# Thermodynamics Lecture Notes

Thermodynamics is the branch of physics which deals with process involving heat, work and internal energy. Its scope is very wide and covers all branches of science in which heat or some other quantities depending on it play an important role.

System and surroundings

When we study thermodynamics, we focus our attention on a particular region of space or a finite portion of matter. This is called a thermodynamic system. Anything outside the system which has got some bearing on the behaviour of the system is called the surrounding.
To investigate a system there are two kinds of approaches. In one approach called microscopic approach, we go into the details of the internal structure of the system. Here we take into account the properties of atoms and molecules, constituting the system. For example in kinetic theory of matter, the behaviour of a system is explained in terms of the properties of molecules.
In another approach called macroscopic approach we take into account only the properties of the system as a whole without reference to internal structure. Volume, pressure, temperature etc are macroscopic quantities which are measurable. Thermodynamics deals with the bulk property of the system and it does not pay attention to the internal structure.

Thermodynamic variables

The thermal state of a simple homogeneous body is defined by its temperature T, pressure P and volume V. These quantities are called thermodynamic variables or co-ordinates. A particular set of such values specify a particular state of the system. The process by which the system goes from one thermodynamic state to the other is called thermodynamic process. Heating a gas contained in a cylinder fitted with a piston or compressing the gas are familiar examples of thermodynamic process.

Heat

When two bodies at different tempratures are brought in contact with each other, the temperature of one body falls while the temperature of the other rises. The process continues until both attain a common temperature. To explain this phenomenon we assume that a certain amount of energy is transferred from the hot body to the cold body. This energy in transit is reffered to as heat. Conventionally heat energy entering a system is said to be positive and that leaving a system is said to be negative. Like any other form of energy, heat energy is measured in Joules.

Work

In mechanics we define work as the product of force and displacement in the direction of force. When we speak of work in thermodynamics we consider only the external work which involves the interaction between the system and its surroundings. Any internal work done by one part of the system on another part is not considered in thermodynamics. In thermodynamics work done by a system is taken as positive and work done on the system is taken as negative. In thermodynamics work is associated with a change in volume.

At the first glance it appears that heat and work are two separate concepts entirely independent of each other; but they are interrelated. Both heat and work are forms of energy and can be transformed from one form into another.

Work done by a thermodynamic system

Consider an ideal gas enclosed in a cylinder fitted with a smooth piston. If the pressure exerted by the gas is P and area of cross section of the piston is A, then force exerted by the gas on the piston is given by, F = PA.

If due to this force the piston moves through a small distance dx, then work done by the gas is given by,

dW = F × dx = P × A × dx = P × dV, where A × dx = dV, the change in volume. If the volume changes from V1 to V2 the total work done is given by,

Internal energy of a thermodynamic system:

According to kinetic theory, a system is made up of large number of particles called molecules. These molecules are constantly in motion and hence possess kinetic energy. Again there exists a force called intermolecular forces between molecules of matter. Due to this force, the molecules possess potential energy. The sum of the kinetic and potential energies of all the molecules of a system is called internal energy. If the temperature of a body increased, its molecular motion increases. Hence the internal energy also increases. When matter changes its phase, its internal energy also changes.