Bas_Con-VK

7/29/2019 Bas_Con-VK

1/5

1

Thermodynamics

Basic ConceptsSession 1

Introduction:

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.

System:

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.


2/5

2

Surroundings:

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.

Properties:

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.


3/5

3

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.

State:

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.

Process:

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.

Heat:

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.

Work:

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. Mathematically,


4/5

4

where f is the force and d the displacement. Work is a scalar quantity.

Let us look at the following piston and cylinder arrangement containing some gas. The

system considered is the gas in the cylinder. It is exerting some pressure on the piston in

the upward direction. This is balanced by the atmospheric pressure plus the weight and

therefore piston though it is free to move, does not move. If the weight is increased, then

the piston moves downwards. The pressure of the gas increases and the movement of the

piston stops when the pressures become equal. The additional weight is doing the work

on the gas. The additional weight has to push the force created by the gas inside to move

the piston. So there is force and distance. But as the piston moves, the pressure

continuously changes. Therefore consider small distance (dL) for which work done (dW)

is also small as shown in the figure which results in small volume change ( dV). Then by

definition of work, we have, if A is the cross sectional area of the piston

dL

dfW

cosdf


5/5

5

Integrating between the two locations of the piston,

This is the equation we use to determine the work. Please note that in general p cannot

be taken out of the integral sign as p may be continuously changing as above.

If we have a p vs. V curve, we can determine the integral as area under the curve.

The area under the curve a21b is the work done on the gas.

We can carry out a process differently with the same initial and final states given

by the points 1 and 2. Since it is a different process, the curve will be different. In that

case the area under the curve will also be different which means to say the work done is

different though the initial and final states are same. Thus the work depends upon the

process or path or how the state is changing. Such quantities are called path functions.

Since heat is also energy in transit as the work is, heat must also be a path function. No

matter what process, as long we have same initial and final states, it brings about the

same pressure difference. Such properties which depend only on state are called state

properties.

1

2

a b

p

V

2

1

21 pdVW

dLfdW

A

VdpA

pdV

Documents

Bas_Con-VK