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Thermodynamic Processes

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Last updated date: 25th Feb 2024
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Definition of Thermodynamic System

We know that there are various methods that can be taken to get a Thermodynamic system from its beginning state to its ultimate state. We'll talk about those Thermodynamic processes in this article. We'll look at what a quasi-static process is first. State variables are defined only when the Thermodynamic system is in equilibrium with its surroundings, as previously explained. A quasi-static process is one in which the system is in Thermodynamic equilibrium with its surroundings at all times.


In a refrigerator, how does food stay cold and fresh? Have you ever noticed that even when a refrigerator's entire inside compartment is chilly, the outside or back of the refrigerator is warm? Here, the refrigerator extracts heat from its interior and transmits it to the surrounding area. This is why a refrigerator's back is warm. Thermodynamic processes are the movement of heat energy within or between systems.


A Thermodynamic system is a specific space or macroscopic region in the universe, whose state can be expressed in terms of pressure, temperature, and volume, and in which one or more than one Thermodynamic process occurs. Anything external to this Thermodynamic system represents the surroundings and is separated from the system by a boundary. The surroundings, system, and the boundary, together constitute the universe. Types of systems in Thermodynamics are as follows:

  • Open System: It allows energy as well as mass to flow in and out of it.

  • Closed System: It only allows energy (work and heat) to be transferred across its boundary.

  • Isolated System: Neither mass nor energy is allowed to interact with it.

In the winter, rubbing your palms together makes you feel warmer. Because touching our palms produces heat, this happens. The heat of the steam is also used in steam engines to move the pistons, which causes the train wheels to rotate. But what is the actual procedure here? This is related to a phenomenon known as 'Thermodynamics.'


The study of the relationship between heat, work, temperature, and energy is known as Thermodynamics. Thermodynamics is concerned with the movement of energy from one location to another and from one form to another in its broadest definition. The essential concept is that heat is a sort of energy that correlates to a specified quantity of mechanical labor.


The heat was not formally recognized as a form of energy until around 1798, when Count Rumford (Sir Benjamin Thompson), a British military engineer, discovered that infinite amounts of heat might be produced while boring cannon barrels, and that the quantity of heat produced is proportionate to the amount of work done in spinning a blunt boring instrument. The foundation of Thermodynamics is Rumford's observation of the relation between heat created and work done. Carnot's research focused on the limits to the maximum amount of work that a steam engine can produce when using a high-temperature heat transfer as its driving force. Rudolf Clausius, a German mathematician, and physicist, refined these ideas into the first and second laws of Thermodynamics later that century.


Types of Thermodynamic Processes

The state of a given Thermodynamic system can be expressed by various parameters such as pressure (P), temperature (T), volume (V), and internal energy (U). If any two parameters are fixed, say, pressure (P) and volume (V) of a fixed mass of gas, then the temperature (T) of the gas will be automatically fixed according to the equation PV =RT. No change can be made to T without altering P and V.


The state of a system can be changed by different processes. In Thermodynamics, types of processes include:

  • Isobaric process in which the pressure (P) is kept constant (ΔP =0).

  • Isochoric process in which the volume (V) is kept constant (ΔV =0).

  • Isothermal process in which the temperature (T) is kept constant (ΔT =0).

  • Adiabatic process in which the heat transfer is zero (Q=0).

Thermodynamic process notes have been discussed later.


Work in Thermodynamic Processes

When the volume (V) of a system alters, it is said that pressure-volume work has occurred. A Thermodynamic process occurring in a closed system in such a way that the rate of volume change is slow enough for the pressure (P) to remain constant and uniform throughout the system, is a quasi-static process. In this case, work (W) is represented as:


δW = PdV, where δW is the infinitesimal work increment by the system, and dV is the infinitesimal volume increment.


Also, W = \[\int\] PdV, where W is the work the system does during the entire reversible process.


Isobaric Process

Since the pressure (P) is constant in this process, the volume of the system changes. The work (W) done can be calculated as W = P (Vfinal - Vinitial).


If ΔV is positive (expansion), the work done is positive. For negative ΔV (contraction), the work done is negative.


Isochoric Process

The volume remains constant in an isochoric process. Therefore, the system does not do any work (since ΔV = 0, PΔV or W is also zero). Such a process in which there is no change in volume can be achieved by placing a Thermodynamic system in a closed container that neither contracts nor expands. Thus, from the first law of Thermodynamics (Q = ΔU + W), the change in internal energy becomes equal to the heat transferred (ΔU = Q) for an isochoric process.


Isothermal Process

The temperature of the system remains constant in an isothermal process. We know,

W = \[\int\] PdV

From Gas Law, 

PV = nRT

P = nRT/V. Using the value of P in the work equation:

W = nRT VB \[\int\]VA (dV/V)

W = nRT ln (VB/VA)

If VB is higher than VA, the work done will be positive, or else negative.

Since internal energy is temperature-dependent, ΔU = 0 because the temperature is constant, and thus, from the first law of Thermodynamics (Q = ΔU + W), we will get Q = W.


Adiabatic Process

No heat is exchanged with the system in an adiabatic process (Q = 0). Its mathematical representation is:

 PVƔ = K (constant).

Also, W =  \[\int\] PdV. Substituting the value of P in the work equation:

W = K Vf \[\int\]Vi (dV/VƔ)

W = K [(Vf1-Ɣ - Vi1-Ɣ)/ 1-Ɣ]

Since Q = 0 for an adiabatic process, from the first law of Thermodynamics (Q = ΔU + W), we will get ΔU = -W. Thus, the internal energy will increase if the work done is negative and vice versa.

