Forms of Energy
Objectives
- The various forms of energy.
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The concept of energy is central to the
study of engineering in general and thermodynamics in particular. A
concise definition of energy is the capacity to do work. If a
system has the capacity to do work, it possesses at least one form of
energy that is available for transformation to another form of energy.
For example, a compressed spring possesses a type of energy referred
to as potential energy. As the term implies, potential
energy is a type of stored energy that has the potential for
producing some useful external effect. Consider a mass attached to a
compressed spring, as illustrated in Figure 7.
When the compressed spring is released, the stored energy in the
spring will begin to resume its original undeformed length, imparting
a velocity to the mass. As the spring elongates, the potential energy
in the spring is converted to kinetic energy. Actually, a small
portion of the potential energy in the spring is converted to thermal
energy (heat) because there is friction between the mass and the
surface and within the spring itself. The important thing to realize
is that all the potential energy in the compressed spring is
converted to other forms of energy; i.e., the total energy of the
transformation is constant. No energy is produced or destroyed during
the energy transformation, in accordance with the first law of
thermodynamics.
. The potential energy in a compressed spring is converted
to kinetic energy.
Energy can exist in many forms. For purposes of
thermodynamic analysis, energy is classified into two broad
categories, macroscopic energy and microscopic energy.
Macroscopic forms of energy are those that a whole system possesses
with respect to a fixed external reference. In thermodynamics, the
macroscopic forms of energy are potential energy and kinetic
energy. Potential and kinetic energy are based on external
position and velocity references, respectively. Microscopic forms of
energy are those that relate to the system on a molecular or atomic
level. There are several types of microscopic energies, so we
conveniently group them together into a single category referred to as
internal energy. Internal energy is the sum of all the various
forms of microscopic energies possessed by the molecules and atoms in
the system. Potential, kinetic, and internal energy warrant further
discussion.
1 Potential Energy
In thermodynamics, there are primarily two forms of
potential energy, elastic potential energy and gravitational
potential energy. Elastic potential energy is the energy stored in
a deformable body such as an elastic solid or a spring.
Gravitational potential energy is the energy that a system
possesses by virtue of its elevation with respect to a reference in a
gravitational field. Elastic potential energy is usually of minor
importance in most thermodynamics work, so gravitational potential
energy is emphasized here. Gravitational potential energy, abbreviated
PE, is given by the relation
where m is the mass of the system (kg), g
is gravitational acceleration (m/s2), and z is the
elevation (m) of the center of mass of the system with respect to a
selected reference plane. The location of the reference plane is
arbitrary but is usually selected on the basis of mathematical
convenience. For example, consider a boulder poised on the edge of a
cliff, as illustrated in Figure 8. The center
of mass of the boulder is 20 m above the ground. A reasonable
reference plane is the ground because it is a convenient origin. If
the boulder's mass is 1500 kg, the gravitational potential energy of
the boulder is
What happens to the boulder's potential energy as
it falls from the cliff?
. A boulder elevated above the ground has gravitational
potential energy.
2 Kinetic Energy
Kinetic energy is the energy that a system
possess as a result of its motion with respect to a reference frame.
Kinetic energy, abbreviated KE, is given by the relation
where m is the mass of the system (kg) and v
is the velocity of the system (m/s). When the boulder in Figure
8 is pushed off the cliff, it begins to fall toward the ground. As
the boulder falls, its velocity increases, and its potential energy is
converted to kinetic energy. If the velocity of the 1500-kg boulder is
10 m/s at a point between the cliff and ground, the boulder's kinetic
energy at this point is
Immediately before the boulder impacts the ground,
all the boulder's potential energy has been converted to kinetic
energy. What happens to the boulder's kinetic energy during the impact
with the ground?
3 Internal Energy
Internal energy
is the sum of all the microscopic forms of energy of a system.
