THE CONCEPTS OF THERMODYNAMICS
Thermodynamics is the science of energy (the ability to cause changes) and an important
engineering tool used to describe processes that involve changes in temperature, transformation
of energy and the relationships between heat and work.
Thermodynamics from Greek words where theme means ‘ heat’ and dynamics means ‘power’ is
the study of energy conversion between mechanical work and heat and macroscopic variables
such as temperature , volume and pressure.
It can be regarded as a generalization of an enormous body of empirical evidence. It is used to
describe the performance of systems, power generation system and refrigerators and to describe
fluid flow , combustion and many other phenomena.
For example : the focus of thermodynamics in aerospace engineering is on the production of
work , often in the form of kinetic energy ( for example in the exhaust of a jet engine) or shaft
power from different sources of heat.
The starting point for most thermodynamic considerations are the laws of thermodynamics ,
which postulate that energy can be exchanged between physical systems as heat or work. They
also postulate the existence of a quantity named entropy, which can be defined for any isolated
system that is in thermodynamic equilibrium.
Thermodynamics is based on two generalizations called the first and second law of
thermodynamics which are based on human experience.
First Law of thermodynamics, about the conservation of energy, it states that energy cannot
created or destroyed. The change in the internal energy of a closed thermodynamic system is
equal to the sum of the amount of heat energy supplied to or removed from the system and the
work done on or by the system or we can say ‘in an isolated system the heat is constant’.
Second Law of thermodynamics, about entropy, the total entropy of any isolated
thermodynamic system always increases over time, approaching a maximum value or we can say
‘in an isolated system, the entropy never decreases’. Another way to phrase this “heat cannot
spontaneously flow from a colder location to a hotter area, work is required to achieve this with
In thermodynamic, dimensions and units is most important part to get accurate data.
1.2 Dimensions and Units
Any physical quantity can be characterized by dimensions. The magnitudes assigned to the
dimensions are called units. Metric SI system is a simple and logical system based on a decimal
relationship between the various units. English system it is has no apparent systematic numerical
base, and various units in this system are related to each other rather arbitrary.
In 1960, an international conference was called to standardize the metric system. The
international System of Units (SI) was established in which all units of measurement are based
upon seven base units: meter (length or distance), kilogram (mass), second (time), ampere
(electrical current), Kelvin (temperature), mole (quantity), and candela (luminous intensity).
In the present discussion, we consider SI units (International System of Units) will be used
throughout this module. It can be divided into fundamental units and derived units. Fundamental
units mean the basic of unit .There no combination with the other units. Derived units mean the
combination units from the basic units. For example in table 1.1 and 1.2. In some cases SI units
prefixes are used to express the multiples of the various units (table 1.3).
Table 1.1 Fundamental Units
Quantity Unit Symbol
Mass kilogram kg
Time second s
Length meter m
Thermodynamic temperature degree Kelvin K
Electric current ampere A
Luminous intensity candela cd
Amount of matter mole mol
Table 1.2 Derived Units
Quantity Unit Symbol Notes
Area meter square m2
Volume meter cube m3
Velocity meter per second m/s
Acceleration Meter per second squared m/s2
Density kilogram /
Force Newton N 1 N = 1 kgm/s2
= 1 Pascal
Table 1.3 Standards Prefixes In SI Units
Multiplying Factor Prefix Symbol
1 000 000 000 000 1012
1 000 000 000 109
1 000 000 106
1 000 103
0.000 001 10-6
0.000000 001 10-9
Calculate the pressure of gas underneath the piston in equilibrium for a 50 kg
mass that reacts to a piston with a surface area of 100 cm2
It is define as a unit of measurement that is determined by combining one or more fundamental
units like force, energy, power, pressure and density.
F = ma
m=mass in unit kg
a=acceleration in unit m/s2
So in the SI unit of force is therefore kgm/s2
. Where of 1 N = 1 kg.m/s2
Heat and work are both forms of energy. Work = Force x Distance
So in the SI unit of work is therefore Newton meter (Nm). Where of 1 Nm = 1 J
which is 1kJ = 103
Power is the rate of energy transfer (or work done) by or to a system. Where of
1 Watt(W) = 1 J/s = 1 Nm/s
Pressure is the force exerted by a fluid per unit area.
P = F/A
The unit of pressure, is N/m2
and this unit is sometimes called the Pascal (Pa).
1 bar =105
Pa = 105
Density is the mass of a substance per unit volume. Unit kg/m3
A density of ρ = 850 kg/m3
of oil is filled to a tank. Determine the amount of
mass m in the tank if the volume of the tank is V = 2 m3
We should end up with the unit of kilograms. Putting the given information into perspective,
ρ = 850 kg/m3
and V = 2 m3
It is obvious that we can eliminate m3
and end up with kg by multiplying these two quantities.
Therefore, the formula we are looking for is
Thus, m = ρV
= (850 kg/m3
= 1700 kg
1.3 Unit Conversions
Conversion of units is the conversion between different units of measurement for the same
quantity.That is the process of converting the standard units from one form to another according
to the requirement. The need for the basic conversion has always existed in the respective fields
for different purposes. The example of unit conversions are:-
1 kg = 1000 g
1 m = 100 cm = 1000 mm
1 km = 1000 m = (100 000 cm @ 105
= (1 000 000 mm @ 106
1 hour = 60 minutes = 3600 seconds
= 1000 litre, or 1 litre = 1 x 10-3
1 bar = 1 x 105
= 1 x 102
1 gallon = 3.786 liter
1 liter = 1000 cm3
1 hp = 746 watt
Convert 1 km/h to m/s.
