Thermodynamics is the physics of energy, heat, work, entropy and the spontaneity of processes. Thermodynamics is closely related to statistical mechanics from which many thermodynamic relationships can be derived.
While dealing with processes in which systems exchange matter or energy, classical thermodynamics is not concerned with the rate at which such processes take place, termed kinetics. For this reason, the use of the term "thermodynamics" usually refers to equilibrium thermodynamics. In this connection, a central concept in thermodynamics is that of quasistatic processes, which are idealized, "infinitely slow" processes. Time-dependent thermodynamic processes are studied by non-equilibrium thermodynamics.
Because thermodynamics is not concerned with the concept of time, it has been suggested that a better name for equilibrium thermodynamics would have been thermostatics.
Thermodynamic laws are of very general validity, and they do not depend on the details of the interactions or the systems being studied. This means they can be applied to systems about which one knows nothing other than the balance of energy and matter transfer between them and the environment.
Examples of this include Einstein's prediction of spontaneous emission around the turn of the 20th century and the current research into the thermodynamics of black holes.
Zeroth law: Thermodynamic equilibrium. When two systems are put in contact with each other, energy and/or matter will be exchanged between them unless they are in thermodynamic equilibrium. Two systems are in thermodynamic equilibrium with each other if they stay the same after being put in contact. The zeroth law is stated as
If A and B are in thermodynamic equilibrium, and B and C are in thermodynamic equilibrium, then A and C are also in thermodynamic equilibrium.
While this is a fundamental concept of thermodynamics, the need to state it explicitly as a law was not perceived until the first third of the 20th century, long after the first three laws were already widely in use, hence the zero numbering. There is still some discussion about its status.
Thermodynamic equilibrium includes thermal equilibrium (associated to heat exchange and parameterized by temperature), mechanical equilibrium (associated to work exchange and parameterized generalized forces such as pressure), and chemical equilibrium (associated to matter exchange and parameterized by chemical potential).
1st Law: Conservation of energy. This is a fundamental principle of mechanics, and more generally of physics. In thermodynamics, it is used to give a precise definition of heat. It is stated as follows:
The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process.
or
The heat flowing into a system equals the increase in internal energy of the system minus the work done by the system.
2nd Law: A far reaching and powerful law, it can be stated many ways, the most popular of which is:
It is impossible to obtain a process such that the unique effect is the subtraction of a positive heat from a reservoir and the production of a positive work.
Specifically,
A system operating in contact with a thermal reservoir cannot produce positive work in its surroundings (Lord Kelvin)
or
A system operating in a cycle cannot produce a positive heat flow from a colder body to a hotter body (Clausius)
3rd Law: This law explains why it is so hard to cool something to absolute zero:
All processes cease as temperature approaches zero.
As temperature goes to 0, the entropy of a system approaches a constant
These laws have been humorously summarised as Ginsberg's theorem: (1) you can't win, (2) you can't break even, and (3) you can't get out of the game.
Or, alternatively: (1) you can't get anything without working for it, (2) the most you can accomplish by working is to break even, and (3) you can only break even at absolute zero.
Or: (1) you can't get out more than you put in (2) even the best-designed machine eventally loses energy and stops (3) you can't get to absolute zero.
The rest of this discussion is about reversible transformation of systems in equilibrium. For irreversible processes or systems out of equilibrium, see non-equilibrium thermodynamics.
