Chapter 1: Introduction and Basic Concepts

Dr. Othman Hassan

1
Dimensions and Units
 Any Physical Quantity can be characterized by a
Dimension
 The magnitudes assigned to the dimensions are
called units
Dimensions and Units
 Primary or Fundamental Dimensions (Mass, Length,
Time, Temperature)
 Derived or Secondary Dimensions (Velocity, Energy,
Volume…)
Dimensional Homogeneity
 Every additive term in an equation must have the same
dimensions.
 For example, conservation of energy of a system states:
Δ𝐸 = Δ𝑈 + ΔEK + Δ𝐸𝑃
 Where,
1
Δ𝑈 = 𝑚 𝑢2 − 𝑢1 , Δ𝐸𝐾 = 𝑚 𝑉22 − 𝑉12 , Δ𝐸𝑃 = 𝑚𝑔 𝑧2 − 𝑧1
2
 All these quantities have the dimensions of energy 𝑚𝐿2 𝑡 −2

An equation that is not dimensionally homogeneous is a sure sign of an error.

British vs Metric Systems
 English system: It has no apparent systematic numerical base, and
various units in this system are related to each other rather
arbitrarily.
 1 Mile = 1760 Yard
 1 yard = 3 ft
 1 ft = 12 in
 1 Ib = 16 oz

 Metric SI system: A simple and logical system based on a decimal

relationship between the various units.
 1 km = 1000 m
 1 m = 100 cm = 1000 mm
 1 kg = 1000 gm
 1 kPa = 1000 Pa
 Conversion Factors between
British and SI Systems
 1 kg = 2.2 Ibm or 1 Ibm = 454 gm
 1 Ib = 16 oz
 1 ft = 30.5 cm
 1 ft = 12 in
 1 gal = 3.78 Litre
 1 C = 1 K = 1.8 F
 1 kgf = 9.8 N
 1 Ibf = 4.45 N
 1 cal = 4.182 J
 1 Btu = 1055 J
 1 hp metric = 735 Watt , 1 hp imperial = 745 Watt
 1 rad = 57.3 degrees
 1 psi = 1 Ibf/in2 = 6891 Pa
Systems and control volumes
System: A quantity of matter or a region in space chosen for study.
Systems may be considered to be closed or open.
Surroundings: The mass or region outside the system
Boundary: The real or imaginary surface that separates the system
from its surroundings.
Systems and control volumes
Closed system (Control mass): A fixed amount of mass, and no
mass can cross its boundary.
 The boundary of a system can be fixed or movable.

A closed system with a moving boundary

Properties of A System
 Any characteristic of a system is called a property.
 Properties are considered to be either intensive or
extensive.
 Intensive properties are those that 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.
Properties of a System
Continuum
 Matter is made up of atoms that are widely spaced.
 Continuum means viewing a substance as a continuous, homogeneous
matter with no voids.
 Continuum allows us to treat properties as point functions and to
assume the properties vary continually in space with no discontinuities.
 Continuum idealization is valid as long as the size of the system we deal
with is large relative to the space between the molecules.
 In this course we will limit our consideration to substances that can be
modeled as a continuum.
Density and specific volume
Density

Specific volume

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Specific gravity and specific weight
 The ratio of the density of a
substance to the density of some
standard substance at a specified
temperature (usually water at 4°C).

 Specific weight: The weight of a

unit volume of a substance.
Example
A 1.5 m3 tank is filled with a certain oil. The tank and the
oil together weigh 1325 kg. If the tank alone weighs 50
kg, find the density, specific weight, specific gravity and
specific volume of the oil.
State and Equilibrium
 A state: set of properties that completely describes the
condition of a system
 Equilibrium: A state of balance.
 In an equilibrium state there are no unbalanced potentials
(or driving forces) within the system. So, all properties can
be calculated or measured throughout the system.
State and Equilibrium
 Thermal equilibrium: If the temperature is the same
throughout the entire system.
 Mechanical equilibrium: If there is no change in
pressure at any point of the system with time.
 Phase equilibrium: If a system involves two phases and
when the mass of each phase reaches an equilibrium level
and stays there.
 Chemical equilibrium: If the chemical composition of a
system does not change with time, that is, no chemical
reactions occur.
Thermodynamics deals with equilibrium states.
The State Postulate
 The state of a simple compressible system is completely specified by two
independent, intensive properties.
 Simple compressible system: If a system involves no electrical, magnetic,
gravitational, motion, and surface tension effects.
 Two properties are independent if one of them can be varied while the other one
is kept constant.
 Temperature and specific volume are independent.
 Temperature and pressure are independent for single phase systems (liquids or
vapors)
 Temperature and pressure are dependent for multiphase systems (mixture of
liquid and vapor)
 Example
 Which of the following systems have their states specified?
 Air at 350 K.

