2. From Brownian to pressure LOs
From: Thinking physics by Epstein
Thermal physics
3. Lesson focus
• Pressure in gases - theory
Learning objectives
At the end of the lesson you will be able to:
• define the term pressure;
• use the kinetic model to explain the pressure exerted by gases.
Thermal physics
4. Learning outcomes
All of you should be able to
• define the term pressure;
• explain in basic terms how a gas exerts a pressure.
Most of you should be able to
• list most of the factors that determine the size of a gas pressure.
Some of you should be able to
• give a detailed description of the way in which a gas exerts a pressure.
Thermal physics
5. Gas pressure LOs
F1 F2
6 cm 3 cm 2 cm
a) b) c)
The diagrams show a cylinder with a freely moving airtight piston. In a) the air inside and
out is at atmospheric pressure. With no force applied the piston stays where it is. In b)
and c) the piston has been pushed in and a force is needed to keep it in place.
To do Comparing situations a) and b) , describe
1. the total number of molecules enclosed (is it, for example, the same, twice, one-
third?)
2. the volume of air in the cylinder;
3. The number of particles per cubic centimetre;
4. The average speed of a molecule;
5. The pressure exerted by the air in the cylinder.
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases
6. Gas pressure? LOs
F1 F2
6 cm 3 cm 2 cm
a) b) c)
The diagrams show a cylinder with a freely moving airtight piston. In a) the air inside and
out is at atmospheric pressure. With no force applied the piston stays where it is. In b)
and c) the piston has been pushed in and a force is needed to keep it in place.
To do Comparing situations a) and c) , describe
1. the total number of molecules enclosed
2. the volume of air in the cylinder;
3. The number of particles per cubic centimetre;
4. The average speed of a molecule;
5. The pressure exerted by the air in the cylinder.
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases
7. Gas pressure? LOs
F1 F2
6 cm 3 cm 2 cm
a) b) c)
The diagrams show a cylinder with a freely moving airtight piston. In a) the air inside and
out is at atmospheric pressure. With no force applied the piston stays where it is. In b)
and c) the piston has been pushed in and a force is needed to keep it in place.
To do Which of these equations correctly describe the changes that took place?
p1 p2 p1 V2 p1 V1
= = = p1 V1 = p2 V2
V1 V2 p2 V1 p2 V2
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases
8. How do we explain gas pressure? LOs
Things you should consider:
• what is pressure?
• why do gases exert a pressure?
• the pressure of a gas in a constant volume
container will depend on …
• animation
Thermal physics LO1: define pressure; LO2: use the kinetic model to explain gas pressure
9. Gas pressure LOs
• The pressure of a gas results from the collisions between its particles
(atoms or molecules) and the walls of its container.
• When a particle collides with the wall it exerts a force on the wall and the
wall, in turn, exerts the same-sized force on it, in the opposite direction
(Newton’s 3rd law of motion).
• The gas pressure is the sum of many forces, due to a vast number of
particles, divided by the area over which these forces are exerted (p = F/ A).
• Although the pressure appears to be constant (because the number of
particles is so large) it varies on a microscopic scale.
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases
10. Gas pressure in more detail LOs
Imagine a single gas particle colliding with the wall of its container.
The force it exerts is equal to its rate of change of momentum.
In symbols,
Δmv before: mv
F = after: - mv
Δt
The total force exerted by a gas is due to the sum of all momentum changes taking
place. From this it can be seen that the pressure of a gas depends on
• the mass of the particles
• the number of collisions taking place per unit time
(this depends on v and ρ).
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases
11. Jan. 2007, 2863 Gas pressure in more detail LOs
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases
12. Jan. 2007, 2863 Gas pressure in more detail LOs
Thermal physics LO 2: use the kinetic model to explain the pressure exerted by gases
