During my second year of studies, I attended a summer school on the physics of low temperatures and there was obviously a reminder of basic thermodynamics. From memory, here is the introductory sentence of our teacher (who is more than true):

"The first time we study the thermodynamics, it does nothing. The second time, we think he understood. And the third time, it is sure to have understood nothing, but it is not serious because it happens all the same to use and it works! "

Thermodynamics is a major discipline of physics and experienced many changes over the centuries. Its function is the first study of heat and different thermal machines but also as a mission the study of systems in equilibrium. The thermodynamics was developed mainly in the 19th century by renowned scientists with, among other Carnot, Clapeyron, Lord Kelvin, Clausius, Boltzmann, Van der Waals etc. (see portrait above, you will notice the importance of the beard).

It is a vast and complex subject, my ticket blog that bears the title "thermodynamics" is ambitious, but I will try as much as possible to give you a concise overview and as fair as possible, which is not clear a spot ... If you simply want to know the principles and applications in thermal machines, see Thermodynamics: Principles and Applications.

The concept of system

Studying the thermodynamics of systems. A system is a portion of the universe that is isolated by the thought and all that is out of this system is called the external environment. There are three types of systems:

- The open systems that exchange of matter and energy with the external environment. Ex: a plastic water bottle open: it can fill or empty it exchanges heat with the ambient air and is deformable.

- Closed systems which do not matter with the external environment but which can exchange energy in the form of mechanical work and heat. Ex: a plastic water bottle closed. It can be distorted and heat exchange with the outside but the quantity of matter inside does not change.

- The isolated systems where there is no exchange with the external environment (or matter, or energy). Ex: a thermos bottle closed ideal rigid.

The state variables

Different quantities (or variables) are used in physical thermodynamics. It is important to understand and to classify them into different categories and then use them. First, one must distinguish between extensive and intensive quantities:

- An extensive quantity is a physical quantity that is proportional to the size of the system (you can add). Eg mass, volume, quantity of material, etc..

- An intensive quantity is a physical quantity does not depend on the amount of material in question (you can not add). Ex: temperature, pressure, density, etc..

Thermodynamics seeks to calculate the physical quantities from one system to a state of equilibrium in terms of other quantities. To do this, we define state variables that are measurable physical quantities to determine the state of a system, regardless of the path followed to reach this state.

Explanation: A few weeks ago I made an exit in skiing in the Swiss Alps and we had to reach the Mont Tellier located 2951 m from a parking lot located at 1925 m. Two groups left the car park by two different routes:

- The first group made a direct ascent regular totaling 2951-1925 = 1026 m elevation gain (blue route).

- The second group has taken a path requiring longer fitted and descents. The total route of 1,500 m elevation gain (red route).

In this example, we can define the state of the system by the variable "altitude" because the difference in elevation between the parking and the top 2 of routes is the same: to DA = 2951-1925 = 1026 m altitude is therefore a state variable.

However, the effort provided by the mechanical red group is significantly higher than the mechanical effort provided by the blue: the variable mechanical effort is not a state variable because it depends on the path followed between 2 states in equilibrium.

In thermodynamics, the main state quantities are:

- The temperature (T) which is measured in degrees kelvin, see ticket Temperature for more details.

- The pressure (P), which is a force per unit area. The pressure is measured in pascal (1 Pa = 1 N / m²) but is often used for convenience the bar (1 bar = 100 000 Pa). A car tire inflated to 2 bars means that the air applies a force of 200 000 newtons per square meter on the tire.

- The volume (V) is measured in cubic meter.

- The internal energy (U), which represents all the energy of a microscopic system and is measured in J / kg. The internal energy is the sum of energies due to thermal agitation of the particles and the links between the nucleons, atoms and molecules.

- The enthalpy (H) is a thermal engineering practice because when that transformation takes place at constant pressure, the change in enthalpy reflects the amount of heat exchanged. The enthalpy is measured in J / kg and is defined by H = U + P * V.

- The entropy (S) which represents a kind of "microscopic chaos" of a system and is measured in J / kg-K. We will come back a little later on the concept of entropy.

The thermodynamic quantities that are not state variables (which depend on the path) are:

- The work (W) which is measured in watts and is a mechanical energy per second.

- The heat (Q) which is also measured in watts, which represents a thermal energy per second.

The functions of state

The different state variables are not independent of each other: they are linked in a more or less complex and thermodynamics then to define the "equation of state" or "state functions" which relations in different quantities at steady state.

The function of thermodynamic state the simplest and is known ideal gas equation can be written:

P * V = n * R * T

where the state quantities are the pressure (P), volume (V) and temperature (T) with n the quantity of matter and "R" the famous gas constant. This equation is a reliable model for gases said perfect, ie the gas consisting of particles far enough apart to neglect the interactions: the
nitrogen and oxygen in our atmosphere is one example.


Transformation is called the passage of a state of initial equilibrium to a final state of equilibrium. It often identifies the transformations by a state variable that remains constant: a transformation at constant temperature (isothermal), at constant pressure (isobaric), constant volume (isochoric), at constant enthalpy (isenthalpic), constant entropy (isentropic).

It also often speak of adiabatic transformation, it simply means that a transformation during which the system was no heat with the surroundings.

More generally, there are two main classes of transformations:

- The reversible transformation: Once the conversion is made, the initial state can be reached from the state by applying a minor on the system. Ex: A closed bottle of water temperature and ambient pressure (20 ° C, 1 bar) is left under a blazing sun. The temperature rises to 30 ° C and the pressure at 2 bar. If the sun disappears under a cloud, the bottle regains its original state after a certain time.

- Transformations irreversible: once the transformation, it is impossible to return to the initial state since the final fast and simple. Ex: A bottle is filled with liquid 2 of the same density and different colors (blue and yellow) separated by a watertight wall. Removing the wall and the 2 liquids mix. It is impossible to find just the initial state.

A transformation is generally considered reversible if intensive quantities are continuous, if the conversion is slow (they say then it is quasi-static) and if there is no dissipative phenomena (diffusion, friction, etc.). .

We can also accept that transformation adiabatic (which was no heat with the external environment) and is a reversible isentropic transformation.

To see more, which explains the principles of thermodynamics with thermal machinery introductions: Thermodynamics: Principles and Applications