 CO2 at 300 K and 100 kPa.

 Steam at 500 kPa

 Water at 10C and atmospheric pressure.

 Example
 Which of the following systems have their states specified?
 Steam in a tank at 200 C and specific volume of 0.4 m3/kg

 A mixture of water and its vapor at 100C

 A 10 m3 tank containing steam at 350 C

Processes and Cycles
 Process: Any change that a system undergoes
from one equilibrium state to another.

 Path: The series of states through which a

system passes during a process.

 To describe a process completely, one should

specify the initial and final states, as well as
the path it follows.

 We will always assume that the process

proceeds in such a manner that the system
remains infinitesimally close to an equilibrium
state at all times. This is called quasistatic or
quasi-equilibrium process.
Processes and Cycles
 Any change that a system undergoes from one equilibrium
state to another is called a process.
 The prefix iso- is often used to designate a process for
which a particular property remains constant.
 An isothermal process, is a process during which the
temperature T remains constant;
 An isobaric process is a process during which the pressure P
remains constant;
 An isochoric (or isometric) process is a process during
which the specific volume v remains constant.
 A system is said to have undergone a cycle if it
returns to its initial state at the end of the process.
Steady flow processes
 A large number of engineering devices operate for
long periods of time under the same conditions,
and they are classified as steady-flow devices.

 Steady-flow process: A process during which a

fluid flows through a control volume steadily.

 The term steady implies no change with time.

The opposite of steady is unsteady, or transient.

 Steady-flow conditions can be closely

approximated by devices that are intended for
continuous operation such as turbines, pumps,
boilers, condensers, and heat exchangers or power
plants or refrigeration systems.
The Zeroth Law of
Thermodynamics
 The zeroth law of thermodynamics:
If two bodies are in thermal equilibrium with a third body, they
are also in thermal equilibrium with each other.
 By replacing the third body with a thermometer, the zeroth law can be
restated as two bodies are in thermal equilibrium if both have the
same temperature reading even if they are not in contact.

In simple words: if two bodies are at

the same temperature, they are in
thermal equilibrium with each other.
Temperature Scale
 The temperature scales used in the SI and in the English
systems today are the Celsius scale (formerly called the
centigrade scale; in 1948 it was renamed after the Swedish
astronomer A. Celsius, 1702–1744, who devised it) and the
Fahrenheit scale (named after the German instrument
maker G. Fahrenheit, 1686–1736), respectively.
 On the Celsius scale, the ice and steam points were
originally assigned the values of 0 and 100°C, respectively.
 The corresponding values on the Fahrenheit scale are 32
and 212°F.
Temperature Scale
 In thermodynamics, it is very desirable to have a
temperature scale that is independent of the
properties of any substance or substances. Such a
temperature scale is called a thermodynamic
temperature scale.
 The thermodynamic temperature scale in the SI is the
Kelvin scale, named after Lord Kelvin (1824–1907).
The temperature unit on this scale is the kelvin,
which is designated by K.
 The lowest temperature on the Kelvin scale is absolute
zero, or 0 K.
 Temperature Scales
 Celsius scale
 Ice point = 0 0C
 Water boiling point = 100 oC
 Fahrenheit scale
 Ice point = 32 F
 Water boiling point = 212 F
 1°C = 1.8 °F
 F = 1.8 x C + 32
 C = (F – 32)/1.8
Temperature Scale
Absolute Temperature Scales
 Zero temperature corresponds to absolute
zero
 Kelvin scale
 K = C + 273.15
 1 K = 1 °C
 Rankine scale
 °R = °F + 459.67
 1 °R = 1 °F
Pressure
 Pressure is defined as a normal force exerted by a fluid
per unit area.
 We speak of pressure only when we deal with a gas or a
liquid. The counterpart of pressure in solids is normal
stress.
 it has the unit of Newtons per square meter (N/m2),
which is called a pascal (Pa)
 Three other pressure units commonly used in practice,
especially in Europe, are bar, standard atmosphere,
and kilogram-force per square centimeter
Pressure
Pressure
 The actual pressure at a given position is called the absolute
pressure, and it is measured relative to absolute vacuum (i.e.,
absolute zero pressure).
 Most pressure-measuring devices, however, are calibrated to read
zero in the atmosphere, and so they indicate the difference
between the absolute pressure and the local atmospheric
pressure.
 This difference is called the gage pressure.
 Pressures below atmospheric pressure are called vacuum
pressures.
Pressure
Pressure variation with depth
 In any fluid, pressure increases with depth due to the weight of the fluid:

 Density of gases can be neglected with respect to liquids when

calculating hydrostatic pressures
 Density of air at standard conditions= 1.25 kg/m3
 Density of lightest liquids  600 kg/m3.
Pascal’s Law
 The pressure applied to a confined fluid increases the
pressure throughout by the same amount.