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10/3/2016
ME 215
Fundamentals of Thermal Systems
CH-2
Basic Concepts of Thermodynamics
ÇANKAYA UNIVERSITY
Mechanical Engineering Department
Asst. Prof. Dr. Ekin Özgirgin Yapıcı
What is Thermodynamics?
• Thermodynamics is the science dealing with
relationships between heat, work and the
properties of materials.
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Definition of a System
• The term system is used to identify the
subject of the analysis. The system is
whatever we want to study. It may be as
simple as a free body or as complex as an
entire chemical refinery.
• The mass or region outside the system is
called surroundings.
• The real or imaginary surface that separates
the system from its surroundings is called the
boundary.
Definition of a System
system
surroundings
boundary
Types of Systems:
A closed system (control mass) is defined when a particular
quantity of matter is under study. A closed system always
contains the same matter. There can be no transfer of mass
across its boundary.
An open system (control volume): With this approach, a
region within a prescribed boundary is studied. The region is
called a control volume. Mass may cross the boundary of a
control volume.
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Definition of a System
Types of Systems:
If no mass can enter or leave a closed system ans also if
energy is not allowed to cross the boundary, that system is
called an isolated system.
Definition of a Sysem
A closed system (control mass)
The thermodynamic relations that
are applicable to closed and open
systems are different. It is important
to recognize the type of system
before you start analyzing it
An open system (control volume)
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Describing Systems and
Properties
Engineers are interested in studying systems and how they
interact with their surroundings. Terms and concepts used to
describe systems and how they behave;

Property (extensive property, intensive property)

State

Process

Phase and Pure Substance

Equilibrium

Steady state

Thermodynamic cycle
Describing Systems and
Properties

A property is a macroscopic characteristic of a
system such as mass, volume, energy, pressure, and
temperature to which a numerical value can be
assigned at a given time without knowledge of the
previous behavior (history) of the system.

Thermodynamic properties can be placed in two
general classes:
extensive and intensive.
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Describing Systems and
Properties

extensive
Dependent of the size and extent of the system
Additive over the system
mVE

Intensive
İndependent of the size and extent of the system
Not Additive over the system
T P 
b=B/m where b is intensive, B is extensive property
Describing Systems and
Properties

State: The word state refers to the condition of a
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.

If a system exhibits the same values of its properties at
two different times, it is in the same state at these
times. A system is said to be at steady state if none of
its properties changes with time.
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Describing Systems and
Properties

When any of the properties of a system change, the
state changes and the system is said to have undergone
a process. A process is a transformation from one state
to another.

A process occurring at constant temperature is an
isothermal process.

A process occurring at constant pressure is an isobaric
process.
Describing Systems and
Properties

Thermodynamic
cycle: is a sequence
of processes that
begins and ends at
the same state. At
the conclusion of a
cycle all properties
have the same
values they had at
the beginning.
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Describing Systems and
Properties

Phase: quantity of matter that is homogeneous throughout
in both chemical composition and physical structure.

Homogeneity in physical structure means that the matter
is all solid, or all liquid, or all vapor (or equivalently all
gas).

When more than one phase is present, the phases are
separated by phase boundaries.

A pure substance is one that is uniform and invariable in
chemical composition. A pure substance can exist in more
than one phase, but its chemical composition must be the
same in each phase.
For example: liquid water and water vapor mixture
Describing Systems and
Properties

Equilibrium:In mechanics, equilibrium means a condition
of balance maintained by an equality of opposing forces.

Isolate the system from its surroundings and watch for
changes in its observable properties. If there are no
changes, we conclude that the system was in equilibrium
at the moment it was isolated. The system can be said to
be at an equilibrium state.
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Units and Dimensions

A unit is any specified amount of a quantity by
comparison with which any other quantity of the same
kind is measured.

EX: meters, centimeters, kilometers, feet, inches, and
miles are all units of length. Seconds, minutes, and
hours are alternative time units.

