Thermodynamic Processes

Definition of Thermodynamic System

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.

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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 = ∫ 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 which neither contracts nor expands. Thus, from the first law of thermodynamics (Q = ΔU + W), change in internal energy becomes equal to the heat transferred (ΔU = Q) for an isochoric process.


Isothermal Process

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

W = ∫ PdV

From Gas Law, 

PV = nRT

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

W = nRT VBVA (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 =  ∫ PdV. Substituting the value of P in the work equation:

W = K VfVi (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.

FAQ (Frequently Asked Questions)

1. 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.

2. 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.