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Thermodynamic processes

We explain what thermodynamic processes are, the types that exist and we give several examples

The thermodynamic processes are processes in which a thermodynamic system under study undergoes a change of state. During this change of state, a movement of energy occurs either within the system or between the system and the surroundings. In fact, this is why they are called thermodynamic processes , since thermo – refers to heat (a form of energy) and – dynamic refers to motion.

By change of state it does not necessarily refer to going from solid to liquid or from liquid to gas (although these are also thermodynamic processes), but to a change in the state functions, that is, in the variables that define the state of the system such as temperature, volume and pressure.

Thermodynamic processes happen everywhere. In fact, practically any process of change for which a system can be defined can be considered a thermodynamic process. From melting ice cream to running an internal combustion engine or running a blender.

Types of thermodynamic processes

There are a great variety of thermodynamic processes, so it is necessary to classify them in order to facilitate their study. The main criteria for classifying thermodynamic processes are:

  1. According to the exchange rate in the system
  2. According to the state function that remains constant
  3. According to the second law of thermodynamics

1. Types of thermodynamic processes according to the exchange rate

  • Changes in the system

This type of thermodynamic process is characterized by the passage of the system from an initial state to a final one, both defined by an adequate number of state variables. In this type of process, the system is in thermodynamic equilibrium both in the initial state and in the final state and the change is caused by some external agent.

The thermodynamic equilibrium condition is a macroscopic condition, which implies that the system will not undergo any change in time unless it is disturbed by some external force. This can consist of the contribution of energy in the form of heat, work, among others.

It should be noted that, when studying change processes in the system, many times (although not always) the process itself is not of importance but only the initial state and the final state.

  • Cyclical processes

In many situations, especially in the construction of machines based on thermodynamic processes, the system is subjected to a set of state change processes that start from an initial state, go through a set of intermediate states and end up again in the same state. original initial, thus completing a cycle.

Cyclical processes do not produce a net change in the state of the system (since they start and end in the same state), but depending on how the cycle is carried out, they can produce the net transfer of energy from a part of the surroundings to another or between two other separate systems.

  • Flow processes

Flow processes differ from the other two types of processes, in that the system is made up of matter that is at a given moment inside a container through which a constant flow of matter passes.

These types of thermodynamic processes are in common use in engineering, and the properties of interest are, in general, the rates of entry and exit of energy to and from the container as the matter that can be a pure liquid, a solution, a reaction mixture, etc.

2. Types of thermodynamic processes according to the state function that remains constant

To facilitate analysis and interpretation, many thermodynamic processes are carried out by keeping one or more variables constant. In this way, the effects on the system of the change of a reduced number of variables can be determined. This makes it possible to define different types of thermodynamic systems according to which variable remains constant. These are:

  • Isothermal processes (T = ctte)

They are those processes in which the temperature remains constant. Despite not sounding intuitive, the fact that the temperature does not change does not mean that there is no heat transfer. During these processes there is no change in the internal energy of the system.

  • Isochoric processes (V = ctte)

They are the processes that occur without volume change. In general they occur in closed systems with rigid walls (which cannot be deformed). They are also called isovolumic and isometric. They are characterized by not involving PV expansion work.

  • Isobaric processes (P = ctte)

It is perhaps the most common type of thermodynamic process that we come across in our daily lives. They are characterized by occurring at constant pressure.

  • Adiabatic processes (q = 0)

These are the processes that occur without any heat transfer. They are often confused with isothermal processes since, since there is no heat transfer, many assume that there is no change in temperature. However, this is not so.

  • Isentropic processes (S = ctte)

They are processes in which the entropy of the system remains constant.

  • Isoenthalpic processes (H = ctte)

They are those processes in which the enthalpy of the system remains constant.

3. Types of thermodynamic processes according to the second law of thermodynamics

The second law is used to predict the spontaneity of thermodynamic processes. Based on that, the following types of processes can be distinguished:

  • Spontaneous processes

They are those that involve an increase in the entropy of the universe and, therefore, occur spontaneously (naturally).

  • Reversible processes

This type of process does not involve a change in the entropy of the universe and, therefore, is in equilibrium. They are reversible processes that can occur in both directions.

  • Irreversible processes

They are those that imply a decrease in the entropy of the universe and, therefore, do not occur naturally from the initial state to the end, but in the opposite direction.

Examples of thermodynamic processes

  • The Carnot cycle is a 4-stage cyclical process , on which refrigeration systems such as refrigerator compressors are based.
  • Evaporation from a pool of water is an example of an irreversible thermodynamic process .
  • Frost formation in a freezer is an irreversible process .
  • The cooling of a gas that is pressurized by letting it escape from the cylinder is an example of a roughly adiabatic process .
  • Cooking gas combustion occurs in air at constant atmospheric pressure, so it is considered an isobaric process .
  • Heating water in a gas heater is a flow process , as the water enters and leaves the pipe at a constant rate, while absorbing the heat from the gas combustion.

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