Unlike potential energy and kinetic energy, which relate to the energy
of a system with respect to external references, internal energy
relates to the energy within the system itself. Internal
energy, denoted by the symbol U, is a measure of the kinetic
energies associated with the molecules, atoms, and subatomic particles
of the system. Suppose the system under consideration is a polyatomic
gas. (A polyatomic gas is a gas that consists of two or more atoms
that form a molecule, such as carbon dioxide, CO2. A
monatomic gas consists of only one atom, such as helium, He, and
argon, Ar.) Because gas molecules move about with certain velocities,
the molecules possess kinetic energy. The movement of the molecules
through space is called translation, so we refer to their kinetic
energy as translational energy. As the gas molecules translate,
they also rotate about their center of mass. The energy associated
with this rotation is referred to as rotational energy. In
addition to translating and rotating, the atoms of polyatomic gas
molecules oscillate about their center of mass giving rise to vibrational
energy. On a subatomic scale, the electrons of atoms “orbit” the
nucleus. Furthermore, electrons spin about their own axis, and the
nucleus also possesses a spin. The sum of the translational,
rotational, vibrational, and subatomic energies constitutes a fraction
of the internal energy of the system called the sensible
energy. Sensible energy is the energy required to change the
temperature of a system. As an example of sensible energy, suppose
that we wish to boil a pan of water on the stove. The water is
initially at a temperature of about 20°C. The stove burner imparts
energy to the water, increasing the kinetic energy of the water
molecules. The increase in kinetic energy of the water molecules is
manifested as an increase in temperature of the water. As the burner
continues to supply energy to the water, the sensible energy of the
water increases, thereby increasing the temperature, until the boiling
point is reached.
If sensible energy is only a fraction of the
internal energy, what kind of energy constitutes the other fraction?
To answer this question, we must recognize the various forces that
exist between molecules, between atoms, and between subatomic
particles. From basic chemistry, we know that various binding
forces exist between the molecules of a substance. When these
binding forces are broken, the substance changes from one phase
to another. The three phases of matter are solid, liquid, and gas.
Binding forces are strongest in solids, weaker in liquids, and weakest
in gases. If enough energy is supplied to a solid substance, ice for
example, the binding forces are overcome and the substance changes to
the liquid phase. Hence, if enough energy is supplied to ice (solid
water), the ice changes to liquid water. If still more energy is
supplied to the substance, the substance changes to the gas phase. The
amount of energy required to produce a phase change is referred to as latent
energy. In most thermodynamic processes, a phase change involves the
breaking of molecular bonds only. Hence, the atomic binding forces
responsible for maintaining the chemical identity of a substance are
not usually considered. Furthermore, the binding energy associated
with the strong nuclear force, the force that binds the protons and
neutrons in the nucleus, is relevant only in fission reactions.
4 Total Energy
The total energy of a system is the sum of
the potential, kinetic, and internal energies. Thus, the total energy,
E, is expressed as
As a matter of convenience, it is customary in
thermodynamics work to express the energy of a system on a per unit
mass basis. Dividing Equation 6-12 by mass, m, and noting
the definitions of potential and kinetic energies from Eqs. (6-10) and
(6-11), we obtain
where e=E/m and u=U/m.
The quantities e and u are called the specific total
energy and specific internal energy, respectively.
In the analysis of many thermodynamic systems, the
potential and kinetic energies are zero or are sufficiently small that
they can be neglected. For example, a boiler containing high
temperature steam is stationary, so its kinetic energy is zero. The
boiler has potential energy with respect to an external reference
plane (such as the floor on which it rests) but the potential energy
is irrelevant because it has nothing to do with the operation of the
boiler. If the potential and kinetic energies of a system are
neglected, internal energy is the only form of energy present. Hence,
the total energy equals the internal energy, and Equation 6-12 reduces
to E=U.
