Basic of Thermodynamics

Basic of Thermodynamics

Thermodynamics deals with heat inter-action and work inter-action with the substances called systems. Work and heat are forms of energy. Transfer of heat or work to a substance brings about certain changes in the substance and whatever change happens is called a process. Thermo means heat. Since work is also a form of energy, thermo is taken to mean heat and work. Dynamics refers to the changes that occur as a result of heat or work transfer.

Biological systems are capable doing work. For example, micro-organism is capable swimming in the body fluid of its host. It needs to do the work. Where does energy for doing this work come from? It is the metabolic activity that converts some form of energy (Nutrition that it takes form host is a form of chemical energy) into work. It is important then to understand how this happens so that we can exploit this to our engineering benefit.

In thermodynamics we have work transfer, heat transfer and then we have a system for interaction which undergoes a process. Let us look at these basic terms.


We need to fix our focus of attention in order to understand heat and work interaction. The body or assemblage or the space on which our attention is focused is called system. The system may be having real or imaginary boundaries across which the interaction occurs. The boundary may be rigid and sometimes take different shapes at different times. If the system has imaginary boundary then we must properly formulate the idea of system in our mind.


Every thing else apart from system constitutes surroundings. The idea of surroundings gets formulated the moment we define system. System and surroundings together form what is known as universe.

Closed system:

If the system has a boundary through which mass or material cannot be transferred, but only energy can be transferred is called closed system. In an actual system, there may not be energy transfer. What is essential for the system to be closed is the inability of the boundary to transfer mass only.

Open system:

If the system has a boundary through which both energy and mass can transfer, then it is called open system.


Variables such as pressure, temperature, volume and mass are properties. A system will have a single set of all these values.

Intensive properties:

The properties that are independent of amount contained in the system are called extensive properties. For example, take temperature. We can have a substance with varying amount but still same temperature. Density is another example of intensive property because density of water is same no matter how much is the water. Other intensive properties are pressure, viscosity, surface tension.

Extensive properties:

The properties that depend upon amount contained in the system are called extensive properties. Mass depends upon how much substance a system has in it therefore mass is an extensive property.


It is defined as condition of a system in which there are one set of values for all its properties. The properties that define the state of a system are called state variables. There is certain minimum number of intensive properties that requires to be specified in order to define the state of a system and this number is uniquely related to the kind of system. This relation is phase rule which we shall discuss little later.

The changes that occur in the system in moving the system from one state to the other is called a process. During a process the values of some or all state variables change. The process may be accompanied by heat or work interaction with the system.


It is a form of energy that exists only in transit. This transit occurs between two points which differ in temperature. Since it exists only in transit, it should be accompanied by changes that occur in the system. The moment this energy cease to move, it appears as internal energy. We shall discuss internal energy when we deal with I law of thermodynamics.


It is also a form energy that exists only in transit. The work cannot be stored. Work is defined as the product of force and distance through the force moves.