The system must be in thermodynamic equilibrium at the initial and final point to study the process. In other words, the quantities that undergo a variation when passing from one state to another must be defined entirely in their initial and final states.
A thermodynamic process can also be seen as the changes of a system, from some initial conditions to other final conditions. According to the first law of thermodynamics, internal energy is the amount of heat supplied externally minus the amount of work done by the system.
Thermodynamic processes can be:
Quasi-static: it is a process that takes place infinitely slowly. Generally, this fact implies that the system goes through successive states of equilibrium, in which case the transformation is also reversible.
Reversible: it is a process that can be reversed (run in the opposite direction) without causing changes to the system or its surroundings once it has taken place.
Irreversible: it is a process that is not reversible. The intermediate states of the transformation are not equilibrium.
The theory of thermal processes is applied to the design of engines, refrigeration units, chemical industry, and meteorology.
What Is the State of a System?
The state of the system is a set of property values of a thermodynamic system that must be specified to reproduce the system. The individual parameters are known as state variables, parameters of state, or thermodynamic variables.
When a sufficient set of thermodynamic variables has been specified, the values of all other system properties are defined. However, the number of values needed to determine the status depends on the system and is not always known.
What Is a Cyclic Process?
A cyclic process is a process in which the initial and final states are the same. In technology, these processes are essential—for instance, repetitive processes, for example, the Carnot cycle, the Rankine cycle.
Reversible Process in Thermodynamics
Reversible processes are idealizations of real processes. A familiar and widely used example is the Bernoulli equation that we have seen to balance input and output flows. Reversible processes are beneficial for defining limits to the system or the behavior of devices. It helps to identify the areas in which inefficiencies occur and allows criteria to be given in the design of machines.
In some kind of reversible process, they represent the maximum work we can extract when going from one state to another. It also can mean the minimum work that is necessary to create a state change.
Types of Thermodynamic Processes
The main thermodynamic processes are the following:
- Isobaric process: it takes place under constant pressure. In other words, the system is dynamically connected, with a movable boundary, to a continual pressure tank. The associated temperature and volume follow Charles's law when a perfect gas evolves isobarically from state A to B.
- Isochoric process: the volume remains constant. Therefore, if the system is at constant volume, the amount of work done by the system will be zero. It implies that the process does not do pressure-volume work. It follows that any thermal energy transferred to the system externally absorbs it in the form of internal energy.
- Isothermal process (or isothermal process): takes place at a constant temperature. In other words, the system is thermally connected, by a thermally conductive boundary, to a constant temperature reservoir.
- Adiabatic process: it is a process in which there is no heat transfer. For a reversible process, this is identical to an isentropic process. Therefore, it can be said that the system is thermally isolated from its surroundings and that it cannot exchange heat with the surroundings.
- Isentropic process: it takes place at constant entropy. For a reversible process, this is identical to an adiabatic process.
- Constant chemical potential process: the system is connected by particle transfer with a permeable particle boundary.
- Constant particle number process: there is no energy added or subtracted from the system by particle transfer. Therefore, the system is isolated by a particle-permeable boundary.
- Polytropic process: a polytropic process is a thermodynamic process during which the heat capacity of gas remains unchanged - there are no heat exchanges.
Sometimes, during the whole process, not one, but several thermodynamic quantities turn out to be unchanged. So, for example, evaporation and condensation in a liquid-vapor system, when both pressure and temperature are constant, there are isobaric-isothermal processes.
What Does It Mean That a System Is in Thermodynamic Equilibrium?
A thermodynamic system is in a state of thermodynamic equilibrium when the main variables of the system remain unchanged. That is, the pressure, volume, and temperature do not experience remain constant over time.
It occurs when two or all of the above variables change. It should be borne in mind that the variation of only one of them is impossible because they are all interconnected by an inverse or direct ratio of proportion. This thermodynamic transformation will take the system towards another point of equilibrium.
For this reason, the initial and final states of a transformation of an ideal gas are identified by two pairs of values out of the three quantities that define a body's state: