tp 5 gas pressure 2 (shared)
DESCRIPTION
TRANSCRIPT
A-level Physics
Unit G484: The Newtonian World
Gas pressure - 2
A-level Physics
Unit G484: The Newtonian World
Gas pressure - 2
Thermal physics
From: Thinking physics by Epstein
From Brownian to pressure LOs
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.
Lesson focus• Pressure in gases - theory
Thermal physics
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
Thermal physics
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) , describe1. 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.
6 cm 3 cm 2 cm
F1 F2
a) b) c)
Gas pressure LOs
LO 2: use the kinetic model to explain the pressure exerted by gases
Thermal physics
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) , describe1. the total number of molecules enclosed2. 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.
6 cm 3 cm 2 cm
F1 F2
a) b) c)
Gas pressure? LOs
LO 2: use the kinetic model to explain the pressure exerted by gases
Thermal physics
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?
6 cm 3 cm 2 cm
F1 F2
p1 p2
V1 V2
=p1
p2 V1
V2=p1
p2
V1
V2
= p1 V1 = p2 V2
a) b) c)
Gas pressure? LOs
LO 2: use the kinetic model to explain the pressure exerted by gases
Thermal physics
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 …
How do we explain gas pressure? LOs
LO1: define pressure; LO2: use the kinetic model to explain gas pressure
• animation
Thermal physics
• 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.
Gas pressure LOs
LO 2: use the kinetic model to explain the pressure exerted by gases
Thermal physics
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,
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
F = ΔmvΔt
• the mass of the particles
• the number of collisions taking place per unit time (this depends on v and ρ).
before: mv
after: - mv
Gas pressure in more detail LOs
LO 2: use the kinetic model to explain the pressure exerted by gases
Thermal physics
Gas pressure in more detail LOsJan. 2007, 2863
LO 2: use the kinetic model to explain the pressure exerted by gases
Thermal physics
Gas pressure in more detail LOsJan. 2007, 2863
LO 2: use the kinetic model to explain the pressure exerted by gases