Unit2

BASIC THERMODYNAMICS J2006/2/1

UNIT 2

BASIC THERMODYNAMICS

OBJECTIVES

General Objective : To understand the basic concept and the First Law of
Thermodynamics

Specific Objectives : At the end of the unit you will be able to:

 Define the fundamental concepts of system, boundary,
surrounding, open system and close system

 explain the property, state and process of the working fluid and
provide example

 state the definitions of the First Law of Thermodynamics

 describe the differences between work and heat transfer

 define the definitions and show the application of internal
energy


INPUT

2.0 Introduction

Every science has a unique vocabulary associated with it, and thermodynamics is no
exception. Precise definition of the basic concepts forms a sound foundation for the
development of science and prevents possible misunderstandings. In this unit, the
systems that will be used are reviewed, and the basic concepts of thermodynamics
such as system, energy, property, state, process, cycle, pressure and temperature are
explained. Careful study of these concepts is essential for a good understanding of
the topics in the following units.

2.1 Definitions of system, boundary, surrounding, open system and close system

A thermodynamic system, or simply a system, is defined as a quantity of matter or
a region in space chosen for study. The fluid contained by the cylinder head,
cylinder walls and the piston may be said to be the system.

The mass or region outside the system is called the surroundings. The surroundings
may be affected by changes within the system.

The boundary is the surface of separation between the system and its surroundings.
It may be the cylinder and the piston or an imaginary surface drawn as in Fig. 2.1-1,
so as to enable an analysis of the problem under consideration to be made.

Boundary
Surrounding

System


Figure 2.1-1 System, surroundings and boundary

A system can either to be close or open, depending on whether a fixed mass or a
fixed volume in space is chosen for study. A close system (also known as a control
mass) consists of a fixed amount of mass, and no mass can cross its boundary. That
is, no mass can enter or leave a close system, as shown in Fig. 2.1-2. But energy,
in the form of heat or work can cross the boundary, and the volume of a close system
does not have to be fixed.

SURROUNDINGS

SYSTEM

BOUNDARY

Fig. 2.1-2 A closed system with a moving boundary

An open system, or a control volume, as it is often called, is a properly selected
region in space. It usually encloses a device, which involves mass flow such as a
boiler, compressor, turbine or nozzle. Flow through these devices is best studied by
selecting the region within the device as the control volume. Both mass and energy
can cross the boundary of a control volume, as shown in Fig. 2.1-3.

Fluid Inlet SURROUNDINGS

QOUT

SYSTEM WOUT

BOUNDARY
Fluid Outlet
Fig 2.1-3 Open system in boiler


2.2 Property, State and Process

Properties are macroscopic characteristics of a system such as mass,
volume, energy, pressure, and temperature to which numerical values can be
assigned at a given time without knowledge of the history of the system.
Many other properties are considered during the course of our study of
engineering thermodynamics. Thermodynamics also deals with quantities
that are not properties, such as mass flow rates and energy transfers by work
and heat. Properties are considered to be either intensive or extensive.

Intensive properties are those which are independent of the size of the
system such as temperature, pressure and density.

Extensive properties are those whose values depend on the size or extent of
the system. Mass, volume and total energy are some examples of extensive
properties.

The word state refers to the condition of system as described by its
properties. Since there are normally relations among the properties of a
system, the state often can be specified by providing the values of a subset of
the properties.

When there is a change in any of the properties of a system, the state changes
and the system are said to have undergone a process. A process is a
transformation from one state to another. However, if a system exhibits the
same values of its properties at two different times, the state remains the
same at these times. A system is said to be at a steady state if none of its
properties changes with time. A process occurs when a system’s state (as
measured by its properties) changes for any reason. Processes may be
reversible or actual (irreversible). In this context the word ‘reversible’ has a
special meaning. A reversible process is one that is wholly theoretical, but
can be imagined as one which occurs without incurring friction, turbulence,
leakage or anything which causes unrecoverable energy losses. All of the
processes considered below are reversible and the actual processes will be
dealt with later.

Processes may be constrained to occur at constant temperature (isothermal),
constant pressure, constant volume, polytropic and adiabatic (with no heat
transfer to the surroundings).


Activity 2A

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE
NEXT INPUT…!

2.1 Fill in the blanks with suitable names for the close system in the diagram
below.
i. _____________

ii. _________

iii. _____________

2.2 Study the statements in the table below and decide if the statements are
TRUE (T) or FALSE (F).

STATEMENT TRUE or FALSE
i. The mass or region inside the system is called
the surroundings.
ii. In a close system, no mass can enter or leave
a system.
iii. Intensive properties are those which are
independent of the size of the system
iv. Mass, volume and total energy are some
examples of intensive properties.


Feedback To Activity 2A

2.1 i. Surroundings
ii. System
iii. Boundary

2.2 i. False
ii. True
iii. True
iv. False

CONGRATULATIONS, IF YOUR ANSWERS ARE CORRECT YOU CAN
PROCEED TO THE NEXT INPUT…..