Because physical quantities are related by definitions
and laws, a relatively small number of them are used
to measure all others. These may be called primary
(or basic) dimensions.

The others may be measured in terms of the primary
dimensions and are called secondary.
Units and Dimensions
Units and Dimensions

Four primary dimensions suffice in
thermodynamics, fluid mechanics, and heat
transfer.

They are mass (M), length (L), time (t), and
temperature (T).

Alternativelyt, Force (F) can be used.
Basic Dimension Systems:

MLTθ (mass length time temperature)

FLTθ (force length time temperature)
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Primary Units and Dimensions
MLT
Secondary Units and Dimensions
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Describing Systems and
Properties; secondary dimensions
Volume(V) m3
 Density ():kg/m3
The density, or local mass per unit volume, is an
intensive property that may vary from point to point
within a system.
 Spesific volume (): m3/kg (1/density)
It is the volume per unit mass. Like density, specific
volume is an intensive property and may vary from
point to point.
In certain applications it is convenient to express
properties such as a specific volume on a molar basis.
kg/kmol

Describing Systems and
Properties; secondary dimensions

Sometimes the density of a substance is
given relative to the density of a wellknown substance. Then it is called
specific gravity, or relative density, and
is defined as the ratio of the density of a
substance to the density of some
standard substance at a specified
temperature:
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Temperature and Zeroth Law
Temperatureis a measure of hottness or coldness.
 Several properties of materials change with
temperatures, so it is very important to measure the
temperature.

A device for temperature measurement is
the liquid-in-glass thermometer
Temperature and Zeroth Law

That is, when a body is brought into contact with
another body that is at a different temperature, heat is
transferred from the body at higher temperature to the
one at lower temperature until both bodies are at same
temperature

At that point, the heat transfer stops, and the two
bodies are said to have reached thermal equilibrium.

The zeroth law of thermodynamics states that if two
bodies are in thermal equilibrium with a third body, they
are also in thermal equilibrium with each other.
http://www.wiley.com/college/moran/0470495901/animat
ions/ext_int_properties/ext_int_properties.html
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Temperature Scales

Celsius scale (formerly called the centigrade
scale)

Fahrenheit scale

Kelvin scale(thermodynamic temperature
scale:that is independent of the properties of any
substance or substances)

Rankine scale
Temperature Measurement
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Pressure
http://www.wiley.com/college/moran/04704
95901/animations/ext_int_properties/ext_int
_properties.html

Pressure = Normal Force / area (1 N/m2= 1 Pa)

Pressure measurement manometer

 barometer
Pressure

The actual pressure at a given position is called the
absolute pressure.

the difference between the absolute pressure and the
local atmospheric pressure is called the gage pressure.

Pressures below atmospheric pressure are called vacuum
pressures
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Variation of Pressure with Depth

The pressure difference between two points in a
constant density fluid is proportional to the vertical
distance z between the points and the density of the
fluid.
Liquids are incompressible
substances, and thus the variation
of density with depth is negligible.
This is also the case for gases
when the elevation change is small
Variation of Pressure with Depth

For fluids whose density changes significantly with elevation, a
relation for the variation of pressure with elevation can be
obtained as:
The pressure is the same at all points on a horizontal
plane in a given fluid regardless of geometry
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The Manometer Measurement
The Manometer Measurement
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The Manometer Measurement
Barometer and atmospheric pressure
The atmospheric
pressure is measured
by a device called a
barometer; thus,
the atmospheric
pressure is often
referred to as the
barometric pressure.
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Example

A closed tank contains compressed air and oil (SGoil 0.90)
as is shown in Fig. A U-tube manometer using mercury
(SGHg = 13.6) is connected to the tank as shown. For
column heights h1=15 cm, h2=7 cm, and h3=10 cm,
determine the pressure reading of the gage.
The pressure at level (1) is
equal to the pressure at level
(2), since these two points are
at the same elevation in a
homogeneous fluid at rest
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