lecture 1 - fundamental concepts of thermodynamics and is system of units
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Mechanical Theory I
MET3405
1. Thermodynamics
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Lecture 1
1.1 Fundamental Concepts of Thermodynamics
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1.1.1 Introduction
Heat
Dynamis
Force
Therme
Thermodynamics
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Thermodynamics
capacity of hot bodies to produce work
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Thermodynamics
It is a branch of engineering science that deals with the relationship between
�energy associated to heat
�and other forms of energy
•••• mechanical
•••• electrical
•••• chemical
•••• ….
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Resources in nature
Fossil Fuels
Radioactive Substances
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People’s needs for energy
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People’s needs for energy
Air conditioning
Heating
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Provision of energy is one
of the main tasks of ME
The branch of science that
�explains how much energy people may extract from various sources
�and predicts how efficiently people may use the extracted energy in a particular situation is called:
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1.1.2 Basic Concepts
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Fluids
Fluids are substances that:
• flow, even under the action of small forces;
• and take the shape of their container.
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GasesLiquids
Incompressible
FluidsVolume = const
Compressible
FluidsVolume ≠≠≠≠ const
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Thermodynamic System
System is the subject of analysis.
It may include:
•••• single simple body,
•••• or very complex assembly of many component and parts.
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Schematic presentation
of a System
System
Surroundings
System Boundary
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Closed System
Closed Reservoir
m = constThe substance in the reservoir cannot leave.New substance cannot enter in the reservoir.
m : mass of the substance
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Cylinder - Piston Assembly
Cylinder
Piston
Boundary
System: Working
Fluid
m = const
V ≠≠≠≠ const
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V2 > V1 Expansion
V1
State 1
V2
State 2
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V1 > V2 Compression
V2
State 2
V1
State 1
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Cylinder - Piston Assembly
This device has very important role in engineering practice and is commonly used in many gas power cycles such as internal combustion engines.
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Closed System ≡ Control Mass
In some textbooks, the closed system is also called Control Massbecause in a closed system the mass of the working substance is under consideration.
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Open System
Pipe
Control Volume
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Open System
Examples of open system include pipes, nozzles and diffusers, boilers, heat exchangers, valves, turbines, pumps and compressors.
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Thermodynamic Property
Properties are quantities that can be measured,
so their values can describe the condition of the system
without knowing how the system came to that condition.
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Examples of Properties
• Mass of the working fluid employed in the process
• Volume occupied by the working fluid
• Temperature of the working fluid
• Pressure of the working fluid
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State
State is a term for a condition of the system
as it is described by its properties.
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Example for a State
• Closed System
Our lecture theatre
• Working Fluid in the lecture theatre:
Air
• Temperature of Air: 20oC
• Pressure of Air: 1 bar
• The state of the system is given with: 20oC and 1 bar.
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Process
Process is a transformation of the system
from one state
to another state.
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Example for a Process
• Closed System: Our Lecture Theatre
• Initial State: 20oC, 1 bar
• Air-conditioning system switched off
• Process: Heating of the air (due to transfer of heat from outside and from students)
• Final State: 30oC, 1 bar
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1.1.3 The SI System of Units in Thermodynamics
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International System of Units: Basic Units
Mass Length Time
Symbol m (Note)
L t
Unit kilogramme metre second
Symbol for unit
kg m s
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Note (Very Important!)
�Quantity mass with unit kg is characteristic for Closed Systems.
�For Open System, corresponding quantity is
called Mass Flow Rate
m&the symbol is
and the unit is kg/s
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Additional Basic Unit
for Heat Interactions:
Temperature
No negative values for temperature
Kelvin Scale
Only positive values for temperature
Absolute Zero Temperature
Temperature (T) expressed in Kelvin degrees: K
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Additional Basic Unit
for Heat Interactions:
TemperatureCelsius Scale
Negative values for temperature
Positive values for temperature
Zero Temperature
Temperature (t) expressed in Celsius degrees: oC
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Relation between Kelvin and Celsius Scales
0 K = - 273.15 oC
0 oC = 273.15 K
T (K) = t (oC) + 273.15
t (oC) = T (K) - 273.15
∆∆∆∆T (K) = ∆∆∆∆t (oC)∆∆∆∆T = T2 - T1; ∆∆∆∆t = t2 - t1
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SI System of Units:
Derived Units
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Examples
• Area
aA=a2
a
b
A=ab d
A=d2ππππ/4
Unit: m 2
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Examples
Volume
ab
c V = abc
Unit: m 3
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Examples: Volume
Liter (l)
• 1 liter = 1 dm3
• 1 dm = 10 cm
• 1 m = 10 dm
• 1 m3 = 1000 dm3 = 1000 l
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Force
F = maF: force acting on the body;
m: mass of the body, kg;
a: acceleration, m/s2;
Unit for force: Newton (N)
2s
m X kg F forcefor Unit =
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Weight
w = mgw: weight of the body, N;
m: mass of the body, kg;
g: acceleration due to gravity, m/s2;
g = 9.81 m/s2
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Density
ρρρρ = m/V
ρρρρ: density, kg/m3;
m: mass of the body, kg;
V: volume occupied by the mass, m3;
m = ρρρρV
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Specific Volume
v = 1/ρρρρ = V/m
v: specific volume, m3/kg;
V: volume, m3;
m: mass of the body, kg;
m = V/v
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Relative Density
s = ρρρρsubstance/ρρρρwaters: relative density, no unit;
Example:
ρρρρmercury = 13595 kg/m3;
ρρρρwater = 1000 kg/m3;
⇒⇒⇒⇒ s = 13595/1000 = 13.595
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Pressure
p = F/A
p: pressure
F: normal component of the force acting over the body, N;
A: area over which the force acts, m2;
Unit for pressure = N/m2 = Pascal = Pa
1 bar = 105 Pa
1 atmosphere pressure = 101325 Pa
= 1.01325 bar
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Reference: Basic Thermodynamics
by K. Iynkaran and D. J. Tandy
�Suggested Additional ReadingChapter 1: Introduction to Thermodynamics
�Suggested Examples1.1 - 1.5
�Suggested Tutorial Problems1.1 - 1.8