The study of the laws that govern the conversion of energy from one form to another, the direction in which heat will flow, and the availability of energy to do work. It is based on the concept that in an isolated system anywhere in the universe there is a measurable quantity of energy called the internal energy (U) of the system. This is the total kinetic and potential energy of the atoms and molecules of the system of all kinds that can be transferred directly as heat; it therefore excludes chemical and nuclear energy. The value of U can only be changed if the system ceases to be isolated. In these circumstances U can change by the transfer of mass to or from the system, the transfer of heat (Q) to or from the system, or by work (W) being done on or by the system. For an adiabatic (Q=0) system of constant mass, DU=W. By convention, W is taken to be positive if work s done on the system and negative if work is done by the system. For nonadiabatic systems of constant mass, DU = Q + W. This statement, which is equivalent to the law of conservation of energy, is known as the first law of thermodynamics.

All natural process conform to this law, but not all processes conforming to it can occur in nature. Most natural processes are irreversible, i.e. they will proceed in one direction. The direction that a natural process can take is the subject of the second law of thermodynamics, which can be stated in a variety of ways. Rudolf Clausius stated the law in two ways: "heat cannot be transferred from one body to a second body at a higher temperature without producing some other effect" and "the entropy of a closed system increases with time". These statements introduce the thermodynamic concepts of temperature (T) and entropy (S), both of which are parameters determining the direction in which an irreversible process can go. The temperature of the body or system determines whether heat will flow into it or out of it; its entropy is a measure of the unavailability of its energy to do work. Thus T and S determine the relationship between Q and W in the statement of the first law. This is usually presented by stating the second law in the form DU = TDS - W.

The second law if concerned with changes in entropy (DS). The third law of thermodynamics provides an absolute scale of values for entropy by stating that for changes involving only perfect crystalline solids at absolute zero, the change of the total entropy is zero. This law enables absolute values to be stated for entropies.

One other law is used in thermodynamics. Because it is fundamental to, and assumed by, the other laws of thermodynamics, it is usually known as the zeroth law of thermodynamics. This law states that when two bodies are each separately in thermal equilibrium with a third body, then all three bodies are in thermal equilibrium.