The analysis of thermodynamic systems involves the
determination of the change of the total energy of the system
because this tells us how energy is converted from one form to
another. It does not matter what the absolute value of the total
energy is because we are interested only in the change of the total
energy. This is paramount to saying that it does not matter what the
energy reference value is because the change of energy is the same
regardless of what reference value we choose. The reference value is
arbitrary. Returning to our falling boulder example, the change of
potential energy of the boulder does not depend on the location of the
reference plane. We could choose the ground as the reference plane or
some other location, such as the top of the cliff or any other
elevation for that matter. The change of potential energy of the
boulder depends only on the elevation change. Hence, if potential and
kinetic energies are neglected, the change of total energy of a system
equals the change of internal energy, and Equation 6-12 is written as E=U.
Professional
Success: Dealing With Engineering Professors
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As a new engineering student, you may
believe that engineering professors are probably not that
much different from professors in other disciplines on
campus. Perhaps you think that they are not even that much
different from people outside higher education who work in
nonteaching occupations. However, after you have taken a few
engineering courses, your opinion will probably change.
Engineering professors are unique. It may even be said that
they are somewhat odd. Some engineering professors are
overly serious, while others may seem rather light-minded.
Some engineering professors dress very neatly, wearing a
suit, tie, polished shoes, etc., while others come to school
looking more like a student, wearing jeans, a sweatshirt,
and sneakers. Regardless of their personalities and personal
appearances, the majority of engineering professors are
genuinely interested in their students and desire to see
them succeed in their engineering studies. Engineering
professors are very knowledgeable people in their
disciplines, and they want to share that knowledge with
students. They were students once, so they understand what
you are going through. Professors are teachers, and
quality instruction is what students expect from them.
However, as a student, you should realize that most
professors are involved in numerous activities outside the
classroom that may or may not relate directly to teaching.
Much of your professor's time is spent developing and
improving the engineering curriculum. Depending on the
availability of graduate teaching assistants, grading may
also occupy a considerable fraction of the professor's time.
Some colleges and universities, particularly the larger
ones, are referred to as research institutions. At
these schools, engineering professors are expected to
conduct research and publish the results of their research.
In addition to publishing research papers, some engineering
professors write textbooks. Because most engineering
professors specialize in a certain aspect of their
discipline, some professors work part-time as consultants to
private or governmental agencies. Most colleges and
universities expect their faculty to render service to the
institution by serving on various campus committees. Some
professors, in addition to their research, writing, and
service activities, serve as department or program advisors
to students. Professors may even be involved with student
recruitment, fund raising, professional engineering
societies, and a host of other activities.
What does all this mean to you, the
engineering student? It means that there are right ways and
wrong ways of dealing with your professors. Here are a few
suggestions:
- Be an active member of your professor's class. Attend
class, arrive on time, take notes, ask questions, and
participate. Being actively engaged in the classroom not
only helps you learn but it also helps the professor
teach!
- If you need to obtain help from your professor outside
of class, schedule an appointment during an office hour
and keep the appointment. Unless your professor
has an “open door” policy, scheduling appointments
during regular office hours is preferred because your
professor is probably involved in research or other
activities.
- Engineering professors appreciate students who give
their best efforts in solving a problem before
asking for help. Before your go to the professor's
office, be prepared to tell your professor how you
approached the problem and where the potential errors
are. Many engineering professors become irritated when
the first thing a student says is, “Look at this
problem, and tell me what I'm doing wrong” or “I
just can't get the answer in the back of the book.”
Preparing to ask the right questions before the visit
will enable your professor to help you more fully.
- Do not call professors at home. If you need assistance
with homework, projects, etc., contact your professor at
school during regular office hours if possible or by
special appointment. Like students, professors try to
have a personal life apart from their day-to-day
academic work. How would you like it if your professors
called you at home to assign additional homework?
- Address professors by their appropriate titles. Do not
call them by their first names. Most engineering
professors have a PhD degree, so it is appropriate to
address these individuals as “Dr. Jones” or
“Professor Jones”. If the professor does not have a
doctorate, the student should address the professor by
“Professor Jones,” “Mr. Jones,” or “Ms.
Jones.”
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