IGCSE Physics
CIE Syllabus
Thermal Physics – Kinetic Theory
Objective: State the distinguishing properties of solids, liquids and gases
In the boxes below, draw diagrams of how molecules would look like in (a) a solid (b) liquid
and (c) gas
Solid
Liquid
Gas
2. List as many properties as you can remember about solids, liquids and gases
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Solids
Liquids
Gases
Objective: Show an understanding of the random motion of particles in a suspension as evidence for the
kinetic molecular model of matter
Describe this motion (sometimes known as Brownian motion) in terms of random molecular bombardment
A Scottish scientist called Robert Brown first showed the motion of pollen particles in water
using a microscope.
Describe what he observed. Draw a diagram of the path of one of the pollen particles when
it moves.
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Space for diagram
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Modern day ‘Brownian Motion’ is seen in a smoke cell. It is a
small glass box with one side open. It is filled with smoke
from the bottom end of a burning straw, and closed quickly
with a thin glass lid such as a microscope slide. The box is
illuminated from one side, and viewed at a perpendicular
direction with a microscope at low power such as ‘x 10 ‘ . It is
focussed to the middle of the box. Smoke particles,
suspended in air, are much more massive than air particles
and are seen as bright specks of light in the microscope’s
field of view. They move haphazardly, without any order in
zig-zag paths of variable lengths. (Imagine following the front
foot of a drunkard walking!) The diagram below illustrates
this.
Explain, using a diagram why the smoke particles move the way they do. Use the
words ’invisible’, ‘air particles’ ‘bombard’, ‘momentum’, ‘resultant force’, ‘speed’
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Space for diagram
Typical path of one smoke
particle in smoke cell
Objective
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Interpret the temperature of a gas in terms of the motion of its molecules
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Describe qualitatively the pressure of a gas in terms of the motion of its molecules
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Describe qualitatively the effect of a change of temperature on the pressure of a gas at
constant volume
Brownian motion established the fact that
1. A gas is made up of very small particles, which were given the name ‘molecules’.
2. The molecules of a gas move endlessly and randomly, in all directions, with all speeds.
3. The molecules collide elastically, amongst themselves and with the walls of the container.
1, 2, and 3 above form part of a particular theory in Physics. What is the name of this theory? Choose one from below:
A. Brown’s Theory
B. Theory of nature
C. Kinetic theory
D. Gas theory
The molecules of a gas are always in motion, so they have kinetic energy. If there are N molecules (N is of the order
of 1023 (Avagadro’s number = 6 x 1023 = the no.of particles in one mole) or more. We can define average k.e. per
molecule as
K.E. ave = < Ek > = (Sum of k.e. of all molecules) ÷ N
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Now imagine heating a gas in a container. The heat energy goes to the particles, which are already
moving. The temperature of the gas increases.
What change will take place to the molecules’ motion?
A. Move faster
B. Move slower
C. Move in circles
D. No change
What change will take place to the average kinetic energy per molecule of the gas?
A. Decreases
decreases
B. Increases
C. Does not change
D. Increases, then
If we want to relate temperature of a gas (or in fact matter in any state) to molecular properties, which
quantity would you recommend?
A. Mass
B. Size
C. Colour
D. < Ek >
In fact the absolute temperature of a gas is defined as a quantity proportional to the average molecular kinetic
energy!
When we study pressure due to solids or liquids we define pressure as force per unit area (p = F/A). Remember,
solids transmit force instantly. Liquids transmit pressure instantly. Here we have clearly recognizable force and
area. In a gas, this is less obvious. A gas cannot transmit pressure instantly.
Imagine pushing the handle of this pump down with the air hole at the end closed with your
thumb. You can feel the pressure from the air. The air is pushing with a force.
Like your thumb, the walls of the pump also experience force. Let us represent this pump
with a simple box. Remember, there are like 1025 molecules all moving chaotically
everywhere. Some are shown here. Average mass of an air particle is 6 x 10-26 kg, and
typical speed of an air particle is 500m/s!
1. Molecule X is travelling as shown. It has a
momentum. Wall AB will ‘feel’ a force when X hits it.
Why?
A
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X
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m
Putting together all these tiny forces due the huge
number of molecules , what would you expect for the
overall effect?
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B
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The pressure of a gas in a container is due to the combined effect (force) due to the bombardment of
a large number of molecules on the walls. The more frequent these bombardments are, the greater
the pressure. We can say: Pressure is proportional to collision frequency.
Now imagine heating a gas in a closed container which itself does not expand due to
heating.
What change will happen to the force exerted by the molecules?
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How will the pressure change?
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Now imagine a gas in a cylinder with a piston as shown below:
Pressure – Volume Relationship
Suppose the temperature T is always kept the same, i.e. the gas is heated if
temperature drops below the set value or cooled if it gets warmer. The piston is
pushed to the right and held , making less space for the molecules to move
about. Remember, the average k.e. per molecule is still the same, as T is
constant.
Weight
What do you expect to happen to
(a) the speed of the molecules?
A. Same as before
B. Greater
C. Less
(b) the number of collisions per second with the walls?
A. Same as before
B. Greater
C. Less
(c ) the pressure on the walls of the cylinder
A. Same as before
B. Greater
C. Less
The diagram on the left helps illustrate simply the consequences of changing
one of
(a) temperature, T
(b) pressure, p
(c) number N of molecules in a cylinder of gas, when all others are kept the
same.
Suppose the weight of the piston is so small compared to the weight in the tray,
so the weight represents pressure.
Length l represents volume.
Suppose the piston is in equilibrium to start with. Some changes are made as
seen below. Choose one of A. Up or B. Down as your answer to the
following questions:
Piston
How will the piston move when
(a) the cylinder is heated to a higher temperature? …………………………………………
At constant pressure, volume is directly proportional to the absolute
temperature.
Gas
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(b) More weights are added, so that the pressure is increased? ……………..…………
At constant temperature, volume is inversely proportional to the
pressure.
(c) More gas is allowed in at the same temperature and pressure? …………………
Heat
At constant temperature and pressure, volume is directly proportional to
the number of molecules.
In the following, the piston does not move.
(d) The gas is heated, but is not allowed to expand. What change has to be
made to the weights to keep the volume the same?
A. Increased
B. Decreased
At constant volume , pressure is directly proportional to the absolute
temperature.
The four statements in (a), (b), (c) and (d) can be combined into one compact
equation pV = nRT which is called the ideal (or perfect) gas equation. n is
the number of moles of gas and R is the universal gas constant.
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