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The Four Laws

Main article: Laws of thermodynamics The present article is focused on classical thermodynamics, which is focused on systems in thermodynamic equilibrium. It is wise to distinguish classical thermodynamics from non-equilibrium thermodynamics, which is concerned with systems that are not in thermodynamic equilibrium. In thermodynamics, there are four laws that do not depend on the details of the systems under study or how they interact. Hence these laws are very generally valid, can be applied to systems about which one knows nothing other than the balance of energy and matter transfer. Examples of such systems include Einstein's prediction, around the turn of the 20th century, of spontaneous emission, and ongoing research into the thermodynamics of black holes. These four laws are: Zeroth law of thermodynamics, about thermal equilibrium: If two thermodynamic systems are separately in thermal equilibrium with a third, they are also in thermal equilibrium with each other. If we grant that all systems are (trivially) in thermal equilibrium with themselves, the Zeroth law implies that thermal equilibrium is an equivalence relation on the set of thermodynamic systems. This law is tacitly assumed in every measurement of temperature. Thus, if we want to know if two bodies are at the same temperature, it is not necessary to bring them into contact and to watch whether their observable properties change with time.[15] This law was considered so obvious it was added as a virtual after thought, hence the designation Zeroth, rather than Fourth. In short, if the heat energy of material A is equal to the heat energy of material B, and B is equal to the heat energy of material C. then A and C must also be equal. First law of thermodynamics, about the conservation of energy: The change in the internal energy of a closed thermodynamic system is equal to the sum of the amount of heat energy supplied to or removed from the system and the work done on or by the system or we can say, "In an isolated system the heat is constant." Second law of thermodynamics, about entropy: The total entropy of any isolated thermodynamic system always increases over time, approaching a maximum value or we can say, "In an isolated system, the entropy never decreases". Another way to phrase this: Heat cannot spontaneously flow from a colder location to a hotter area - work is required to achieve this. Third law of thermodynamics, about the absolute zero of temperature: As a system asymptotically approaches absolute zero of temperature all processes virtually cease and the entropy of the system asymptotically approaches a minimum value; also stated as: "the entropy of all systems and of all states of a system is zero at absolute zero" or equivalently "it is impossible to reach the absolute zero of temperature by any finite number of processes". Absolute zero, at which all activity would stop if it were possible to happen, is −273.15 °C (degrees Celsius), or −459.67 °F (degrees Fahrenheit) or 0 K (kelvins, formerly sometimes degrees absolute).