What is the thermodynamic process?

The word 'thermodynamics' can be divided into two words, 'thermo' and 'dynamics'. The word thermo deals with the heat and thermal phenomenon, whereas dynamics deals with forces or external agent that drives the heat flow or aids in the motion of heat.

In mechanical engineering, thermodynamics primarily deals with the conversion of heat into useful work and interconversion between them. In other words, thermodynamics connects with the relationship between heat and work for the systems which are in equilibrium. When a system changes its state from one equilibrium condition to another, the system is said to have undergone a thermodynamic process.

Laws of thermodynamics

There are generally three laws of thermodynamics. They are:

Zeroth law

The zeroth law forms the basis of the temperature measurement of a system. It simply states that, if a body is in thermal equilibrium with respect to a second body, and the second body is thermal equilibrium with respect to a third body, then all the three bodies are in thermal equilibrium with each other.

First law

The first law forms the basis of the conversion of total heat into total work. The first law introduces the concept of internal energy. If a system in its initial state moves to a final state by undergoing a thermodynamic process and does energy interactions with the surroundings in the form of heat and work, then the difference between all the heat interactions and work interactions is equal to the system's internal energy. Mathematically, $∆Q-∆W=∆U$, where, $∆Q$ = change in the heat energy, $∆W$ = change in work energy, $∆U$ = change in the internal energy of the system. Internal energy is a property of the system and is a path function.

Second law

The second law of thermodynamics explains all the spontaneous thermodynamic process that happens in nature. Like, temperature always changes from a higher thermal gradient to a low thermal gradient, but the reverse is not possible. To initiate the reverse, some constraints have to be sacrificed. The second law governs these constraints. The second law introduces the concept of entropy. 

Thermodynamic processes

The different thermodynamic processes are generally represented in a pressure-volume (PV) diagram. Some of the thermodynamic processes are outlined below,

Adiabatic process

If in a thermodynamics process, the system undergoes a change of state from its initial state to final state without any energy interactions to and from the surroundings in the form of heat, the process is known as the adiabatic process. The value, $∆Q=0$ for an adiabatic process, where $Q$ represents the heat exchange between the system and the surrounding.

Isothermal process

It is also known as the constant temperature process. When a system in thermodynamic equilibrium undergoes a change of state from its initial position to a final state, such that the temperature of the system remains constant throughout, the process is known as the isothermal process or constant-temperature process. For an isothermal process, $∆T=0$, where $T$ represents the temperature of the system. 

Isobaric process

In this process, the pressure of the system remains constant throughout the process while undergoing a thermodynamic change. For an isobaric process, $∆P=0$, where $P$ represents the pressure of the system.

Isochoric process

In this process, the volume of the system remains unchanged or constant when the system is undergoing a change. Isochoric processes are also known as constant volume processes or isovolumetric processes. For an isochoric process, $∆V=0$, where $V$ represents the volume of the system.

Reversible process

When a system undergoes a change from its initial state to a final state such that after the termination of the process both the system and the surroundings can be brought back to its initial conditions, such processes are known as reversible processes. 

Isenthalpic process

When during a thermodynamics process, the enthalpy of the system remains constant, the process is known as the isenthalpic process. For an isenthalpic process, $∆H=0$, where $H$ represents the enthalpy of the system.

Quasi-static process

A thermodynamic process when happens infinitesimally slowly such that the path is undertaken by the system can be represented by locus of equilibrium points. Such kind of process is known as a quasi-static process. 

Polytrophic process

It is known as a universal process. The Polytrophic process follows the mathematical relation of $P{V}^n=C$, where, $P$ is the pressure of the system, $V$ is the volume of the system, and $n = 1,2,....\infty$. 

State of a system

The condition of a system at a certain amount of time is known as the state of a system in thermodynamics. The state of a system is characterized by the associated thermodynamic properties of the system, designated as the state variables. Due to the energy changes of the system, the state of the system changes. These energy changes are associated with the change in equilibrium conditions of the system.

In the above image, a system undergoing a cyclic process A-B-C-D-A is shown by the T-S diagram. Here, $Q_H$ is the heat gained by the system while moving from point A-B, and $Q_C$ is the heat released by the system while moving from C-D. 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Science (Physics)
• Bachelors in Technology (Mechanical Engineering)
• Masters in Science (Physics)
• Masters in Technology (Mechanical Engineering)

Practice Problems

1. For which of the following process ∆Q=0?

1. Adiabatic process
2. Isothermal process
3. Isobaric process
4. Reversible process

Correct option- a

Explanation: An Adiabatic process is a process where the heat interaction is zero when a system undergoes a thermodynamic process. 

2. Which of the following processes is known as the isochoric process?

1. Constant volume process
2. Constant temperature process
3. Constant pressure process
4. Quasi-static process

Correct option- a

Explanation: The other term for a constant volume process is the isochoric process.

3. If a process happens isothermally, which of the following parameters remains constant?

1. Pressure, P
2. Volume, V
3. Temperature, T
4. None of these

Correct option- c

Explanation: For an isothermal process, the temperature remains constant throughout the thermodynamic process.

4. Which of the following process is also known as the constant volume process?

1. Isovolumetric process
2. Isochoric process
3. Isobaric process
4. Isopiestic process

Correct option: Both a and b are correct

Explanation: A constant volume process is also known as an isochoric or isovolumetric process.

5. Which of the following holds true for the quasi-static process?

1. The process happens very fast
2. The process so happens that the heat exchange is zero
3. The process happens infinitesimally slowly.
4. None of these

Correct option- c

Explanation: The quasi-static process is that process where the process happens infinitesimally slowly so that each of the points represents equilibrium points in the path followed by the system.