The four laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems. The laws describe how these quantities behave under various circumstances, and forbid certain phenomena (such as perpetual motion).The four laws of thermodynamics are:[1]HYPERLINK \l "cite_note-Kittel_and_Kroemer_1980-2"[2]HYPERLINK \l "cite_note-Adkins_1968-3"[3]HYPERLINK \l "cite_note-Kondepudi_2008-4"[4]HYPERLINK \l "cite_note-LJCV_2008-5"[5]HYPERLINK \l "cite_note-6"[6]Zeroth law of thermodynamics: If two systems are in thermal equilibrium with a third system, they must be in thermal equilibrium with each other. This law helps define the notion of temperature./wiki/First_law_of_thermodynamics"First law of thermodynamics: Because energy is conserved, the internal energy of a system changes as heat flows in or out of it. Equivalently, machines that violate the first law (perpetual motion machines) are impossible. Heat is not a form of energy. It is merely the flow of thermal energy from one object to another./wiki/Second_law_of_thermodynamics"Second law of thermodynamics: The entropy of any isolated system not in thermal equilibrium almost always increases. Closed systems spontaneously evolve towards thermal equilibriumthe state of maximum entropy of the systemin a process known as "thermalization". Equivalently, machines that violate the second law (perpetual motion machines) are impossible./wiki/Third_law_of_thermodynamics"Third law of thermodynamics: The entropy of any pure substance in thermodynamic equilibrium approaches zero as the temperature approaches zero.[7] The entropy of a system at absolute zero is typically zero, and in all cases is determined only by the number of different ground states it has.There have been suggestions of additional laws, but none of them achieve the generality of the four accepted laws, and they are not mentioned in standard textbooks.[1]HYPERLINK \l "cite_note-Kittel_and_Kroemer_1980-2"[2]HYPERLINK \l "cite_note-Adkins_1968-3"[3]HYPERLINK \l "cite_note-Kondepudi_2008-4"[4]HYPERLINK \l "cite_note-LJCV_2008-5"[5]HYPERLINK \l "cite_note-DGM_1962-8"[8]HYPERLINK \l "cite_note-Glansdorff_and_Prigogine_1971-9"[9]The laws of thermodynamics are important fundamental laws in physics and they are applicable in other natural sciences