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Thermodynamics is the study of the effects of work, heat, and energy on a system. Thermodynamics is only concerned with so-called macroscopic observations, which are observations on large numbers of particles.[1]

Thermodynamics and statistical mechanics

For systems consisting of many particles (such as a gas consisting of many molecules), it is possible to describe and predict the properties of the system accurately, even though it is virtually impossible to know what individual particles do. In essence, because of the large number of particles, one can apply statistics to learn about the "average" behavior of the system. The branch of physics known as statistical mechanics does just this.

"Classical" thermodynamics, on the other hand, predates this field, and makes no explicit reference to the constituent particles of a system. It consists of a number of "empirical" laws, which are derived purely from observations on thermodynamical systems, such as vessels of gas, or steam engines. Well-known "laws" of thermodynamics are:[2]

The Zeroth Law of Thermodynamics
allows us to define the concept of temperature, by stating that "Two systems in thermal equilibrium with a third one are in thermal equilibrium with each other". This law is called "zeroth" because it was only formulated after the three others, but is actually more fundamental, and hence deserves a lower number.
The First Law of Thermodynamics
states that energy is conserved and that heat and work are transfers of energy.
The Second Law of Thermodynamics
states (in one of its various formulations) that entropy in an isolated system cannot decrease, and that irreversible processes can only make it increase.[3] An equivalent formulation states that heat cannot spontaneously flow from a cooler body to a hotter body.
The Third Law of Thermodynamics
also known as Nernst's Law, states that it is not possible to bring any system to the absolute zero of temperature in a finite number of operations. Also stated as follows: The entropy of a perfect crystal at absolute zero is zero.

These laws tell us to what constraints any system is subject. For example, it allows us to calculate the maximum possible efficiency of an engine once we know the temperature at which it operates.

The particular properties of a specific system cannot be calculated from these laws alone. More information is required: so-called thermodynamic equations of state tell us how a particular system will behave under thermodynamic processes. A simple example of such an equation is the Ideal Gas Law that applies to dilute gases.


  2. Mark W. Zemansky, Heat and Thermodynamics, McGraw-Hill, New York, 1957
  3. Gregory H. Wannier, Statistical Physics, John Wiley & Sons, New York, 1966