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Engg Thermodynamics L1 - Basics and Terms

  • Introduction

  • System and Surroundings

  • Thermodynamic Properties

  • State of a System

  • Equilibrium

  • Process

  • Quasi-equilibrium Process

  • Cycle

  • Pure Substance

  • Continuum

  • Zeroth Law of Thermodynamics


Thermodynamics consists of two parts: therm (comes from Greek meaning heat) + dynamics (motion). In a broad way, thermodynamics is study of energy transformations and its effect on the physical properties of substances.

Thermodynamics has a wide range of applications such as:

  • Power Production such as in power plants

  • Refrigeration and Air Conditioning

  • Internal Combustion engines

  • Jet/Rocket Engines

  • Heating, Ventilation and Air Conditioning (HVAC)

Thermodynamics is basically concerned with macroscopic analysis of thermal systems.

System and Surroundings

A volume of interest or a quantity of matter on which experiments are performed is called a system. Everything outside the system is called its surroundings. System and surroundings are separated by a system boundary.

Systems are generally of two types:

1) Closed System: Closed system is a fixed mass system. Only energies in the form of heat and work crosses the system boundary.


2) Open System: Mass as well as energies (heat and work) crosses the system boundary.


3) Isolated System: An isolated system is a system on which surroundings have no effect and neither mass nor energies (heat and work) crosses the system boundary.

Thermodynamic Properties

1) Extensive Properties: Dependent on mass/size of system for example volume, energy, enthalpy etc.

2) Intensive Properties: Independent of mass/size of system for example temperature, pressure etc.

State of a System

Description of condition of a system at a given time defined by set of independent properties.

According to 'Two Property Rule' state of a system may completely be described by two independent properties.


A system is said to be in equilibrium if there is no driving force for change of state. So in a state of equilibrium there will not be any change in macroscopic properties of the system.


Transformation of a system from one state to another state of equilibrium. Process diagram may be plotted by taking any set of two properties as x and y axis such as p-v plot, T-s plot etc.


This process diagram shows that system has undergone a process from state 1 (p1, v1) to state 2 (p2, v2).

Characteristics of the system which does not depend on path taken by system and depends on end states only is called point functions. All the properties such as pressure, temperature, internal energy, entropy, enthalpy etc are point functions.

Quasi-equilibrium Process

Since the properties of a system are defined at equilibrium only and during the process there is no equilibrium, an actual process can not be plotted on property diagram. To resolve this, an ideal process called quasi-equilibrium process is defined to analyze thermal systems.

A quasi-equilibrium process is an infinitely slow process so that each state the system passes through may be considered as an equilibrium state.


A process path where start and end state are same.


Pure Substance

A Pure substance is homogeneous and invariable in chemical composition throughout its mass. State of pure substance is fixed by two property rule.


Specific volume is defined as volume per unit mass and density is mass/volume. Hence specific volume and density are reciprocal to each other. For a infinitesimal volume of δV and mass δm, specific volume may be written as:


where δV’ is the minimum volume for which matter may be treated as continuum. Below this, molecular effects will be dominant and there will be large variation in density inside the molecule and atoms.


Zeroth Law of Thermodynamics

When two systems each are in thermal equilibrium with a third system, these two systems are in thermal equilibrium with each other. This is called zeroth law of thermodynamics.

This law is used for temperature measurement.

Suppose two bodies A and B each are in thermal equilibrium with a body C, then A and B will be in thermal equilibrium with each other.

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