about the eventual fate of the universe.
The three laws of thermodynamics govern the transfer and behavior of all matter and energy in the universe as science commonly understands it. In summary, the first law states that the amount of energy and heat in an isolated system remains constant; no more energy can come from a system than is being input. This ties into the Law of Conservation of Energy which states that matter and energy cannot be created or destroyed. The second law of thermodynamics is one of the most important foundations of nature, responsible for the arrow of time and the irreversibility of nature.
No matter how ideal a system may seem, there is no perfect system. Some of the energy, electrical, thermal or mechanical, will be lost through friction and will be converted into waste heat. This means that over time the total energy of a system will gradually approach zero unless external energy is added. The amount of entropy, or disorder, in a system will approach a maximum, meaning that no useful work can be done in or by the system because its molecules and particles are too disordered. The second law of thermodynamics states that systems tend towards disorder starting from a more orderly state; an ice cube in a glass of water will melt and slightly heat the surrounding water, bringing the two systems into balance.
The second law of thermodynamics overrides the first law; where the first law holds that energy output can never exceed energy input, the second law holds that energy output can never equal energy input due to the constant loss of energy and the tendency of systems to approach equilibrium. The less energy a system has, the more entropy there is because it takes energy to restore order and decrease total entropy. When the energy within a system reaches an equilibrium, the entropy is at its maximum. For example, although a cup of hot water naturally cools, a constant application of heat energy is required to keep the water at a hotter temperature.
Cosmologists look to the second law of thermodynamics to create theories about the eventual fate of the universe. The most common theory is that since the universe itself is an isolated thermodynamic system, its entropy will approach its maximum as the stars burn out and become black holes. These will eventually evaporate after 100 trillion years, leaving the universe as a sterilized vacuum for the rest of eternity, barring quantum fluctuations.
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