the kinetic theory of gases and the gas laws. kinetic theory/ideal gas we can understand the...
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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 Newton’s laws of motion.
Please tell me you remember!
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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)
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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 (perpendicular) 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 over billions of collisions per second macroscopically is measured as the pressure of the gas
Change of momentum
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Explaining the behavior of gaseshttp://phet.colorado.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 behavior 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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Explaining the behavior of gases
When we heat a gas a constant pressure, the volume increases. Why?
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Explaining the behavior of gases
When we heat a gas at 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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Explaining the behavior of gases
When we compress (reduce the volume) a gas at constant temperature, the pressure increases. Why?
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Explaining the behavior 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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Explaining the behavior 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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Temperature and KE
KEave = (3/2) kBT
KEave – Average Kinetic Energy
kB – Boltzmann constant 1.38 x 10-23 J/K
T – Temperatue (K)