Inside Story

# THERMODYNAMICS

### INTRODUCTION

Thermodynamics is constituted from two Greek words. Therm which means heat & dynamis which means power. So from here we can conclude that thermodynamics is the study of interaction between heat and power & it’s effects on physical properties of matter.

• Energy
• Entropy
• Equilibrium
Most of us know very well about energy & equilibrium but it is little difficult to understand about entropy. So we will discuss briefly about entropy later on in this article.

### Thermodynamic system

It is defined as the quantity of matter or region in space on which study is focused. It can be termed as both system or thermodynamic system.

### Surrounding

Everything outside the system is known as surrounding.

### Boundary

It is a thin wall which separates system from surrounding. It may either be real or imaginary.
There are two methods of studying a thermodynamic system :-

1. Control mass approach.
2. Control volume approach.

Control mass approach :- If there is no mass interaction takes place but there may be energy interaction takes in a system then we use control  mass approach method.

Control volume approach :- If there is both energy as well as mass interaction takes place in a system then we use control volume approach.

### Types of system :-

1. Closed system
2. Open system
3. Isolated system.

### Closed System

If there is no mass interaction takes place but there may be energy interaction between system and surrounding it is known as closed system. While studying a closed system we always use control mass approach method.
If  there is both energy and mass interaction takes place between system and surrounding then it is known as open system. While studying an open system we always use control volume approach method.
If there is no interaction between system and surrounding then it is known as isolated system. Isolated system can also be treated as control volume approach.
2. Extensive property.

### Intensive property

The property which is independent of mass is known as intensive property. Eg – pressure, temperature etc.

Extensive property :- The property which is dependent of mass is known as extensive property. Eg – mass, volume etc.

Note :- Extensive property per unit mass is known as intensive property or specific property. Eg – specific volume, specific heat etc.

Thermodynamics Process :- To define a thermodynamic process we need atleast two thermodynamical properties. A thermodynamic process can be defined as continuous change of states. Let’s try to understand it more accurately looking into the graph below.

The above graph is plotted by using two thermodynamical properties pressure and volume. The point 1 & 2 are the states states the curve which link state 1 & 2 is known as path. There are number of continuous states between state 1 & 2. So this continuous change of states shown in the graph by using a certain path is known as process.

Path :- The distance between two continuous states in a thermodynamic system may be defined as path.

Types of process :- We may define process on two different basis.
1. On the basis of path
2. On the basis of nature of system.
On the basis of path there are two types of process :-
1. Reversible process
2. Irreversible process.

Reversible process :- A reversible process is a process whose direction can be returned into it’s original position by inducing infinitesimal changes to some properties of the system via its surrounding. It is a very slow process. In this process all the intermediate states are in equilibrium.
Irreversible process :- An irreversible process is a process in which both system and surrounding cannot be returned to their original process. It is a very fast process.
On the basis of nature of system there are five types of process :-

1. Isobaric  or constant pressure process
2. Isochoric or constant volume process
3. Isothermal or constant temperature process.
4. Polytropic process
Isobaric process :- The process in which pressure remains constant at every point is known as isobaric process. So we can conclude that in isobaric process dp/dt = constant.

Isochoric process :- The process in which volume remains constant at every point is known as isochoric process. So we can conclude that in isochoric process dv/dt =constant.

Isothermal process :- The process in which temperature remains constant at every point is known as isothermal process. So we can conclude that in isothermal process dt = constant.

Polytropic process :- Any process which satisfies the equation given below is known as polytropic process.

P×V^n = constant

Adiabatic process :- The process in which heat is constant throughout is known as adiabatic process. So we can conclude that in adiabatic process dq/dt =constant. In adiabatic process the boundary of system is surrounded by adiabatic walls.

## LAWS OF THERMODYNAMICS

First law of thermodynamics :- The first law of thermodynamics states that the energy of a system is conserved during any thermodynamic process. In other words we can say that energy can neither be created nor be  destroyed. Let’s see the mathematical expression for first law of thermodynamics.

W = Q – ∆U

Where, W = work done
Q = heat
∆U = change in internal energy
Second law of thermodynamics :- The second law of thermodynamics states that the total Entropy of an isolated system can never decrease over time, & is constant if & only if all the process are reversible. It also tells us about the direction of heat flow.

Zeroth law of thermodynamics :- The zeroth law of thermodynamics states that if two bodies A & B are in thermal equilibrium with 3rd body C then body A & B are in thermal equilibrium with each other as well.

## ENTROPY

The measure of systems thermal energy per unit temperature that is unavailable for doing useful work. Because work is obtained from ordered molecular motion, the amount of entropy is also a measure of the molecular disorder or randomness of a system.