Convert 25 g/mm3
1 kg = 1000 g
1 m = 1000 mm
= 1000 x 1000 x 1000 mm3
1.4 Definitions Of System, Boundary, Surrounding, Open System And Close System.
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.
Surroundings is defined the mass or region outside the system. The surroundings may be
affected by changes within the system.
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 Fig. 1.1 below, so as to enable an
analysis of the problem under consideration to be made.
Fig. 1.1 : System, surroundings and boundary
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 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.
Fig. 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
Fig. 1.3: Open system in boiler
1.5 Energy Conversion
The process of changing energy from one form to another. There are many conversion processes
that appear as routine phenomena in nature, such as the evaporation of water by solar energy or
the storage of solar energy in fossil fuels. In the world of technology the term is more generally
applied to operations of human origin in which the energy is made more usable; for instance, the
burning of coal in power plants to convert chemical energy into electricity, the burning of
gasoline in automobile engines to convert chemical energy into propulsive energy of a moving
vehicle, or the burning of a propellant for ion rockets and plasma jets to provide thrust.
1.6 Properties Of Systems, State And Equilibrium
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. Properties are considered to be either intensive or extensive. Intensive
properties are those which are independent of the mass of a 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. Equilibrium implies a state of balance. In an equilibrium
state there are no unbalanced potentials ( or driving forces) within the system. A system in
equilibrium experiences no changes when it is isolated from its surrounding. Example:
i. Thermal equilibrium,
ii. Mechanical equilibrium,
iii. Phase equilibrium,
iv. Chemical equilibrium
1.7 Process and Cycle
Any change that a system undergoes from one equilibrium state to another is called a process,
and the series of states through which a system passes during a process is called the path of the
process. To describe a process completely, we should specify the initial and final states of the
process, as well as the path it follows, and the interactions with the surrounding.
Several commonly studied thermodynamic processes are:-
i. Isobaric process: occurs at constant pressure
ii. Isochoric process: occurs at constant volume (also called isometric/isovolumetric)
iii. Isothermal process: occurs at a constant temperature
iv. Adiabatic process: occurs without loss or gain of energy by heat
v. Isentropic process: a reversible adiabatic process, occurs at a constant entropy
vi. Isenthalpic process: occurs at a constant enthalpy
vii. Steady state process: occurs without a change in the internal energy
A system is said to have undergone a cycle if it returns to its initial state at the end of the process.
That is, for a cycle the initial and final states are identical. For example, the piston of car engine
undergoes Intake stroke, Compression stroke, Combustion stroke, Exhaust stroke and goes back
to Intake again. It is a cycle.
1.1 Zeroth’s Law of thermodynamics
The measurement of the degree of hotness or coolness is temperature.
If two bodies at different temperatures are brought together, the hot body will warm up the cold
one. At the same time, the cold body will cool down the hot one. This process will end when the
two bodies have the same temperatures. At that point, the two bodies are said to have reached
The Zeroth Law of thermodynamics states:
Two bodies each in thermal
equilibrium with a third body will
be in thermal equilibrium with each
The Zeroth Law of thermodynamics is a basis for the validity of temperature measurement
If A and C are each in thermal equilibrium with B, A also in thermal equilibrium with C
To establish a temperature scale, two fixed, easy duplicated points are used. The
intermediate points are obtained by dividing the distance between into equal
subdivisions of the scale length.
Temperature Scale Fixed Point 1 Fixed Point 2
Fahrenheit Scale (o
F) Freezing Point of Water = 32.0
Boiling Point of Water
Celsius Scale (o
C) Freezing Point of Water = 0.0
Boiling Point of Water =
The pressure of an ideal gas is
zero = 0.0
The Triple Point of Water =
The relations between the above temperature scales are:
T (K) = T(o
C) + 273.15
F) = 1.8T(o
C) + 32.0
F) = 1.8 (T(K)-273.15) + 32.0
Ex: If T(A) = T (B)
And T (B) = T (C)
Then T(A) = T (C)
The thermodynamic temperature scale in the English system is the Rankine scale. The
temperature unit on this scale is the rankine, which is designated by R. The
thermodynamic temperature scale in S.I. system (K) and English system (R) are related
T(R) = 1.8 T(K)
1.1 Define thermodynamics.
1.2 Distinguish between fundamental units and derived units and state the examples.
1.3 What is the work done by an expanding gas if the force resisting the motion of the
piston is 700 N and the length of the stroke is 0.5 m ?
1.4 What is the force required to accelerate a mass of 30 kg at a rate of 15 m/s2
1.5 The fuel tank of a large truck measures 1.2m x 0.9m x 0.6m. How many litres of
fuel are contained in the tank when it is full?
1.6 A weather research instrument is suspended below a helium filled balloon which
is a 3.8m diameter sphere. If the specific volume of helium is 5.6m3
/kg, what is
the weight of helium in the balloon? Explain briefly why the balloon rises in the
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3. Kamaruzzaman Daud and Roslan Hashim. Module Thermodynamics 1. Malaysia Polytechnics, Ministry Of Education.
4. Article Source: http://EzineArticles.com/6388269
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