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Chapter 2

ENGG 3260 Chapter Notes - Chapter 2: Vehicle-To-Vehicle, Control Volume, Inq Mobile


Department
Engineering
Course Code
ENGG 3260
Professor
Linda Gerber
Chapter
2

Page:
of 9
1
Summary of Lecture
Chapter 2
Energy, Energy Transfer, and General Energy Analysis
2
Unit and Symbol of Energy:
Energy, E = unit of force × unit of distance = N × m = Joul or J
Specific form:
== kg
mN
or
kg
J
;
m
E
mass
Energy
e
Rate form:
== or Wor watt
s
J
;
t
E
time
Energy
E
&
Forms of Energy
: (a)
Macroscopic
: The macroscopic forms of energy are those a system possesses as a
whole with respect to some outside reference frame. For example,
system’s
kinetic energy
and
potential energy
.
(b)
Microscopic
: 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. For example,
molecular translational, rotational, vibrational kinetic energies.
Internal Energy
: The sum of all the microscopic forms of energy is called the internal energy (U) of a
system.
Total Energy
: Sum of all Macroscopic and Microscopic forms of energy. Total energy is denoted by E.
[ ]
+++==
+++=
+
+
+
=
kg
kJ
or
kg
J
;energy specific of formsother zgV
2
1
u
m
E
Energy, Specific
kJor J ; etc.) chemical, (magnetic,energy of formsother zgmVm
2
1
UE Energy, Total
etc.)
chemical,
(magnetic,
energy
of
forms
other
KE
U
E
Energy,
Total
2
2
e
Energy Transfer Processes: Energy transfer processes from a system to its surrounding or from
surrounding to a system or between two systems, Examples,
1. Energy transfer by Heat (Closed or Open system)
2. Energy transfer by Work (Closed or Open system)
3. Energy transfer by Mass (Open system only)
Heat: Heat is defined as the form of energy that is transferred between two systems (or a system and its
surroundings) by virtue of a temperature difference. Heat is easy to recognize: Its driving force is a
temperature difference between the system and its surroundings or between two systems.
Symbol used for Heat = Q (J or kJ) and Specific Heat = q = Q/m (J/kg or kJ/kg)
Symbol used for Heat per unit time or Heat transfer rate = Q
&
=Q/t (J/sec or watt)
Heat transfer from the system to
the surrounding
Heat transfer from the
surrounding to the system
Thermal energy vs. heat
3
Work: Energy can cross the boundary of system in the form of heat or work. Therefore, if the energy
crossing the boundary of a system is not heat, it must be work. Then we can simply say that an
energy interaction that is not caused by a temperature difference between a system and its
surroundings is work. Example, a rising piston, a rotating shaft, and an electric wire crossing the
system boundaries are all associated with work interactions.
Electric
Work
Shaft
Work
Piston or moving
boundary Work
Symbol used for Work = W (J or kJ) and Specific Work = w = W/m (J/kg or kJ/kg)
Symbol used for Work per unit time or Power = W
&=W/t (J/sec or watt)
Old Sign Convention (positive or negative) for Heat and Work:
Modern Sing Convention:
Energy goes into the system (Positive or Negative) and Energy leaving the system (Negative or Positive)
Important Notes on Heat and Work:
Heat and work are not system’s properties like internal energy, specific volume, enthalpy, etc. Heat and
work are energy transfer mechanisms between a system and its surroundings, and there are many
similarities between them:
1. Both are recognized at the boundaries of a system as they cross the boundaries. That is, both heat and
(Positive)
(Positive)
(Negative)
(Negative)
Example:
W=30 kJ means 30 kJ work done by the
system
W=-20 kJ means 20 kJ work done on
the system
Q=10 kJ means 10 kJ heat transferred to
The system
Q=-15 kJ means 15 kJ rejected by the
system