Thermodynamics is the study of the macroscopic behaviour of physical systems under the influence of exchange of work and heat with other systems or their environment. It is not concerned with the microscopic properties of these systems, such as the movements of atoms.
- 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.
- 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 the flow of thermal energy from one object to another.
- Second law of thermodynamics: The entropy of any isolated system not in thermal equilibrium almost always increases. Closed systems spontaneously evolve towards thermal equilibrium—the state of maximum entropy of the system—in a process known as "thermalization". Equivalently, machines that violate the second law (perpetual motion machines) are impossible.
- Third law of thermodynamics: The entropy of any pure substance in thermodynamic equilibrium approaches zero as the temperature approaches zero. 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.
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).
