2. ENERGY, ENERGY TRANSFER, AND
GENERAL ENERGY ANALYSIS
Objectives of Ch. 2
Introduce the concept of energy and define its
various
forms.
Discuss the nature of internal energy.
Define the concept of heat and the terminology
associated with energy transfer by heat.
Discuss the three mechanisms of heat transfer:
Jan 14, 2016 2
3. Jan 14, 2016 3
Conduction ,convection , and radiation.
Define the concept of work, including electrical work and
several forms of mechanical work.
Introduce the first law of thermodynamics, energy
balances, and mechanisms of energy transfer to or from a
system.
5. FORMS OF ENERGY
Energy can exist in numerous forms such as thermal,
mechanical, kinetic, potential, electric, magnetic,
chemical, and nuclear, and their sum constitutes the
total energy E of a system. The total energy of a
system on a unit mass basis is denoted by e and is
expressed as
Jan 14, 2016 5
6. Thus the total energy of a system can be assigned a
value of zero (E=0) at some convenient reference
point. The change in total energy of a system is
independent of the reference point selected.
Jan 14, 2016 6
7. Types of Energy
The macroscopic forms of energy are those a system
possesses as a whole with respect to some outside
reference frame, such as kinetic and potential
energies.
The microscopic forms of energy are those related to
the molecular structure of a system and the degree of
the molecular activity, and they are independent of
outside reference frames.
Jan 14, 2016 7
8. Internal Energy
The sum of all the microscopic forms of energy is
called the internal energy of a system and is denoted
by U.
The macroscopic energy of a system is related to
motion and the influence of some external effects
such as gravity, magnetism, electricity, and surface
tension.
Jan 14, 2016 8
9. Kinetic and Potential Energy
Jan 14, 2016 9
The energy that a system possesses as a result of its
motion is called kinetic energy (KE). When all parts of
a system move with the same velocity, the kinetic
energy is expressed as
10. The energy that a system possesses as a result of its
elevation in a gravitational field is called potential
energy (PE)
Control volumes typically involve fluid flow for long
periods of time, and it is convenient to express the
energy flow associated with a fluid stream in the rate
form. This is done by incorporating the mass flow rate
m., which is the amount of mass flowing through
a cross section per unit time. It is related to the
volume flow rate V,
Jan 14, 2016 10
11. Internal Energy
Internal energy is defined earlier as the sum of all the
microscopic forms of energy of a system. It is related
to the molecular structure and the degree of
molecular activity and can be viewed as the sum of
the kinetic and potential energies of the molecules.
Jan 14, 2016 11
12. The molecules of a gas move through space with
some velocity, and thus possess some kinetic energy.
This is known as the translational energy. The
atoms of polyatomic molecules rotate about an axis,
and the energy associated with this rotation is the
rotational kinetic energy. The atoms of a
polyatomic molecule may also vibrate about their
common center of mass, and the energy associated
with this back-and-forth motion is the vibrational
kinetic energy.
Jan 14, 2016 12
13. 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 is also associated with various
binding forces between the molecules of a substance,
between the atoms within a molecule, and between
the particles within an atom and its nucleus.
Jan 14, 2016 13
14. If sufficient energy is added to the molecules of a
solid or liquid, the molecules overcome these
molecular forces and break away, turning the
substance into a gas. This is a phase-change
process.
The internal energy associated with the phase of a
system is called the latent energy.
Jan 14, 2016 14
15. The internal energy associated with the atomic bonds
in a molecule is called chemical energy.
The tremendous amount of energy associated with
the strong bonds within the nucleus of the atom itself
is called nuclear energy.
Jan 14, 2016 15
FIGURE 1–25
Nitrogen
T = 25°C
v = 0.9 m3/kg
fix the state of a simple compressible system (Fig. 1–25). Temperature and pressure, however, are independent properties for single-phase systems, but are dependent properties for multiphase systems. Higher altitudes Example
