Download 17.1assign - Advancing Physics

CHAPTER 17: PROBING DEEP INTO MATTER
The world is full of variety and dynamic change – startling beauty and violent destruction. The brute force
of nuclear fusion inside stars contrasts with the delicacy of life and the subtleties of thought itself. And
yet, beneath the surface there is astonishing simplicity. In this chapter we will describe how:
 matter is made from just a few different types of fundamental particles
 scattering experiments reveal the deep structure of matter
 interactions take place by the exchange of particles
 conservation laws determine what can and cannot happen
 quantum effects result in complex atomic structures
What are the fundamentals?
We think that the really fundamental ideas and issues in modern quantum thinking include the following:
 The existence of pair creation and annihilation, leading necessarily through the conservation laws
to the existence of antiparticles.
 The basic contrast between fermions and bosons, with fermions obeying the Pauli exclusion
principle and functioning as ‘matter-like’, whilst carriers of interactions are bosons.
 The basic contrast between leptons and hadrons, with only hadrons experiencing the strong
(‘colour’) interaction.
 The idea of a fundamental particle as one without components, and so as ‘point-like’.
 The current state of the ‘Standard Model’ and some of its problems.
 The continuing use of wave-mechanical thinking, in problems for which it is the simplest tool to
use.
 The fundamental notion that the probabilities of all processes are obtained after superposing
complex quantum amplitudes (‘arrows’) for all the possible sets of unseen virtual processes
which could form part of them.
This chapter is designed as a set of assignments for you to complete and learn from. For each
assignment you need to do the following:
o Read the whole set of tasks.
o Plan which ones you will do when, including lesson time and homework time.
o Organise the resources you need, when you need them.
o Complete all the tasks (and produce evidence to show that you have completed them).
o Ask for help when necessary to ensure that your work meets all the stated learning outcomes
(see below).
o Hand in the ‘evidence’ (the completed tasks), either on paper or electronically.
ASSIGNMENT: 1
Learning outcomes
 Particles can be created and annihilated in matter–antimatter pairs.
 Particles and their antiparticles have opposite charges and lepton or baryon numbers.
 Electromagnetic interactions arise from the exchange of photons.
 Electrons are fermions, which obey the Pauli exclusion principle.
 Leptons are particles like the electron, neutrino and their antiparticles. Baryons are particles like
the proton and neutron and their antiparticles.
 Total energy, momentum and charge are conserved in interactions, as are lepton and baryon
number.
As a straightforward introduction, look at the pictures in Activity 10S Software Based 'Bubble chamber
photographs' and answer the questions about what you have seen.
Activity 20P Presentation 'Who, what and when?' (described below) should be started early on as it can
be an ongoing activity that develops throughout the chapter, work with other students to make it bigger
and better!
The people who made quantum, nuclear and particle physics
The physics of the atom and beyond is the product of many hands and minds. In 1890, hardly anything
was known about atoms, not even how big they are. By the year 2000, people had probed inside protons
and neutrons to reach a fundamental level of matter. This work has led both to the deepest
understanding of matter so far achieved, and to frightening threats to civilisation and humanity.
In this activity, you can build up a picture of who did what, and when. And you can add to it something
about them as people.
Make a blank time line, divided into the decades from 1890 to the year 2000. A long strip to be pinned on
the laboratory wall will make it easy for everyone in the class to make a contribution:
Pick a discovery or pick a person
Here is a list of some of the discoveries, inventions, ideas and things done in creating this new physics.
Following it is a list of people who were involved in some way: discovering something in an experiment,
proposing a theory, having an idea, leading a team.
You could start with a discovery, or with a person. You might want to know who found out about
antiparticles, for example. Or you may have heard of Lise Meitner, and want to find out what she did.
Discoveries: Antiparticles, Bubble chamber, Cloud chamber, Cyclotron, Director at CERN, Electron, E =
m c2, Existence of nucleus, First fission bomb, First fission reactor, First quantum theory of atom
People: Carl Anderson, Henri Becquerel, Niels Bohr, James Chadwick, Georges Charpak, Marie Curie,
Louis de Broglie, Paul Dirac, Enrico Fermi, Richard Feynman
Find out who, what, when
You’ve chosen a starting point: a person or a discovery. Look up books and/or search the Internet to find
out what the person did, or who found something out. Match the person and the discovery.