INPUT

2.3 The First Law of Thermodynamics

Energy can exist in
many forms such as
thermal, kinetic,
potential, electric,
chemical,…

Figure 2.3 Pictures showing types of energy

The first law of thermodynamics is simply a statement of conservation of
energy principle and it asserts that total energy is a thermodynamic property.
Energy can neither be created nor destroyed; it can only change forms. This
principle is based on experimental observations and is known as the First
Law of Thermodynamics. The First Law of Thermodynamics can therefore be
stated as follows:

When a system undergoes a thermodynamic cycle then the
net heat supplied to the system from its surroundings is
equal to the net work done by the systems on its surroundings.
……The First Law of Thermodynamics

In symbols,
Σ dQ = Σ dW (2.1)

where Σ represents the sum of a complete cycle.


2.4 Work and Heat Transfer

Work transfer is defined as a product of the force and the distance moved in
the direction of the force. When a boundary of a close system moves in the
direction of the force acting on it, then the system does work on its
surroundings. When the boundary is moved inwards the work is done on the
system by its surroundings. The units of work are, for example, Nm or J. If
work is done on unit mass of a fluid, then the work done per kg of fluid has
the units of Nm/kg or J/kg. Consider the fluid expanding behind the piston
of an engine. The force F (in the absence of friction) will be given by
F = pA (2.2)
where
p is the pressure exerted on the piston and
A is the area of the piston
If dx is the displacement of the piston and p can be assumed constant
over this displacement, then the work done W will be given by,
W = F x dx
= pA x dx
= p x Adx
= p x dV
= p(V2 – V1) (2.3)
where dV = Adx = change in volume.

F
IS
PRESSURE

dx

Figure 2.4 Work transfer

When two systems at different temperatures are in contact with each other,
energy will transfer between them until they reach the same temperature (that
is, when they are in equilibrium with each other). This energy is called heat,
or thermal energy, and the term "heat flow" refers to an energy transfer as a
consequence of a temperature difference.

Heat is a form of energy which crosses the boundary of a system during a
change of state produced by the difference in temperature between the system


and its surroundings. The unit of heat is taken as the amount of heat energy
equivalent to one joule or Nm. The joule is defined as the work done when
the point of application of a force of one newton is displaced through a
distance of one meter in the direction of the force.

2.5 Sign Convention for Work Transfer

It is convenient to consider a convention of sign in connection with work
transfer and the usual convention adopted is:
• if work energy is transferred from the system to the surroundings, it is
donated as positive.
• if work energy is transferred from the surroundings to the system, it is
donated as negative.

SURROUNDINGS BOUNDARY

WORK W2
+ ve
SYSTEM
WORK W1
- ve

Figure 2.5 Sign Convention for work transfer


2.6 Sign Convention for Heat Transfer

The sign convention usually adopted for heat energy transfer is such that :
• if heat energy flows into the system from the surroundings it is said to be
positive.
• if heat energy flows from the system to the surroundings it is said to be
negative. It is incorrect to speak of heat in a system since heat energy
exists only when it flows across the boundary. Once in the system, it is
converted to other types of energy.

SURROUNDINGS

HEAT ENERGY
Q2
HEAT SYSTEM -ve
ENERGY
Q1
+
ve

BOUNDARY
Figure 2.6 Sign convention for heat transfer

2.7 Internal Energy

Internal energy is the sum of all the energies a fluid possesses and stores
within itself. The molecules of a fluid may be imagined to be in motion
thereby possessing kinetic energy of translation and rotation as well as the
energy of vibration of the atoms within the molecules. In addition, the fluid
also possesses internal potential energy due to inter-molecular forces.

Suppose we have 1 kg of gas in a closed container as shown in Figure 2.7.
For simplicity, we shall assume that the vessel is at rest with respect to the
earth and is located on a base horizon. The gas in the vessel has neither
macro kinetic energy nor potential energy. However, the molecules of the gas
are in motion and possess a molecular or 'internal' kinetic energy. The term is
usually shortened to internal energy. If we are to study thermal effects then


we can no longer ignore this form of energy. We shall denote the specific
(per kg) internal energy as u J/kg.

Now suppose that by rotation of an impeller within the vessel, we add work
dW to the closed system and we also introduce an amount of heat dQ. The
gas in the vessel still has zero macro kinetic energy and zero potential
energy. The energy that has been added has simply caused an increase in the
internal energy.

The change in internal energy is determined only by the net energy that has
been transferred across the boundary and is independent of the form of that
energy (work or heat) or the process path of the energy transfer. In molecular
simulations, molecules can of course be seen, so the changes occurring as a
system gains or loses internal energy are apparent in the changes in the
motion of the molecules. It can be observed that the molecules move faster
when the internal energy is increased. Internal energy is, therefore, a
thermodynamic property of state. Equation 2.4 is sometimes known as the
non-flow energy equation and is a statement of the First Law of
Thermodynamics.

dU = dQ - dW
or, U 2 − U 1 = Q12 − W12 (2.4)

dW

dQ

Figure 2.7 Added work and heat raise the internal energy of a close system


Example 2.1

Qin = +10 kJ Wout = (+) ?

