Lecture

1. Lecture 2 - Kinetic theory and gases

2. We Are Learning To
Use a simple kinetic theory model to describe movement
of particles in the three states of matter
Explain the pressure of a gas in terms of the motion of its
particles
Describe the effect of changing the temperature of a gas
on the speed of its particles

3. States of matter –
a particle view
solid liquid gas
Close packed in a
regular pattern,
occupy least space
Particles (molecules)
slightly further apart
than in solids
Very far apart and
occupy any available
space
Vibrate about fixed
positions
Particles slide past one
another
Move randomly at
high speed
Held by very strong
attractive forces
Attractive forces
between particles
(inter-molecular forces)
Weak forces of
attraction between
particles

4. Moving particles
• If a gas is cooled down its molecules have less energy and
move more slowly. If it is heated up they have more energy
and move faster.
• This idea about moving molecules is called the kinetic theory.
• Further cooling causes condensation into a liquid and eventually
freezing into a solid.
• Heating a solid causes it to melt and a liquid to evaporate.
Solid
Liquid
Gas
gas condenses
cooling
liquid freezes
cooling
heating
liquid boils solid melts
heating

5. Pressure = Force
Area
Pressure

6. Temperature:
a measure of how hot or cold the gas is
Volume: how much space a gas occupies
In this topic we will be investigating how the volume,
temperature and pressure of a gas are related to
each other.
Pressure:
The force exerted per unit area. The pressure of a gas is caused by
its particles hitting the walls of its container. The more frequent the
collisions and the harder they hit the greater the pressure.

7. Boyle’s law
For a fixed mass of gas, the pressure is inversely proportional to
the volume if the temperature remains constant.
V1P1 = V2 P2

8. We Are Learning To
Unit P3: Applications of physics
Topic 5
Kinetic theory and gases
5.7 Investigate the temperature and volume relationship for a gas
5.4 Describe the term absolute zero, -273°C, in terms of the lack
of movement of particles

9. Investigating the temperature
and volume relationship of a gas
heat

10. Absolute zero
The temperature -2730C is called absolute zero.
This is the temperature at which the pressure of a
gas would be zero and the particles would not
move.

11. We Are Learning To
Unit P3: Applications of physics
Topic 5
Kinetic theory and gases
5.5 Convert between the Kelvin and Celsius scales
5.6 Recall that the average kinetic energy of the particles in a gas
is directly proportional to the Kelvin temperature of the gas

12. Convert 27 0C, -3 0C, 150 0C
and -90 0C to Kelvin
Convert 373 K, 200 K
and 1000 K to Celsius
Convert between the Kelvin and Celsius scales
27 0C
-3 0C
150 0C
-90 0C
300 K
270 K
423 K
183 K
373 K
200 K
1000 K
100 0C
-73 0C
727 0C

13. If you double the kelvin temperature, the average
kinetic energy of the particles in a gas also doubles.
Directly proportional!
Kinetic energy
of the particles
in a gas

14. We Are Learning To
Unit P3: Applications of physics
Topic 5
Kinetic theory and gases
5.8 Use the relationship:
V1 = V2T1/T2
to calculate volume for gases of fixed mass at constant pressure
(rearranging not required)

15. Charles’ law states that if a given quantity of gas is held
at a constant pressure, its volume is directly proportional
to the absolute temperature (Kelvin).
V1 = V2T1
T2
NOTE:
Temperature must be in Kelvin (K)
Charles’ law

16. A hot air balloon contains 2000 m3 of air at
1000C. What volume of air at 50C is needed to fill
the balloon?
Question
1000C
50C
373K
278K
V1 = V2T1
T2
V1 = 2000 x 278
373
1491 m3

17. We Are Learning To
Unit P3: Applications of physics
Topic 5
Kinetic theory and gases
5.11 Use the equation:
initial pressure (pascal, Pa) x initial volume (metre3, m3) /
initial temperature (kelvin, K) = final pressure (pascal, Pa) x
final volume (metre3, m3) / final temperature (kelvin, K)
P1V1/T1 = P2V2/T2
5.12 Apply an understanding of the equation in 5.11 to the use of
bottled gases in medicine, including the need for a pressure
above atmospheric and the calculation of the volume of gas
released at atmospheric pressure

19. Air particles are moving (they have kinetic energy)
.. they are colliding with walls of the balloon…
… exerting a force
0 C 273 K
-273 C 0K 290

20. Gases take up large
volumes at atmospheric
pressure and so they need
to be compressed and
stored a higher pressures.

21. Gases take up large volumes at atmospheric pressure
and so they need to be compressed and stored a higher
pressures.