Phases and Phase Changes

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Chapter 17

• Phases and Phase Changes

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Outline

• Ideal gas
• Origin of Pressure
• Kinetic energy and temperature
• Evaporation

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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.

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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.

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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

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Example equation of state for an ideal gas

• A persons 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.

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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.

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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.

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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

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Homework

• See online assignment on www.masteringphysics.com