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Dr. Jie Zou PHY 1151G Department of Physics
1
Chapter 17
Phases and Phase Changes
Dr. Jie Zou PHY 1151G Department of Physics
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Outline Ideal gas Origin of Pressure Kinetic energy and temperature Evaporation
Dr. Jie Zou PHY 1151G Department of Physics
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Ideal gases Phases of matter: The matter we come into
contact with every day is in one of three forms: solid, liquid, or gas. In this chapter, we consider these three phases of matter in detail.
Ideal gas: One that has no interactions between its molecules. Ideal gas is a good approximation to real gases.
Equation of state: In general, a relationship between the thermal properties of a substance is referred to as an equation of state.
Dr. Jie Zou PHY 1151G Department of Physics
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Some observations The pressure of an ideal gas varies
linearly with temperature, when the volume and number of molecules in the gas are held constant.
The pressure of an ideal gas increases as the number of molecules increases, when the temperature and volume of the gas in the ball are held constant.
The pressure varies inversely with volume, when the number of molecules and temperature are held constant.
Dr. Jie Zou PHY 1151G Department of Physics
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Equation of state for an ideal gas The pressure of an ideal gas:
k = Boltzmann constant = 1.38 10-23 J/K.
N = the number of molecules in the gas. T = the temperature of the gas. V = the volume of the gas.
Equation of state for an ideal gas: PV = NkT
PkNT
V
Dr. Jie Zou PHY 1151G Department of Physics
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Example: equation of state for an ideal gas
A person’s lungs might hold 6.0 L (1 L = 10-3 m3) of air at body temperature (310 K) and atmospheric pressure (101 k Pa). Given that their air is 21 % oxygen, find the number of oxygen molecules in the lungs.
Dr. Jie Zou PHY 1151G Department of Physics
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The origin of pressure The origin of pressure: the pressure exerted
by a gas is due to the innumerable collisions between gas molecules and the walls of their container.
Pressure in the kinetic theory of gases:
Physical meaning: The pressure of a gas is directly proportional to the average kinetic energy of its molecules. This is the key connection between microscopic behavior and macroscopic observables.
P1
3
N
V
2Kav
2
3
N
V
1
2mv2
av
Dr. Jie Zou PHY 1151G Department of Physics
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Kinetic energy and temperature Kinetic energy and temperature:
This is one of the most important results of kinetic theory: The average kinetic energy of a gas molecule is directly proportional to the Kelvin temperature, T.
1
2mv2
av
Kav 3
2kT
Dr. Jie Zou PHY 1151G Department of Physics
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Evaporation Evaporation: the release of molecules
from the liquid into the gas. It is a change of phase from liquid to gas that takes place at the surface of a liquid.
Basic mechanism of evaporation: Molecules at the surface that gain kinetic energy by being bumped from below may have enough energy to break free of the liquid, leaving the surface and flying into the space above the liquid, becoming molecules of vapor.
Evaporation is a cooling process
Dr. Jie Zou PHY 1151G Department of Physics
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Homework Chapter 17, Page 564, Problems:
#2, 3, 7.
