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## The kinetic theory of gases and the gas laws

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### The kinetic theory of gases and the gas laws * * * Kinetic theory/ideal gas We can understand the behaviour of gases using a very simple model, that of an ideal ... – PowerPoint PPT presentation

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Title: The kinetic theory of gases and the gas laws

1
The kinetic theory of gases and the gas laws
2
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

3
Ideal gas assumptions
• The particles of gas (atoms or molecules) obey
Newtons laws of motion.

You should know these by now!
4
Ideal gas assumptions
• The particles in a gas move with a range of speeds

5
Ideal gas assumptions
• The volume of the individual gas particles is
very small compared to the volume of the gas

6
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)

7
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?
8
Ideal gas assumptions
• The duration of a collision is small compared to
the time between collisions.

9
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)

10
Pressure A reminder
• What is origin of the pressure of a gas?

11
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
12
Explaining the behaviour of gaseshttp//phet.colo
• When we heat a gas at constant volume, the
pressure increases. Why?

13
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.

14
Explaing the behaviour of gases
• When we heat a gas a constant pressure, the
volume increases. Why?

15
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.

16
Explaing the behaviour of gases
• When we compress (reduce the volume) a gas at
constant temperature, the pressure increases. Why?

17
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.

18
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)

19
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!