Thermodynamic transformation. A system is said to undergo thermodynamic transformation when it passes from one state of thermodynamic equilibrium to another.
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- 1 Quasi-static transformations
- 2 Irreversible transformations
- 3 Cyclical transformation
- 4 Work on a thermodynamic transformation
- 5 Sources
While a thermodynamic transformation takes place there is no equilibrium and no equation of state can be applied. However, transformations that can be assumed as a succession of very close equilibrium states are generally considered ; they are called quasi-static transformations and they are the only ones that can be represented by a Clapeyron diagram .
A reversible transformation is a quasi-static transformation in which an infinitesimal change can cause the system to revert to its initial equilibrium configuration. Otherwise it is said to be an irreversible transformation .
An example of reversible transformation can be the vaporization of a liquid under its own vapor pressure in a cylinder with a frictionless piston and in contact with an isothermal enclosure .
Real processes are always irreversible to a greater or lesser extent. The heat flow from one body to another, the free expansion of a gas, the dissolution of a solid or spontaneous chemical reactions are irreversible transformations.
A cyclic (or cycle) transformation is one in which the final state of the system coincides with its initial state. Otherwise it is said to be an open transformation . Also very important concepts in thermodynamics are those of isothermal transformation, which is the one that takes place at constant temperature, adiabatic transformation, which is the one that takes place without heat exchanges with the external environment, isobaric transformation , which is the one that takes place at constant pressure , and isochoric transformation. which is the one that takes place at constant volume .
Work on a thermodynamic transformation
When a system that is subjected to pressure from external forces expands, it does work against those forces. On the other hand, if it is compressed, it is the external forces that carry out work on the system. In thermodynamics, the work done by the system ( expansion ) is considered positive and the work done by external forces (compression) is negative.
To calculate this work, a fluid contained in a rigid cylinder hermetically closed by a piston is considered.
Initially the fluid is in equilibrium under pressure P. If the area of the piston is S. the force exerted on it by the fluid will be F = PS. If the piston experiences a displacement Ae, the elemental work done by the fluid will be: AW = PS-Ae = P-AV
since the S-Ae product is precisely the volume variation.
A reversible expansion can be decomposed into a succession of elemental expansions in the opposite direction to pressure, therefore the expansion work of a fluid can be expressed as: W = IP-AV
In general, to calculate this sum of elementary works it will be necessary to resort to the integral calculation, since the pressure and the volume will be variables linked by a functional relationship. The work is given by the area of the shaded surface in the volume-pressure diagram. Particular case: for an expansion to be isobaric, the summation (shaded area) is easily calculated: W = P- (V1-V0)