The kinetic theory of gases and the gas laws

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The kinetic theory of gases and the gas laws
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Kinetic theory/ideal gas

• We can understand the behaviour of gases using a
  very simple model, that of an ideal gas.
• The model makes a few simple assumptions

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Ideal gas assumptions

• The particles of gas (atoms or molecules) obey
  Newtons laws of motion.

You should know these by now!
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Ideal gas assumptions

• The particles in a gas move with a range of speeds

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Ideal gas assumptions

• The volume of the individual gas particles is
  very small compared to the volume of the gas

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Ideal gas assumptions

• The collisions between the particles and the
  walls of the container and between the particles
  themselves are elastic (no kinetic energy lost)

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Ideal gas assumptions

• There are no forces between the particles (except
  when colliding). This means that the particles
  only have kinetic energy (no potential)

Do you remember what internal energy is?
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Ideal gas assumptions

• The duration of a collision is small compared to
  the time between collisions.

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Pressure A reminder

• Pressure is defined as the normal (perpendiculr)
  force per unit area
• P F/A
• It is measured in Pascals, Pa (N.m-2)

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Pressure A reminder

• What is origin of the pressure of a gas?

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Pressure A reminder

• Collisions of the gas particles with the side of
  a container give rise to a force, which averaged
  of billions of collisions per second
  macroscopically is measured as the pressure of
  the gas

Change of momentum
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Explaining the behaviour of gaseshttp//phet.colo
rado.edu/sims/ideal-gas/gas-properties.jnlp

• When we heat a gas at constant volume, the
  pressure increases. Why?

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Explaining the behaviour of gases

• When we heat a gas at constant volume, the
  pressure increases. Why?
• Increased average kinetic energy of the
  particles means there are more collisions with
  the container walls in a period of time and the
  collisions involve a greater change in momentum.

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Explaing the behaviour of gases

• When we heat a gas a constant pressure, the
  volume increases. Why?

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Explaing the behaviour of gases

• When we heat a gas a constant pressure, the
  volume increases. Why?
• Increasing the volume reduces the chance of
  particles colliding with the container walls,
  opposing the effect of the particles increased
  kinetic energy.

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Explaing the behaviour of gases

• When we compress (reduce the volume) a gas at
  constant temperature, the pressure increases. Why?

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Explaing the behaviour of gases

• When we compress (reduce the volume) a gas at
  constant temperature, the pressure increases.
  Why?
• A smaller volume increases the likelihood of a
  particle colliding with the container walls.

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Explaing the behaviour of gases

• In this way we are explaining the macroscopic
  behaviour of a gas (the quantities that can be
  measured like temperature, pressure and volume)
  by looking at its microscopic behaviour (how the
  individual particles move)

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The Gas Laws

• There are mathematical relationships between
  pressure, volume and temperature of a gas (at a
  fixed mass).
• We will study these next year at higher level!