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Thermal physics
A-level Physics
Unit G484: The Newtonian World
The ideal gas law
A-level Physics
Unit G484: The Newtonian World
The ideal gas law
Thermal physics
To do
Copy and complete these graphs to show the
relationships between the pairs of variables.
p
V
T T
V p
The gas laws: what do you remember? LOs
Thermal physics
Learning objectives
At the end of the lesson you will be able to:
• select and apply pV/T = constant;
• state the basic assumptions of the kinetic theory of gases;
• use the ideal gas equation expressed as pV = nRT or pV = NkT
to solve problems.
Lesson focus• The ideal gas law
Thermal physics
Learning outcomes
All of you should be able to
• explain what is meant by the term ideal gas;
• list the simplifying assumptions used when we describe an ideal gas;
• use the relationship for a fixed mass of gas to solve problems;
• state two forms of the ideal gas equation.
Thermal physics
The combined gas law
This equation describes the
behaviour of an ideal gas.
The gas laws: summary LOs
Figure taken from Hyperphysics
Thermal physics
Ideal gases
• obey Boyle’s law (p 1/V) at all temperatures;
• cannot be liquefied;
• because there are no forces between particles, the internal energy of an ideal gas is entirely present as the kinetic energy of its particles.
Ideal gas behaviour LOs
Thermal physics
Ideal gases (delete words that do not apply)
• consist of a small / large number of particles (atoms or molecules) in
constant motion at high speed;
• collisions between particles and between particles and the walls of a
container are inelastic / elastic (kinetic energy is / is not conserved);
• attractive forces between particles are present / do not exist;
• intermolecular forces between particles exist at all times / are zero except
during instantaneous collisions;
• the total volume of particles is very small / significant compared with the
volume of the container.
LO 2: state the basic assumptions of the kinetic theory of gases
Ideal gases: simplifying assumptions LOs
Thermal physics
An ideal gas is one that obeys Boyle’s law at all temperatures.
On a microscopic scale, an ideal gas:
• consists of a large number of particles (atoms or molecules) in constant motion at high speed;
• collisions between particles and between particles and the walls of a container are perfectly elastic (kinetic energy is conserved);
• attractive forces between particles do not exist;
• intermolecular forces are zero except during instantaneous collisions;
• the total volume of particles is very small compared with the volume of the container.
A gas fitting this description is called an ‘ideal gas’. Normal gases (especially dilute gases) come close to meeting the description.
Ideal gases: simplifying assumptions LOs
LO 2: state the basic assumptions of the kinetic theory of gases
Thermal physics LO 2: state the basic assumptions of the kinetic theory of gases
Ideal gases: simplifying assumptions LOs
Thermal physics
pVT
amount of gas=
It can be shown by experiment that
When the amount of gas is given in moles
pVT
nR=
where n – number of molesR – molar gas constant (8.31 ________ )
pV = nRT
To do Work out the units of R
LO 3: use the ideal gas equation expressed as pV = nRT or pV = NkT to solve problems
The ideal gas law LOs
Thermal physics
When the amount of gas is given in molecules
pVT
Nk=
Where N – number of molecules
k – Boltzman constant (1.38 x 10-23 ________ ; the ‘gas constant per molecule’)
pV = NkT
To do
1. Work out the units of R2. What is the link between R and k?
The ideal gas law LOs
LO 3: use the ideal gas equation expressed as pV = nRT or pV = NkT to solve problems
Thermal physics
pV = nRT and pV = NkT are both forms of the ‘ideal gas law’.
Exercises
• Philip Allen worksheet: Qs 1 - 5
The ideal gas law LOs
LO 3: use the ideal gas equation expressed as pV = nRT or pV = NkT to solve problems
Thermal physics
Oxford University entrance exam for physics, 2008
The ideal gas law LOs