Chapter_2_lecture.pdf

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Thermodynamics: An Engineering Approach, 6 th Edition
Yunus A. Cengel, Michael A. Boles
McGraw-Hill, 2008
Chapter 2
ENERGY, ENERGY
TRANSFER, AND GENERAL
ENERGY ANALYSIS
Created by Mehmet Kanoglu
Modified by Kai Ming Kiang
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Objectives
• 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: 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.
• Determine that a fluid flowing across a control surface of a control
volume carries energy across the control surface in addition to any
energy transfer across the control surface that may be in the form of
heat and/or work.
• Define energy conversion efficiencies.
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INTRODUCTION
•
If we take the entire room—including the air and the refrigerator (or
fan)—as the system, which is an adiabatic closed system since the
room is well-sealed and well-insulated, the only energy interaction
involved is the electrical energy crossing the system boundary and
entering the room.
•
As a result of the conversion of electric energy consumed by the
device to heat, the room temperature will rise .
A fan running in a
well-sealed and
well-insulated room
will raise the
temperature of air in
the room.
A refrigerator
operating with its
door open in a well-
sealed and well-
insulated room
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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.
•
Thermodynamics deals only with the change of the total energy.
•
Macroscopic forms of energy : Those a system possesses as a whole
with respect to some outside reference frame, such as kinetic and
potential energies.
•
Microscopic forms of energy : Those related to the molecular
structure of a system and the degree of the molecular activity.
•
Internal energy, U : The sum of all the microscopic forms of energy.
•
Kinetic energy, KE : The energy
that a system possesses as a result
of its motion relative to some
reference frame.
•
Potential energy, PE: The energy
that a system possesses as a result
of its elevation in a gravitational
field.
The macroscopic energy of an
object changes with velocity and
elevation.
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Kinetic energy
Kinetic energy
per unit mass
Mass flow rate
Potential energy
Potential energy
per unit mass
Energy flow rate
Total energy
of a system
Energy of a system
per unit mass
Total energy
per unit mass
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