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