Thermodynamics-I (ME-121)
Teacher :Engr. Muhammad Sumair
B.Sc. Mechanical Engineering (UET Lahore 2014-2018)
M.Sc. Thermal Power Engineering (UET Lahore 2018-2020)
Introduction
• Thermodynamics can be defined as the science of energy. The name
thermodynamics is derived from the Greek words therme (heat) and dynamis
(power), which is most descriptive of the early efforts to convert heat into power.
Today the same name is used for all aspects of energy and energy changes,
including power generation, refrigeration, and relationships among the properties
of matter.
• Applications of Thermodynamics: Thermodynamics is commonly encountered
in many engineering systems and other aspects of life. The heart is constantly
pumping blood to all parts of the human body, various energy conversions occur
in trillions of body cells, and the body heat generated is constantly rejected to the
environment. Many ordinary household utensils and appliances are designed, in
whole or in part, by using the principles of thermodynamics. Some examples
include the electric or gas range, the heating and air-conditioning systems, the
refrigerator, the humidifier, the pressure cooker, the water heater, the shower, the
iron, and even the computer and the TV.
Introduction (Cont’d)
Introduction (Cont’d)
• It is well-known that a substance consists of a large number of particles called
molecules. The properties of the substance naturally depend on the behavior of
these particles. For example, the pressure of a gas in a container is the result of
collision between the molecules and the walls of the container. However, we do
not need to know the behavior of the gas particles to determine the pressure in the
container. We only need to attach a pressure gage to the container. This
macroscopic approach to the study of thermodynamics that does not require a
knowledge of the behavior of individual particles is called classical
thermodynamics.
• On the other hand, a microscopic approach which is more detailed approach and
is based on the behavior of large groups of individual particles, is called statistical
thermodynamics.
Thermodynamic Systems
• A system is defined as a quantity of matter or a region in
space which is chosen for study. The mass or region outside
the system is called the surroundings. The real or imaginary
surface that separates the system from its surroundings is
called the boundary. The boundary of a system can be fixed
or movable.
• Systems may be considered to be closed, open or isolated. A
closed system (also known as a control mass or fixed system)
consists of a fixed amount of mass, and no mass can cross its
boundary. That is, no mass can enter or leave a closed system.
But energy, in the form of heat or work, can cross the
boundary. If, as a special case, even energy is not allowed to
cross the boundary, that system is called an isolated system.
Thermodynamic Systems (Cont’d)
• An open system, or a control
volume, as it is often called, is a
properly selected region in
space. Both mass and energy
can cross the boundary of a
control volume.
• It usually encloses a device that
involves mass flow such as a
compressor, turbine, or nozzle.
Properties of a System
• The matter contained within the boundaries of the system can be liquid,
vapour, or gas, and is known as the working fluid. At any instant, the state
of the working fluid may be defined by certain characteristics called its
properties. Some common properties in thermodynamics are pressure P,
temperature T, volume V, and mass m.
• 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 e.g., total mass, total
volume, and total energy.
• Generally, uppercase letters are used to denote extensive properties (with
mass m being a major exception), and lowercase letters are used for
intensive properties (with pressure P and temperature T being the obvious
exceptions).
Properties of a System (Cont’d)
• Specific Properties: Extensive properties per unit mass are called specific
properties. Some examples of specific properties are specific volume (v =
V/m) and specific total energy (e =E/m).
• As we know that density is defined as mass per unit volume, and specific
volume has been defined as volume per unit mass, so they are reciprocal to
each other and thus their product is unity.
State of a System and Equilibrium
• State of a system is the condition of the system which is
defined with the help of certain properties of the system. At
a given state, all the properties of a system have fixed
values. If the value of even one property changes, the state
will change to a different one. Consider a system not
undergoing any change. The word equilibrium means a
state of balance. In an equilibrium state there are no forces
within the system. A system in equilibrium experiences no
changes.
State of a System and Equilibrium (Cont’d)
• Types of equilibrium: There are many types of equilibrium, for example, a
system is in thermal equilibrium if the temperature is the same throughout the
entire system.
• Mechanical equilibrium is related to pressure, and a system is in mechanical
equilibrium if there is no change in pressure at any point of the system with time .
• If a system involves two phases, it is in phase equilibrium when the mass of each
phase reaches an equilibrium level and stays there. Finally, a system is in chemical
equilibrium if its chemical composition does not change with time, that is, no
chemical reactions occur.
The State Postulate
• As noted earlier, the state of a system is described by its properties. But we know
from experience that we do not need to specify all the properties in order to fix a
state. The number of properties required to fix the state of a system is given by the
state postulate: “The state of a simple compressible system is completely
specified by two independent, intensive properties”.
• A system is called a simple compressible system in the absence of electrical,
magnetic, gravitational, motion, and surface tension effects. Two properties are
independent if one property can be varied while the other one is held constant.
• Since any two independent properties are sufficient to define the state of a system,
it is possible to represent the state of a system by a point situated on a diagram of
properties. For example, a cylinder containing a certain fluid at pressure P1 and
specific volume v1, is at state 1, defined by point 1 on a diagram of P against v.
The State Postulate (Cont’d)
• At any other instant, the piston may be moved in the cylinder such that the
pressure and specific volume are changed to P2 and v2 . State 2 can then be marked
on the diagrams accordingly.
Thermodynamic Processes
• Any change that 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 the path of the process. To describe a
process completely, one should specify the initial and
final states of the process, as well as the path it follows,
and the interactions with the surroundings.
• The prefix iso- is often used to designate a process for
which a particular property remains constant. An
isothermal process, for example, is a process during
which the temperature T remains constant; an isobaric
process is a process during which the pressure P
remains constant; and an isochoric (or isometric)
process is a process during which the specific volume v
remains constant.