FAQs on Thermodynamic Processes

1. What are the types of Thermodynamic processes?

The types of Thermodynamics are-

1. Reversible Process

A reversible process is one that can be reversed by producing tiny changes to some property of the system, according to Thermodynamics. It makes no changes to the system or its surroundings as a result of this. The entropy of the system does not grow during the reversible process, and the system is in Thermodynamic equilibrium with its surroundings.


2. Irreversible Process

In Thermodynamics, an irreversible process is one that cannot be reversed and does not return the system or its surroundings to their initial condition. During irreversible processes, the system's entropy increases.


3. Cyclic Process

A cyclic process is one that returns a system to its initial state over time. All properties have the same value at the end of a cycle as they had at the start. Because the ultimate state of such a process is identical to the initial state, the total internal energy change must be zero.


4. Isentropic Process

A Thermodynamic process in which the entropy of the fluid or gas remains constant is known as an isentropic process. It's an adiabatic process that can be reversed. A constant entropy process is another name for an isentropic process. Such an idealised process is quite valuable in engineering for comparing genuine processes.

2. What are the laws of Thermodynamics?

The laws of Thermodynamics are:

  • The zeroth law of Thermodynamics- The first two systems are in thermal equilibrium with each other when they are each in thermal equilibrium with a third system. This trait makes using thermometers as a "third system" and defining a temperature scale meaningful.

  • The first law of Thermodynamics, or the law of conservation of energy- The difference between heat added to the system from its surroundings and work done by the system on The change in a system's internal energy equals the change in its surroundings.

  • The second law of Thermodynamics- Heat does not flow naturally from a colder to a hotter region, or, to put it another way, heat at a specific temperature cannot be converted entirely into work. As a result, a closed system's entropy, or the quantity of heat energy per unit of temperature, increases over time until it reaches a maximum value. As a result, all closed systems converge to a state of equilibrium in which entropy is maximum and no energy is available to perform useful work.

  • The third law of Thermodynamics- As the temperature approaches absolute zero, the entropy of a perfect crystal of an element in its most stable state tends to zero. This allows for the establishment of an absolute entropy scale, which determines the degree of randomness or disorder in a system from a statistical aspect.

3. What is the application of the first law of Thermodynamics?

The conservation of energy principle is critical if we want to understand how heat transmission is translated into work. The first law of Thermodynamics applies the conservation of energy concept to systems in which energy is transferred into and out of the system through a heat transfer and work. According to the first rule of Thermodynamics, the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system. The first law of Thermodynamics is written as U = Q W in equation form.

4. Where can I find notes on Thermodynamic Processes - Types and Equations?

Students can find notes on the website of Vedantu. All the notes, as well as additional study materials of Thermodynamic Processes - Types and Equations, can be found on Vedantu. Students can go through them and clear their concepts. Vedantu also organizes online live sessions where you can attend them and clear your doubts. You can get a free pdf of the important question of Thermodynamic Processes - Types and Equations on Vedantu. The plus point is you can download them and practice them anywhere anytime.

5. What are the highlights of Thermodynamic Processes - Types and Equations?

The highlights are-

  • Thermodynamic variables are used to explain a system's thermal behaviour. These variables for an ideal gas are pressure, volume, temperature, and the number of molecules or moles in the gas.

  • An equation of state connects the Thermodynamic variables in a system in Thermodynamic equilibrium.

  • The temperature of a heat reservoir does not vary when it exchanges heat with other systems because it is so big.

  • The system involved in a quasi-static process moves at such a slow rate that it is always in Thermodynamic equilibrium.

  • A reversible process is one in which the temperature and pressure are uniform throughout the system and may be made to retrace its steps.

  • Thermodynamic processes include (a) isothermal, in which the system's temperature remains constant; (b) adiabatic, in which no heat is exchanged; (c) isobaric, in which the system's pressure remains constant; and (d) isochoric, in which the system's volume remains constant.

6. What are the first two laws of Thermodynamics?

First law: 

  • It is also known as the law of conservation of energy. According to this law, neither can energy be created nor be destroyed, but can only be changed from one to another form. It is expressed by the equation Q = ΔU + W. If an amount of heat Q is given to a system, a part of it is used in increasing the internal energy (ΔU) of the system and the rest in doing work (W) by the system.

Second law: 

  • Kelvin-Planck Statement: It is not possible to convert all the heat extracted from a hot body(QH) into work (W), without losing some heat (QC) to a cold body.

  • Clausius Statement: It is impossible to transfer heat from a hot body to a cold body without the expenditure of work by an external energy source.

  • Entropy: For a cyclic process, the entropy will either remain the same or will increase.

7. What is meant by internal energy, enthalpy, and entropy of a thermodynamic system?

  • The internal energy of a system is a measure of the total energy content of a closed system. It is a sum of the total kinetic energy of the system and its potential energy. A change in internal energy (ΔU)  is given as the difference between the heat (Q) supplied to a system and the work (W) done by it (ΔU = Q - W).

  • Enthalpy (H) is one of the Thermodynamic Properties of a system. It is a measure of the total heat contained in a Thermodynamic system. Enthalpy is expressed as the sum of the product of pressure (P) and volume and the internal energy (U) of the system. Thus, H = U + PV.

  • Entropy (S) quantifies the amount of energy unavailable for doing any work and measures the system's disorder. Change in entropy (ΔS) = (Q/T)reversible, where Q = heat transfer and T = temperature.

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