Download Adiabatic process

Chapter 20
Heat and the First Law of
Thermodynamics (cont.)
Dr. Jie Zou
PHY 1361
1
Outline
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The first law of thermodynamics (20.5)
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Some special cases: an isolated system and a cyclic process
Definitions for some idealized thermodynamic
processes (20.6)
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Adiabatic process
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Adiabatic free expansion
Isobaric process
Isovolumetric process
Isothermal process
Example: Isothermal expansion of an ideal gas (20.6)
Energy transfer mechanisms (conceptual) (20.7)
Dr. Jie Zou
PHY 1361
2
The first law of thermodynamics
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The first law of thermodynamics: Eint = Q + W
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Eint: Change in the internal energy of the system
Q: Energy transferred by heat to the system
W: Work done on the system
The first law of thermodynamics is a special case of the law of
conservation of energy that relates the change in internal
energy of a system to the net transfer of energy by heat and
work.
Internal energy is a state variable (like P, V, and T): its value is
determined by the state of the system, independent of the path.
Some special cases:
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Isolated system: one that does not interact with its
surroundings. Since Q = W = 0, Eint = 0 and Eint, i = Eint, f.
Cyclic process: a process that starts and ends at the same
state. Since Eint = 0, Q = -W.
Dr. Jie Zou
PHY 1361
3
Definitions for some idealized
thermodynamic processes
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Reading assignment: P. 619 – 623
Adiabatic process: one during
which no energy enters or leaves
the system by heat, i.e. Q = 0.
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Adiabatic free expansion: both Q
= 0 and W = 0.
Quick Quiz 20.5
Characterize the paths
below as isobaric,
isovolumetric, isothermal,
or adiabatic. Note that Q
= 0 for path B.
Isobaric process: one that
occurs at constant pressure.
Isovolumetric process: one that
takes place at constant volume.
Isothermal process: one that
occurs at constant temperature.
Dr. Jie Zou
PHY 1361
4
Example 20.6 An isothermal
expansion
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A 1.0-mol sample of an ideal gas is kept at
0.0°C during an expansion from 3.0 L to
10.0 L.
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(A) How much work is done on the gas during
the expansion?
(B) How much energy transfer by heat occurs
with the surroundings in this process?
(C) If the gas is returned to the original volume
by means of an isobaric process, how much
work is done on the gas?
What can we learn from this example?
 Vi
(1) Isothermal expansion of an ideal gas (proof): W  nRT ln 
V
 f
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(2) Isothermal process involving an ideal gas: Eint = 0 since the internal
energy of an ideal gas is a function of temperature only.
Dr. Jie Zou
PHY 1361
5
Energy transfer mechanisms
(conceptual)
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Reading assignment: P. 623 – 629
Thermal conduction (on an atomic scale):
Exchange of kinetic energy between microscopic
particles, in which less-energetic particles gain
energy in collisions with more energetic particles.
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Dr. Jie Zou
The law of thermal conduction: P = kA|dT/dx|; P:
the rate of energy transfer by heat; k: thermal
conductivity of the material; |dT/dx|: temperature
gradient; A: cross-sectional area.
Convection (a form of matter transfer): Energy
transferred by the movement of a warm
substance is said to have been transferred by
convection.
Electromagnetic radiation: All objects radiate
energy continuously in the form of
electromagnetic waves (see Ch. 34). When an
object is hotter than its surroundings, it radiates
more energy than it absorbs, and its temperature
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decreases. PHY 1361
Homework
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Ch. 20, P. 634, Problems: #28, 30, 32,
34, 39.
Dr. Jie Zou
PHY 1361
7