Thermodynamic Cycles
• A system is said to have undergone a cycle if it returns to its initial state at
the end of the process. That is, for a cycle the initial and final states are
identical (same).
Temperature and Zeroth Law of Thermodynamics
• It is a common experience that a cup of hot coffee left
on the table eventually cools off and a cold drink
eventually warms up. 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 attain the 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”.
Temperature and Zeroth Law of Thermodynamics (Cont’d)
• The zeroth law serves as a basis for the validity of temperature
measurement. 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.
Forms of Energy
• Energy can exist in numerous forms such as thermal, mechanical( kinetic and
potential), electric, magnetic, chemical, and nuclear etc., and their sum makes the
total energy E of a system (is it extensive or intensive property?).
• The total energy of a system on a unit mass basis is denoted by “e” and is
expressed as
• In thermodynamic analysis, it is often helpful to consider the various forms of
energy that make up the total energy of a system in two groups: macroscopic and
microscopic.
• The macroscopic energy is that form of energy which a system possesses as a
whole, such as kinetic and potential energies (see figure).The microscopic energy
is that form of energy which is related to the molecular structure of a system and
the degree of the molecular activity. The sum of all the microscopic forms of
energy is called the internal energy of a system and is denoted by U.
Forms of Energy (Cont’d)
• The energy that a system possesses as a result of its motion is called
kinetic energy (KE).
or, on a unit mass basis
• The energy that a system possesses as a result of its height in a
gravitational field is called potential energy (PE) and is expressed as
or, on a unit mass basis
• The total energy of a system consists of the kinetic, potential, and internal energies
and is expressed as
or, on a unit mass basis,
• Most closed systems remain stationary during a process and thus experience no
change in their kinetic and potential energies. Closed systems whose velocity and
elevation of the center of gravity remain constant during a process are called
stationary systems. The change in the total energy ∆E of a stationary system is
same as the change in its internal energy ∆ U.
∆𝐸 = ∆𝑈 𝑜𝑟 ∆𝑒 = ∆𝑢
Thermal Energy versus Mechanical Energy
• As defined earlier, internal energy as the sum of all the microscopic forms of
energy of a system. It is related to the molecular structure and can also be
defined as the sum of the kinetic and potential energies of the molecules.
• The portion of the internal energy of a system associated with the kinetic
energies of the molecules is called the sensible energy. The internal energy
associated with the phase of a system is called the latent energy.
• Note that sensible (due to K.E. of molecules) and latent (due to phase change)
forms of internal energy are called Thermal Energy in thermodynamics.
• Difference should be made between the overall macroscopic kinetic energy of an
object and the microscopic kinetic energies of its molecules. The macroscopic kinetic
energy of an object is an organized form of energy. On the opposite side, the kinetic
energies of the molecules (microscopic kinetic energy) are completely random and
highly disorganized.
Thermal Energy versus Mechanical Energy (Cont’d)
• The mechanical energy can be defined as the form of energy that can be
converted to mechanical work completely by an ideal mechanical device
such as an ideal turbine. Kinetic energy, potential energy and work are the
forms of mechanical energy. Thermal energy is not mechanical energy,
because it cannot be converted to work completely (the second law of
thermodynamics).
• The organized energy (macroscopic energy/mechanical energy) is much
more valuable than the disorganized energy, and a major application area of
thermodynamics is the conversion of disorganized energy (heat) into
organized energy (work).
Mechanisms of Energy Transfer
• Energy can be transferred to or from a system in three
forms: heat, work, and mass flow.
1. Energy Transfer by Heat (Q): Heat is defined as
the form of energy that is transferred between two
systems (or a system and its surroundings) because of
temperature difference. There cannot be any heat
transfer between two systems which are at the same
temperature. Heat is energy in transition. It exists
only as it crosses the boundary of a system.
• Consider the hot baked potato. The potato contains
energy, but this is energy not heat. We can say that
system has energy but cannot say that system has heat.
Heat only appears as it passes through the skin of the
potato (the system boundary) to reach the air. Once in
the surroundings, the transferred heat becomes part of
the internal energy of the surroundings.
Mechanisms of Energy Transfer (Cont’d)
2. Energy Transfer by Work (W): Work, like heat, is an energy interaction between a
system and its surroundings. As mentioned earlier, energy can cross the boundary of a
closed system in the form of heat or work. Therefore, if the energy crossing the
boundary of a closed system is not heat, it must be work.
• If energy transfer is not caused by a temperature difference between a system and its
surroundings, it must be through work. More specifically, work is the energy transfer
2
associated with a force acting through a distance. 𝑊 = − ‫׬‬1 𝑃𝑑𝑣
• Work is also a form of energy transferred like heat and, therefore, has energy units
such as J. The work done during a process between states 1 and 2 is denoted by W12,
or simply W. The work done per unit mass of a system is denoted by w. The work
ሶ The SI unit of power is J/s, or
done per unit time is called power and is denoted 𝑾.
W. Like heat, work is also energy in transition. Therefore, heat and work are not
the properties of the system.
Mechanisms of Energy Transfer (Cont’d)
3. Energy Transfer by Mass (for open systems
only): When mass enters a system, the energy of
the system increases because mass carries energy
with it (in fact, mass is energy). Likewise, when
some mass leaves the system, the energy
contained within the system decreases because
the leaving mass takes out some energy with it.
For example, when some hot water is taken out
of a water heater and is replaced by the same
amount of cold water, the energy content of the
hot-water tank (the control volume) decreases as
a result of this mass interaction.
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