9 julio, 2024

Thermodynamic processes: what they are, types, examples

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

What are thermodynamic processes?

The thermodynamic processes They 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 the reason why they are called thermodynamic processesgiven that thermos– refers to heat (a form of energy) and –dynamic refers to movement.

By change of state, reference is not necessarily made to going from solid to liquid or from liquid to gas (although those 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 occur everywhere. In fact, virtually any process of change for which a system can be defined can be considered a thermodynamic process. From melting ice cream to the operation of an internal combustion engine or the operation of a blender.

Types of thermodynamic processes

There are a wide 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:

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

1. Types of thermodynamic processes according to the exchange rate

This type of thermodynamic process is characterized by the passage of the system from an initial state to a final state, 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 processes of change in the system, often (but not always) the process itself is not of importance but only the initial state and the final state.

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.

Cyclic 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 one part of the surroundings to another or between two other separate systems.

An example of cyclical processes is what happens inside the pistons of an internal combustion engine, as well as the set of processes that occur inside the compressor of refrigeration systems.

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

These types of thermodynamic processes are commonly used in engineering, and the properties of interest are, in general, the rates of energy input and output to and from the container as matter passes through it, which can be a pure liquid, a solution, a reactant mixture, etc.

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

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

Isothermal processes (T=ctte)

They are those processes in which the temperature remains constant. Despite not seeming 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 change of volume. In general, they occur in closed systems with rigid walls (which cannot be deformed). They are also called isovolumetric 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 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.

Isenthalpic 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 this, the following types of processes can be distinguished:

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

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

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

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