Thermodynamics is the science of energy and its transformation. The first law of thermodynamics states that energy is conserved during any process as it changes from one form to another. The second law states that the quality or usefulness of energy degrades as it is transformed. Thermodynamics applies to all natural processes and is important in engineering systems like power plants, refrigerators, and automobiles. Properties can be analyzed from a macroscopic or microscopic perspective, with classical thermodynamics taking a macroscopic approach useful for engineering applications. The state of a simple compressible system can be fully described by two intensive properties such as pressure and temperature.

1. Basic Concepts of Thermodynamics
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2. 1-1 Thermodynamics and Energy
• Thermodynamics is the science of energy (Probably we
may say the power of heat) .
• Energy is the ability to do work.
 Conservation of energy principle (first law of
thermodynamics): During an interaction (i. e. a process),
energy can change from one form to another but the total
amount of energy remains constant.
 Energy has quality as well as quantity; the second law
of thermodynamics.
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3. Importance of thermodynamics
 All activities in nature involve interaction between energy and matter
(thermodynamics).
 Thermodynamics is encountered in many engineering systems and
other aspects of life.
 Energy conversion occurs in the cells of human body.
 Examples in an ordinary house: electric or gas range, the heating
and air-conditioning systems, the refrigerator, the pressure cooker,
the water heater, the shower, the iron, and even the computer and
the TV set.
 In automotive industry thermodynamics plays an essential role
regarding design and analysis.
 Power plants.
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4. Micro vs. Macro
• Thermodynamic properties can be analyzed from a
microscopic or a macroscopic perspective:
- Classical thermodynamics treats matter as a ‘continuum’ –
macroscopic approach; a direct and easy way to the solution
of engineering problems.
- Statistical thermodynamics studies the statistical (i.e., random)
behavior of individual molecules – microscopic approach – and
then averages over all the molecules.
- We will focus entirely on the classical approach.
- Fortunately, the two approaches converge to the same answer.
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5. 1-2 Dimensions and Units
•
Dimension: physical properties or characteristics of a system
-
Primary Dimensions: mass m, length L, time t, and temperature T
-
Secondary Dimensions (or derived dimensions): velocity V, energy E,
force F, and volume V
•
Units: magnitudes assigned to dimensions
•
Two Unit Systems: English and SI or System Internationale
English
SI
Conversion Factors
mass:
pound mass, lbm
kilogram, kg
1 lbm = 0.45359 kg
length:
foot, ft
meter, m
1 ft = 0.3048 m
force:
pound force, lbf
newton, N
1 lbf = 4.4482 N
work / energy:
British Thermal Unit, Btu
joule, J
1 Btu = 1055.06 J*
power:
Btu/h
watt, W
3.41214 Btu/h = 1 W
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6. Important Note for Using British Units!
•
A conversion factor is needed when relating mass to either force, work,
or energy, i.e.,
Force = mass*acceleration/gc, where
gc = 1.0 (kg.m/s2) / N
= 32.17 (lbm.ft/s2) / lbf
•
A conversion factor is also used when dealing with ‘thermal energy’ such
that
One Btu (British thermal unit) = 778.169 ft-lbf
•
Horsepower ‘hp’ is also used as a unit such that
1 hp = 550 ft-lbf / s
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7. Important terms:
Pound-force: the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate
of 1 ft/s2
Weight is a force (not mass) = mass x gravitational acceleration.
Specific Weight (w) is the weight per unit volume of a substance; w = density x
gravitational acceleration.
Joule is a unit of energy in the SI system; 1 J = 1 N . m
British thermal unit (BTU) is the unit of energy in the English system; 1 BTU =
the amount of energy required to raise the temperature of 1 lbm of water at 68 oF by 1
degree Fahrenheit.
1 Calorie (cal) is a unit of energy in the SI (metric) system defined as the amount
of energy required to raise the temperature of 1 g of water at 68 oC by 1 degree
Celsius.
Dimensional Homogeneity: Quantities added or subtracted from each other
must have the same units. The terms on both sides of an equation (separated
by either + or – sign) must have the same units for the equation to be correct
(see examples 1-1 and 1-2).
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8. Standard Prefixes in SI units
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9. Nomenclature
specific thermodynamic property, where x = u, h, s, …(J/kg)
x (lower case)
molar thermodynamic property, where X = U, H, S, …(J/mol)
X
total thermodynamic property, where X = U, H, S, …(J)
X
work, power (J, J/s)
W, W
heat, heat rate (J, J/s)
Q, Q
velocity (m/s)
V
volume, volumetric flow rate (m3, m3/s)
V,V
weight (N)
W
mole fraction: liquid or solid phase, vapor phase
x, y
molecular weight
M
mole, molar flow rate (mol, mol/sec)
n, n
mass, mass flow rate (kg, kg/s)
m, m
Dimension (example units)
Symbol
.
.
.
.
.
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10. Closed and Open Systems
(a) Closed Systems:
controlled (fixed) mass.
Moving
boundary
Fixed
boundary
GAS
2 kg
3 m3
GAS
2 kg
1 m3
Closed
System
energy YES
(m= constant)
mass NO
If a closed system is not allowed to
exchange energy with its
surroundings, then it is called an
isolated system.
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11. (b) Open system: controlled (fixed) volume
Water
Heater
(control
volume)
Control surface
Hot Water
Out
Cold Water
IN
Note:
Volume
is fixed
Examples of open systems: Pumps, compressors, turbines, boilers, Lungs of
humans, Nozzles, etc.
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12. Properties of a System
Name of property Symbol definition Units (SI)
Density
Specific volume
Specific gravity, or
relative density
kg/m3
m3/ kg
s =

H2O
H2O=1000 kg/m3 @ 4oC
• Property: Any characteristic of a system.
Examples:
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13. Classification of properties:
m
V
T
P

m/2 m/2
V/2 V/2
T T
P P
 
- Intensive properties: independent on system size (e.g., P, T, , e)
- Extensive properties: depend on system size (e.g., m, V, E)
Extensive
properties
Intensive
properties
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14. Thermodynamic State and Equilibrium
• Thermodynamic state of a system – the condition of the system as
characterized by the values of its properties.
• There are many different types of equilibria that can be attained:
- Thermal: the temperature is the same throughout the system.
- Mechanical: the pressure is the same.
- Phase: no driving force for the total mass in each phase to change.
- Chemical: no driving force for chemical composition to change.
• We can show a state as a point on a phase diagram as long as the
continuum theory applies; thermodynamics deals only with
equilibrium states.
• Stable equilibrium state – a state in which the system is not capable of
any spontaneous change to another state without a finite change in
the surrounding. There are no driving forces to carry out a change.
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15. Simple compressible system
A system is said to be a simple compressible system if it is not influenced
by electrical, magnetic, gravitational, motion, and surface tension effects.
- Typically, properties chosen are either :
≫ P & T
≫ P & specific volume (v = Volume / mass), or
≫ T & v
- The above 3 variables are the easiest to measure.
- The state of a two-phase system cannot be fully defined by P & T only.
State postulate: The state of a single-component, simple compressible
system, can be fully described by two intensive properties.
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