SYSTEM
Qout = -3 kJ
Win= -2 kJ

The figure above shows a certain process, which undergoes a complete cycle
of operations. Determine the value of the work output for a complete cycle,
Wout.

Solution to Example 2.1

ΣQ = Qin + Qout Qin is +10 kJ
= (10) + (-3)
= 7 kJ Qout is –3 kJ

Win is –2 kJ
ΣW = Win + Wout
= (-2) + (Wout) Wout is +ve

Hence ΣQ - ΣW = 0
ΣW = ΣQ
(-2) + (Wout) = 7
Wout = 9 kJ


Example 2.2

A system is allowed to do work amounting to 500 kNm whilst heat energy
amounting to 800 kJ is transferred into it. Find the change of internal energy
and state whether it is an increase or decrease.

Solution to Example 2.2

U2 – U1 = Q12 – W12

now,
W12 = +500 kNm = 500 kJ
Q12 = +800 kJ

∴ U2 – U1 = 800 – 500
= 300 kJ

Since U2 > U1, the internal energy has increased.


Activity 2B

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE
NEXT INPUT…!

2.3 During a complete cycle operation, a system is subjected to the
following: Heat transfer is 800 kJ supplied and 150 kJ
rejected.
Work done by the system is 200 kJ.
Calculate the work transferred from the surrounding to the system.

2.4 Each line in Table 2.4 gives information about a process of a closed system.
Every entry has the same energy unit i.e. kJ. Fill in the empty spaces in the
table with the correct answers.

PROCESS Q12 W12 (U2 – U1)

a. +50 -20 i. ________
b. +100 ii. _______ -30
c. iii. _______ -70 +130
d. -50 +20 iv. _______

2.5 A close system undergoes a process in which there is a heat transfer of 200 kJ from
the system to the surroundings. The work done from the system to the surroundings
is 75 kJ. Calculate the change of internal energy and state whether it is an increase
or decrease.


Feedback To Activity 2B

2.3 ΣQ = Qin + Qout = (800) + (-150) = 650 kJ
ΣW = Win + Wout = (Win) + (200)

Hence ΣQ - ΣW = 0
ΣW = ΣQ
(Win) + (200) = 650
Win = 450 kJ

2.4 i. 70
ii. 130
iii. 60
iv. -70

2.5 U2 – U1 = Q12 – W12

now,
Q12 = -200 kJ
W12 = 75 kJ

∴ U2 – U1 = (-200) – (75) = -275 kJ
(Since U2 - U1 = -ve, the internal energy is decreased)

CONGRATULATIONS, IF YOUR ANSWERS ARE CORRECT THEN YOU
ARE SUCCESSFUL.


! Tips
Problem Solving Methodology
There are several correct and effective steps to problem solving. There are many
variations as to what various authors give for their problem solving strategy.
Some of these steps are:

 Read the ENTIRE problem carefully and all the way through before starting work on the
problem. Make sure that you understand what is being asked.

 List the data based on the figures given in the question. This will include both explicit and
implicit data items. Note that not all of the explicitly given data are always necessarily
involved in the problem solution. Be wary of introducing implicit conditions that may be
unnecessary for the problem solution.

 Draw a diagram of the physical situation. The type of drawing will depend upon the
problem.

 Determine the physical principles involved in the particular problem. What are the pertinent
equations and how can they be used to determine either the solution or intermediate
results that can be further used to determine the solution. Often one equation will be
insufficient to solve a particular problem.

 Simplify the equations as much as possible through algebraic manipulation before
plugging numbers into the equations. The fewer times numbers are entered into equations,
the less likely numerical mistakes will be made.

 Check the units on the quantities involved. Make sure that all of the given quantities are in
a consistent set of units.

 Insert the given data into the equations and perform the calculations. In doing the
calculations, also manipulate the units. In doing the calculations, follow the rules for
significant figures.

 Is the result reasonable and are the final units correct?

GOOD LUCK, TRY YOUR BEST.


SELF-ASSESSMENT

You are approaching success. Try all the questions in this self-assessment
section and check your answers with those given in the Feedback to Self-
Assessment on the next page. If you face any problem, discuss it with your
lecturer. Good luck.

1. A thermodynamic system undergoes a process in which its internal energy decreases
by 300 kJ. If at the same time, 120 kJ of work is done on the system, find the heat
transferred to or from the system.

2. The internal energy of a system increases by 70 kJ when 180 kJ of heat is transferred
to the system. How much work is done by the gas?

3. During a certain process, 1000 kJ of heat is added to the working fluid while 750 kJ
is extracted as work. Determine the change in internal energy and state whether it is
increased of decreased.

4. If the internal energy of a system is increased by 90 kJ while the system does 125 kJ
of work to the surroundings, determine the heat transfer to or from the system.


Feedback to Self-Assessment

Have you tried the questions????? If “YES”, check your answers now.

1. Q = - 420 kJ
2. W = 110 kJ
3. U2 – U1 = 250 kJ (Since U2 - U1 = +ve, the internal energy is increased)
4. Q = 215 kJ

CONGRATULATIONS!!!!
…..May success be with
you always….

Unit2

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