This document discusses particle theory and the states of matter. It begins by describing the arrangement of particles in solids, liquids, and gases. Experiments demonstrating Brownian motion and diffusion are then described, providing evidence for the kinetic motion of molecules. These experiments show random particle movement due to collisions with smaller, faster molecules. Finally, the document discusses Einstein's analysis of Brownian motion and how it supported the kinetic theory of matter by revealing evidence of molecular motion.
1. A-level Physics
Unit G484:
The Newtonian
World
Solids, liquids & gases - 3
(2)
Thermal physics
2. The kinetic theory of matter LOs
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).
Thermal physics
3. Lesson focus
• solids, liquids & gases - 2
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.
Thermal physics
4. 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
5. Evidence for the kinetic nature of matter LOs
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?)
Thermal physics LO1: describe s, l & g in terms of the spacing, ordering and motion of particles
6. Evidence for the kinetic nature of matter LOs
bromine vapour
Diffusion of bromine vapour
Again,
1. Record your observations
2. How do you explain your observations?
liquid bromine
Extension
How could you make this diffusion happen more
quickly?
before after
where T – absolute temperature
m – mass of particle
Thermal physics LO1: describe s, l & g in terms of the spacing, ordering and motion of particles
7. Evidence for the kinetic nature of matter LOs
Diffusion of ammonia and hydrogen chloride vapours
cotton wool cotton wool
soaked in HCl vapour of soaked in NH3
ammonium
chloride
1. Record your observations
2. How do you explain your observations?
where T – absolute temperature
mA – mass of particle A
Thermal physics LO1: describe s, l & g in terms of the spacing, ordering and motion of particles
8. Modelling the motion of scent particles LOs
You are going to model the ‘random
walk’ of a scent particle. 1
How this works 6 2
1. Roll the die.
2. Use the key to decide which
way to move.
3. Draw a pencil line joining the 5 3
start and end points.
4. Repeat the process. 4
Thermal physics LO1: describe s, l & g in terms of the spacing, ordering and motion of particles
9. Evidence for the kinetic nature of matter LOs
particles colliding
Diffusion
with a partition
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 A
Why is A more likely than B?
particles diffusing
B after the partition is
removed
Thermal physics
10. Evidence for the kinetic nature of matter LOs
Mean free path
The mean free path of a particle (atom or molecule) in a gas is the
average distance between collisions.
Molecular diameter of
bromine ≈ 0.24 nm Mean free path calculator
Thermal physics LO1: describe s, l & g in terms of the spacing, ordering and motion of particles
11. Brownian motion LOs
Thermal physics LO3: describe an experiment that demonstrates Brownian motion
12. Brownian motion LOs
Brownian motion is the seemingly random
movement of particles suspended in a fluid.
Thermal physics LO3: describe an experiment that demonstrates Brownian motion
13. Brownian motion LOs
Fat droplets in milk
Brownian motion is the seemingly random
movement of particles suspended in a fluid.
Thermal physics LO3: describe an experiment that demonstrates Brownian motion
14. Brownian motion LOs
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).
Thermal physics
17. Brownian motion LOs
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
Thermal physics such an experiment.
18. Brownian motion LOs
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
Thermal physics such an experiment.
19. Brownian motion LOs
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?]
Thermal physics LO3: describe an experiment that demonstrates Brownian motion
