Thermal physics

A-level Physics

Unit G484: The Newtonian World

Solids, liquids & gases (2)

A-level Physics

Unit G484: The Newtonian World

Solids, liquids & gases - 3

Thermal physics

To do

Sketch the arrangement of particles in solids, liquids and gases.

Now do this in detail for a gas. Attach a velocity vector (arrow) to each

particle (atom or molecule).

The kinetic theory of matter LOs

Thermal physics

Learning objectives

We are learning to

• describe solids, liquids and gases in terms of the spacing, ordering and

motion of atoms or molecules;

• describe a simple kinetic model for solids, liquids and gases;

• describe an experiment that demonstrates Brownian motion and discuss the evidence for the movement of molecules provided by such an experiment.

Lesson focus• solids, liquids & gases - 2

Thermal physics

Learning outcomes

All of you should be able to• draw diagrams to show the arrangement of particles in solids, liquids

and gases;

• describe the motion of particles in solids, liquids and gases;

• describe in writing the experimental evidence for particle motion in liquids and gases.

Most of you will be able to

• describe in detail the motion of particles in gases

- list factors that affect the speed of particles

- describe variations within a population of particles.

- describe a simple model that explains diffusion.

Thermal physics

Perfume demonstration

1. Write down your observations.

2. What name is given to this phenomenon?

3. How do you explain your observations? (what do you imagine

is happening on a small scale?)

LO1: describe s, l & g in terms of the spacing, ordering and motion of particles

Evidence for the kinetic nature of matter LOs

Thermal physics

Diffusion of bromine vapour

Again,

1. Record your observations

2. How do you explain your observations?

Extension

How could you make this diffusion happen more

quickly?

before after

liquid bromine

bromine vapour

where T – absolute temperature

m – mass of particle

LO1: describe s, l & g in terms of the spacing, ordering and motion of particles

Evidence for the kinetic nature of matter LOs

Thermal physics

vapour of ammonium

chloride

Diffusion of ammonia and hydrogen chloride vapours

cotton wool soaked in HCl

cotton wool soaked in NH3

1. Record your observations

2. How do you explain your observations?

where T – absolute temperature

mA – mass of particle A

LO1: describe s, l & g in terms of the spacing, ordering and motion of particles

Evidence for the kinetic nature of matter LOs

Thermal physics

You are going to model the ‘random

walk’ of a scent particle.

How this works

1. Roll the die.

2. Use the key to decide which

way to move.

3. Draw a pencil line joining the

start and end points.

4. Repeat the process.

1

2

3

4

5

6

LO1: describe s, l & g in terms of the spacing, ordering and motion of particles

Modelling the motion of scent particles LOs

Thermal physics

Diffusion

Diffusion is the movement of a substance (gas, liquid or solid in a

solution) from a region of high _______________ to low

______________ . It happens because of continuous, random

____________ between particles. Diffusing particles move with

a ‘__________ _______’ motion. The rate of diffusion is

directly proportional to the square root of the absolute

________________ and inversely proportional to the square

root of the diffusing particle’s ________ .

Extension

Why is A more likely than B?

particles colliding with a partition

particles diffusing after the partition is

removed

A

B

Evidence for the kinetic nature of matter LOs

Thermal physics

Mean free path

Mean free path calculatorMolecular diameter of bromine ≈ 0.24 nm

The mean free path of a particle (atom or molecule) in a gas is the average distance between collisions.

LO1: describe s, l & g in terms of the spacing, ordering and motion of particles

Evidence for the kinetic nature of matter LOs

Thermal physics LO3: describe an experiment that demonstrates Brownian motion

Brownian motion LOs

Thermal physics

Brownian motion is the seemingly random

movement of particles suspended in a fluid.

LO3: describe an experiment that demonstrates Brownian motion

Brownian motion LOs

Thermal physics

Fat droplets in milk

LO3: describe an experiment that demonstrates Brownian motion

Brownian motion is the seemingly random

movement of particles suspended in a fluid.

Brownian motion LOs

Thermal physics

Einstein’s view:

‘In this paper it will be shown that, according to the molecular-kinetic theory of

heat, bodies of a microscopically visible size suspended in liquids must, as a result of

thermal molecular motions, perform motions of such magnitudes that they can be

easily observed with a microscope. It is possible that the motions to be discussed

here are identical with so-called Brownian molecular motion; however, the data

available to me on the latter are so imprecise that I could not form a judgment on

the question.’

(John Stachel, ed., Einstein's Miraculous Year: Five papers that changed the face of physics,

Princeton University Press, 1998, 85; Einstein's original papers are included in the Collected

Papers of Albert Einstein, vol. 2).

Brownian motion LOs

Thermal physics

Einstein and Brownian motion

Click to link

Thermal physics

Click to link

Brownian motion LOs

Thermal physics

Brownian motion is the random movement of microscopic particles

suspended in a fluid (e.g. a liquid, such as water, or air).

Examples are:

• smoke particles (ash) in air

• coal dust on the surface of water.

LO3: describe an experiment that demonstrates Brownian motion and discuss the evidence for the movement of molecules provided by

such an experiment.

Brownian motion LOs

Thermal physics

Key points

Fluid particles

• cannot be seen and are, therefore, much smaller, and have a lower mass, than the visible suspended particles;

• move in random directions and with a range of speeds.

Suspended particles

• move because they are struck by randomly moving fluid particles;

• move continuously because fluid particles are continuously moving;

• move by small amounts because they are struck on all sides by fluid particles.

LO3: describe an experiment that demonstrates Brownian motion and discuss the evidence for the movement of molecules provided by

such an experiment.

Brownian motion LOs

Thermal physics LO3: describe an experiment that demonstrates Brownian motion

Question

The speed of smoke particles in thermal equilibrium in air can be measured

from Brownian motion observations. It is found to be about 10 mm s-1. Of

course, the direction of motion changes many times per second.

Estimate the speeds of air molecules at room temperature, given that the

mass of an air molecule is 5 x 10-26 kg and that of an average smoke particle is

1 x 10-16 kg.

[Hint: what does ‘in thermal equilibrium’ imply about the particles of air and

air molecules?]

Brownian motion LOs