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Basic Concepts of Thermodynamics
Every science has its own unique vocabulary associated with it. Precise definition of basic
concepts forms a sound foundation for development of a science and prevents possible
misunderstandings. Careful study of these concepts is essential for a good understanding
of topics in thermodynamics.
Thermodynamics and Energy
Thermodynamics can be defined as the study of energy, energy transformations and its
relation to matter. The analysis of thermal systems is achieved through the application of
the governing conservation equations, namely Conservation of Mass , Conservation of
Energy (1st law of thermodynamics), the 2nd law of thermodynamics and the property
relations. Energy can be viewed as the ability to cause changes.
First law of thermodynamics: one of the most fundamental laws of nature is the
conservation of energy principle. It simply states that during an interaction, energy can
change from one form to another but the total amount of energy remains constant.
Second law of thermodynamics: energy has quality as well as quantity, and actual
processes occur in the direction of decreasing quality of energy.
Whenever there is an interaction between energy and matter, thermodynamics is
involved. Some examples include heating and air‐conditioning systems, refrigerators,
water heaters, etc.
Dimensions and Units
Any physical quantity can be characterized by dimensions. The arbitrary magnitudes
assigned to the dimensions are called units. There are two types of dimensions, primary or
fundamental and secondary or derived dimensions.
Primary dimensions are: mass, m; length, L; time, t; temperature, T
Secondary dimensions are the ones that can be derived from primary dimensions such as:
velocity (m/s 2 ), pressure (Pa = kg/m.s 2 ).
There are two unit systems currently available SI (International System) and USCS (United
States Customary System) or English system. We, however, will use SI units exclusively in
this course. The SI units are based on decimal relationship between units. The prefixes
used to express the multiples of the various units are listed in Table 1 ‐1.
Table 1: Standard prefixes in SI units.
MULTIPLE 1012 109 106 103 10 ‐^2 10 ‐^3 10 ‐^6 10 ‐^9 10 ‐^12
PREFIX tetra, T giga, G mega, M kilo, k centi, c mili, m micro, μ nano, n pico, p
Important note: in engineering all equations must be dimensionally homogenous. This
means that every term in an equation must have the same units. It can be used as a sanity
check for your solution.
Example 1: Unit Conversion
The heat dissipation rate density of an electronic device is reported as 10.72 mW/mm
2 by
the manufacturer. Convert this to W/m
2 .
2
2
2 10720 1000
m
W
mW
W
m
mm
mm
mW
Closed and Open Systems
A system is defined as a quantity of matter or a region in space chosen for study. The
mass or region outside the system is called the surroundings.
Fig. 1: System, surroundings, and boundary
Boundary: the real or imaginary surface that separates the system from its surroundings.
The boundaries of a system can be fixed or movable. Mathematically, the boundary has
zero thickness, no mass, and no volume.
Closed system or control mass: consists of a fixed amount of mass, and no mass can cross
its boundary. But, energy in the form of heat or work, can cross the boundary, and the
volume of a closed system does not have to be fixed.
Open system or control volume: is a properly selected region in space. It usually encloses
a device that involves mass flow such as a compressor. Both mass and energy can cross
the boundary of a control volume.
Important note: some thermodynamics relations that are applicable to closed and open
systems are different. Thus, it is extremely important to recognize the type of system we
have before start analyzing it.
Isolated system: A closed system that does not communicate with the surroundings by
any means.
Rigid system: A closed system that communicates with the surroundings by heat only.
SYSTEM
BOUNDARY
SURROUNDINGS
Kinetic energy: energy that a system posses as a result of its relative motion relative to some reference frame, KE
kJ
mV KE 2
2
where V is the velocity of the system in (m/s).
Potential energy: is the energy that a system posses as a result of its elevation in a gravitational field, PE
PE mgz^ kJ^
where g is the gravitational acceleration and z is the elevation of the center of gravity of the system relative to some arbitrary reference plane.
Microscopic forms of energy: are those related to molecular structure of a system. They
are independent of outside reference frames. The sum of microscopic energy is called the
internal energy, U.
The total energy of a system consists of the kinetic, potential, and internal energies:
mgz kJ
mV E U KE PE U 2
2
where the contributions of magnetic, electric, nuclear energy are neglected. Internal
energy is related to the molecular structure and the degree of molecular activity and it
may be viewed as the sum of the kinetic and potential energies of molecules.
