Introduction to Thermodynamics

Thermodynamics

The word ‘thermodynamics’ is composed of two words, namely ‘Thermo’ and ‘dynamics’. ‘Thermo’ stands for heat while ‘dynamics’ is used in connection with mechanical motion which involves ‘work’ done. Thus, thermodynamics is the branch of physics which deals with process involving heat, work and internal energy.

There are two ways of studying the behavior of the system. One is microscopic behavior which deals with the study of atoms and molecules of the substance, constituting that system. Second way is to study the macroscopic behavior. In this way we study the average behavior of extremely large study the average behavior of extremely large number of atoms/molecules constituting the system.

In thermodynamics we deal with quantities like temperature, pressure, volume, internal energy, etc. These quantities, actually, describe the average behaviorof a large number of atoms/molecules or the pressure exerted by a few molecules. Instead we talk about temperature of pressure of a gas which contains a large number of molecules. Thus, thermodynamics is concerned with macroscopic behavior rather than microscopic behavior of the system.

Basic Terminology

Internal Energy (U)-“Heat” 

Atoms/molecules, constituting matter, are constantly in motion. Type of motion depends upon the state of matter, i.e., whether it is a solid, a liquid or a gas. Energy possessed by the atoms/molecules by virtue of their motion is called kinetic energy.

Since there is a force of attraction between any two molecules, they possess some potential energy due to that.

The sum of total kinetic and potential energies of atoms or molecules constituting a system is called the internal energy of the system.

Internal energy of a system depends upon the parameters of the system. It has a definite value for a definte thermodynamic state. It is not a measurable quantity. In actual practice, we shall deal with change in internal energy which is a measurable quantity.

Heat

When a cold body ‘A’ is placed in contact with a hot body ‘B’ something is transferred from hot body ‘B’ to the cold body ‘A’ which results in a rise in temperature of the cold body. This transference stops when the two have acquired same temperature. This indicates that only a part of energy of ‘B’ is transferred to ‘A’. This part is called heat. Heat is that part of internal energy which is transferred from one body to another an account of the temperature difference.

By convention, heat is given to a body is taken as positive while that taken out of the body is taken as negative.

Sign conventions for cahnge ‘ΔU’ in internal energy:-

(a) ΔU is taken as positive if the internal energy of the system increases.

(b) ΔU is taken as negative if the internal energy of the system decreases.

WorK

Work is said to be done when a force acting on a system displaces the body in its own direction. Work ‘W’ done on or by a system is the product of force and displacement.

So, W = (F) (x) = (P) (A) = P (V_f – V_i)

Sign conventions for cahnge ‘ΔU’ in internal energy:-

(a) If the gas expands, work is said to be done by the system. In this case V_f > V_i, therefore, W will be positive.

(b) If the gas is compressed, work is said to be done on the system. In this case V_f < V_i, therefore, work done is negative.

Thermodynamic Variables or Parameters

The thermodynamic state of system can be determined by quantities like temperature (T), volume (V), pressure (P), internal energy (U) etc. These quantities are known as thermodynamic variables, or parameters of the system.

Any change in one of the variables results in a change in the thermodynamic state of the system. 

Equation of State

A relation between the values of any of the three thermodynamic variables for the system, is called its equation of state.

Equation of state for an ideal gas is, PV = RT

Here, ‘R’ is a gas constant for 1 mole of the gas.

Any one of the three variables P,V,T can be expressed in terms of the other two. Therefore, any two of these can be taken as independent variables, the third being dependent variable. The state of a system is, therefore, uniquely determined from a knowledge of two independent thermodynamic variables.

Equilibrium of a System

A system is said to be in equilibrium  if its macroscopic quantities do not change with time. An equilibrium can be classified into following three categories.

(a) Mechanical Equilibrium:- If there is no macroscopic movement between the system and its surroundings, the system is said to mechanical equilibrium.

(b) Chemical Equilibrium:- If the composition of the system remains constant with time, it is said to be in chemical equilibrium.

(c) Thermal Equilibrium:- If the temperature of the system remains constant withy time, it is said to be in thermal equilibrium.

In thermodynamics, we shall deal only with the concept of thermal equilibrium. 

Relation Between Joule and Calorie

Work and heat belong to the same category of physical quantity i.e., energy. They have same dimensions and are measured in same unit i.e., joule. However, heat is some times expressed in calorie also. To convert joule into calorie and vice-versa, we shall have to use a relation called joule-cal relation which is given below.

1 joule = 4.186 cal

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