Basic concepts of thermodynamics

Definition of Thermodynamics

Thermodynamics is an axiomatic science which deals with the relations among heat, work, and properties of systems which are equilibrium. It describes state and changes of state in physical systems.

Macroscopic Vs Microscopic Viewpoint

Thermodynamics studies are undertaken by two different approaches.

Macroscopic approach:

In the macroscopic approach, a certain quantity of matter is considered without the events occurring at the molecular level is taken into account. Macroscopic thermodynamics is only concerned with the effects of the action of many molecules, and these effects can be perceived by human senses. For example, the macroscopic quantity, pressure, is the average rate of change of momentum due to all the molecular collisions made on a unit area. The effects of pressure can be felt. The macroscopic point of view is not concerned with the action of individual molecules, and the force on a given unit area can be measured by a pressure gauge. macroscopic
observations are completely independent of the assumptions regarding the nature of matter.

Microscopic approach:

The matter is composed of myriads of molecules. If it is gas, each molecule at a given instant has a certain position, velocity, and energy, and for each molecule, these change very frequently as a result of collisions. The behavior of the gas is described by summing up the behavior of each molecule. Such a study is made in microscopic or statistical thermodynamics. A large no. of variables are needed to describe a system. So the approach is very complicated.

Pure Substance:

A homogeneous ( simply composed of only one type of molecule ) and invariable chemical composition even though there may be a change of phase. A mixture of liquid gas and gaseous gas is not a pure substance.

Thermodynamic systems

A thermodynamic system is defined as a quantity of matter or a region in space upon which attention is concentrated in the analysis of a problem.

System: A system is a finite quantity of a matter or a prescribed region of space.

Boundary: Everything external to the system is called the surroundings or the environment. The system is separated from the surroundings by the system boundary. The boundary may be either fixed or moving. A system and its surroundings together comprise a universe. The actual or hypothetical envelope enclosing the system is the boundary of the system.

There are six classes of systems as follows:

Closed System: The closed system is a system of fixed mass. There is no mass transfer across the system boundary. There may be energy transfer into or out of the system.

Open System: A system in which matter flows into or out of the system. Most of the systems are open.

Isolated System: A system that doesn’t exchange either energy or matter with the environment or any other system.

Adiabatic system: A System that is thermally insulated from its surroundings. However, it can exchange work with its surroundings. If it does not, it becomes an isolated system.

Homogeneous System: A system that consists of a single-phase is termed as a homogeneous system.

Heterogeneous System: A system that consists of two or more phases is called a heterogeneous system.

Thermodynamics Equilibrium:

A chemical equilibrium system in which temperature and pressure at every point are the same with no velocity gradient. The following three types of equilibrium states must be achieved to get a state of thermodynamic equilibrium.

  • Thermal Equilibrium: The temperature of the system does not change with time and has equal value at all the points of the system.
  • Mechanical Equilibrium: There are no unbalanced forces within the system or surroundings. The pressure in the system at all points and does not change with respect to time.
  • Chemical Equilibrium: No chemical reaction takes place inside the system and the chemical composition must be the same throughout the system and must not change with the time.


The state is the condition of a system at an instant of time as measured by its properties. Each unique condition of a system is called a state.

Properties of the system:

A property of a thermodynamic system is a characteristic of a system that depends upon the state, but not upon how the state is achieved. There are two kinds of properties of a system as follows:

Intensive properties: These properties do not depend on the mass of the system. temperature and pressure are examples of intensive properties.

Extensive Properties: These properties depend on the mass of the system. Example: The volume of a system of mass M is V, then the specific volume of the matter within the system is  \frac{V}{M} = v which is an extensive property.

Point function:

When two properties locate a point on the co-ordinated axes graph then those properties are called point function.

Examples: Temperature, volume or pressure, etc.
 \int_{0}^{1}dV = V_1 - V_0   {an exact value on graph.}

Path function:

There are certain quantities which can be located by the area on a graph, not by a point. So the area on the graph, pertaining to the particular process, is a function of a path of the process. Such quantities are known as Path Functions.

Examples: Heat, work, etc.

Thus  \int_{1}^{2} \delta Q \neq Q_2 - Q_1 and is shown as  Q_{1-2}

and       \int_{1}^{2} \delta W \neq W_2 - W_1 and is shown as  W_{1-2}

Note: δ is used to denote inexact differentials.


Temperature is a thermal state of a body that distinguishes a hot body from a cold body. The temperature of a body is directly proportional to the stored molecular energy i.e, the average molecular kinetic energy of the molecules in a system. It can be depicted as a thermal state that depends upon the internal or molecular energy of a body.

A particular molecule does not have the temperature, it has energy. The gas as a system has a temperature.

There are three most commonly used units or scales for measurement of temperature i.e. Celsius, Fahrenheit, and Kelvin. Symbols C, F, and K are used to denote the readings on these three Celsius, Fahrenheit, and Kelvin Scales.

The relation between Temperature scale Celsius C and Fahrenheit F as follows:

 \boxed{ \frac{^{\circ}C}{100} = \frac{^{\circ}F-32}{180} }

The relation between Temperature scale Celsius C and Kelvin K as follows:

 \boxed{ K =  {^{\circ}C +273 }}


The pressure is defined as force per unit area. Pressure may be exerted by Liquid or gases. Generally, pressure measuring instruments records pressure as a difference between two pressures. It is the difference between the pressure exerted by the fluids of interest and the ambient atmospheric pressure. When the recorded pressure is above atmospheric pressure, it is called Gauge Pressure or Positive Pressure. When it is below atmospheric pressure, it is called Vacuum or Negative Pressure.


Absolute pressure = Atmospheric pressure + Gauge pressure

 P_{abs} = P_{atm} + P_{gauge}

Vacuum pressure = Atmospheric pressure – Absolute pressure

The absence of pressure is called the Vacuum. When the absolute pressure is zero, it is said to be a perfect vacuum.

Unit of Pressure: SI unit of pressure is N/m^2  or Pascal, Pa another unit of pressure is bar

 1 bar = 10^5 N/m^2 = 10^5 Pa

Specific Volume

The specific volume of a system is the volume occupied by the unit mass of the system. The symbol used is v and units are ;  m^3/kg. The symbol V will be used for volume. (Specific volume is reciprocal of density).

Reversible Process

A process that can be stopped at any stage and reversed so that the system and surroundings are exactly restored to their initial states known as Reversible process or Quasi-static Process.

Characteristics of reversible processes:

  1. After reversed, This process will leave no history of events in the surroundings.
  2. Reversible processes must pass through the exact same states on the reverse path as were initially visited on the forward path.
  3. This process must pass through a continuous series of equilibrium states points.

Examples: Frictionless relative motion & compression of fluids

Irreversible Process

A process in which heat is transferred through a finite temperature. examples: Relative motion with friction & Combustion, Throttling and heat transfer.