The sum of translational, vibrational, and rotational energies of molecules is the kinetic energy of molecules, and it is also called the sensible energy. At higher temperatures, system will have higher sensible energy.
Internal energy associated with the phase of a system is called latent heat. The intermolecular forces are strongest in solids and weakest in gases.
The internal energy associated with the atomic bonds in a molecule is called chemical or bond energy. The tremendous amount of energy associated with the bonds within the nucleolus of atom itself is called atomic energy.
Energy interactions with a closed system can occur via heat transfer and work.
Fig. 1 ‐4: Forms of energy.
Mechanical equilibrium: when there is no change in pressure at any point of the system. However, the pressure may vary within the system due to gravitational effects.
Phase equilibrium: in a two phase system, when the mass of each phase reaches an equilibrium level.
Chemical equilibrium: when the chemical composition of a system does not change with time, i.e., no chemical reactions occur.
Processes and Cycles
Any change 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 a path.
Fig. 6: To specify a process, initial and final states and path must be specified.
Quasi‐equilibrium process: can be viewed as a sufficiently slow process that allows the
system to adjust itself internally and remains infinitesimally close to an equilibrium state
at all times. Quasi‐equilibrium process is an idealized process and is not a true
representation of the actual process. We model actual processes with quasi‐equilibrium
ones. Moreover, they serve as standards to which actual processes can be compared.
Process diagrams are used to visualize processes. Note that the process path indicates a
series of equilibrium states, and we are not able to specify the states for a non‐quasi‐
equilibrium process.
Prefix iso ‐ is used to designate a process for which a particular property is constant.
Isothermal: is a process during which the temperature remains constant
Isobaric : is a process during which the pressure remains constant
Isometric: is process during which the specific volume remains constant.
A system is said to have undergone a cycle if it returns to its initial state at the end of the
process.
State 2
State 1
Process path
A
B
Fig. 1 ‐7: A four‐process cycle in a P‐V diagram.
The state of a system is described by its properties. The state of a simple compressible
system is completely specified by two independent , intensive properties.
A system is called simple compressible system in the absence of electrical, magnetic,
gravitational, motion, and surface tension effects (external force fields).
Independent properties: two properties are independent if one property can be varied
while the other one is held constant.
Pressure
Pressure is the force exerted by a fluid per unit area.
Pa m
N
Area
Force Pressure
In fluids, gases and liquids, we speak of pressure ; in solids this is stress. For a fluid at rest,
the pressure at a given point is the same in all directions.
Fig. 8: Pressure of a fluid at rest increases with depth (due to added weight), but constant in horizontal planes.
V
P
P(z)
z
Area = A
P gh
A
Ahg
A
mg P
P
Area
Weightofliquid
h
A
Bourdon Tube is a device that measures pressure using mechanical deformation. Pressure
Transducers are devices that use piezoelectrics to measure pressure.
very accurate and robust
can measure from 10
‐ 6 to 10
5 atm
can measure Pgauge or Pabs
Barometer is a device that measures atmospheric pressure. It is a manometer with a near
vacuum on one end
Fig. 11: Burdon gauge.
Example 2: Pressure
The piston of a cylinder‐piston device has a mass of 60 kg and a cross‐sectional area of
0.04 m 2 , as shown in Fig. 12. The depth of the liquid in the cylinder is 1.8 m and has a
density of 1558 kg/m
3
. The local atmospheric pressure is 0.97 bar, and the gravitational
acceleration is 9.8 m/s
2
. Determine the pressure at the bottom of the cylinder.
Solution: the pressure at the bottom of the cylinder can be found from the summation of
the forces due to atmospheric pressure, piston weight, and the weight of the liquid in the
cylinder.
gh A
mg P P
W P A W W
bottom atm
bottom atm liquid Piston
bars N m
bar
kg ms
N m
kg m m s m m
kg m s Pbottom bar
2 5 2
2
3 2 2
2
^
Fig. 12: Sketch for example 2.
Temperature
Temperature is a pointer for the direction of energy transfer as heat.
Fig. 13: Heat transfer occurs in the direction of higher‐to‐lower‐temperature.
When the temperatures of two bodies are the same, thermal equilibrium is reached. The
equality of temperature is the only requirement for thermal equilibrium.
The 0th 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.
The 0th law makes a thermometer possible.
A = 0.04 m
2
h = 1.8 m
P =?
mPiston = 60 kg
Patm = 0.97 bar