There are more people than discoveries in the list, so expect more than one person for some
discoveries. But there are some people associated with more than one discovery, too.
Try to find out something interesting about the people involved. They come from many countries. A good
number worked in countries other than their own. Some were on opposite sides during a war. Some are
women. Several are Jewish. Do these kinds of facts have any bearing on what happened?
Find out when the discovery was made and locate the person and discovery on the time line.
Try to get a photograph of the person, or a photograph of the experiment or apparatus.
Find out how and why
 How was it found out? An experiment? Just an idea? A theory? A large team effort?
 What had to be done? How did the discovery depend on what went before?

Why was it done? By chance or lucky accident? Because it was now possible? Because of the dangers of
war? Because a lot of money had been spent? Because it was what the people were thinking about?
Use what you find out to write a brief story about the discovery.
Find out where
 What nationality were the people involved?
 In what country was this work done?
 Is there a reason why it was done there?
Read pages 173 - 180 in the A2 Advancing Physics textbook and make your own notes on:
Antimatter, PET, annihilation & creation, particle interactions, Pauli’s exclusion principle, neutinos
Short Answer 'Things that don’t change' and check
Do the simple, practice question Question 10S
your answers – ask about any you find difficult.
Take some time to read through Reading 30T
Text to Read 'Early ideas about elementary particles'
and Reading 40T
Text to Read 'Quantum theory in the twentieth century'.
If you need a little “light relief” read Reading 20T Text to Read 'Three poems about particles'. Why were
these poems written? Could you do one of your own?
Use Activity 30S Software Based 'Interactions in particle physics' to visualise the way in which particles
interact with a set of Modellus models. Draw a few of your own Feynman diagrams of simple
interactions.
Answer the 5 questions set in Activity 40S Software Based 'Time and the Muon' based on the
spreadsheet of data given.
Have a go these multiple-choice questions below:
Question 40M: Multiple Choice
Choose from the list of kinds of particle below the one that best fills the empty slot in each sentence.
Here is a list of kinds of particle: Electron(s) Positron(s) Photon(s) Boson(s) Fermion(s) Neutrino(s)
Proton(s) Neutron(s) Nucleon(s) Lepton(s) Antiparticle(s)
In each sentence below choose the kind of particle which best fills the empty slot. Try to give a reason
why your choice is correct.
1.
The electrical force between an electron and a proton is carried by exchange of virtual [
].
2.
In particle physics, forces are understood as exchange of virtual [
].
3.
The antiparticles of electrons are [
4.
Electrons and neutrinos but not protons and neutrons are both [
5.
[
6.
Particles which are [
7.
Together, protons and neutrons are called [
8.
In a beta decay, both the [
9.
In b+ decay, a [
10.
Because a neutrino has no electric charge it cannot interact by exchange of [
11.
A photon with enough energy can create a particle–[
12.
Electrons and positrons annihilate to produce [
13.
Electrons, neutrinos, protons and neutrons are all [
].
].
] are leptons which have zero charge and mass.
] all obey the Pauli exclusion principle.
].
] number and the [
] number are conserved.
] as well as a positron is emitted, thus conserving the [
] pair.
].
].
] number.
].
14.
The strength of the electromagnetic interaction is indicated by the probability of interaction of
electrons and [
].
15.
A neutron decays into a(n) [
], a(n) [
16.
Charge would not be conserved if [
pair creation.
] and an anti[
].
] were not produced at the same time as particles in
Question 60M: Multiple Choice
Electron–positron annihilation
Events in which the total energy or the total linear momentum would change never happen. A photon of
energy E has momentum p = E / c in its direction of travel. You are asked to identify events in which the
total linear momentum does or does not change. Choose the corresponding letters, and justify your
choice.
Pictures of events
Electron-positron annihilation
KEY
The pictures show examples of electron–positron
annihilation. Assume that the two photons in each
picture have the same energy.
electron
Consider whether momentum is conserved
in each picture
positron
1.
In which picture(s) is the total linear
momentum conserved?
photon
annihilation
both at rest
A
both at rest
2.
In which picture(s) is the initial linear
momentum
zero?
B
3.
In which picture(s) is the final linear
momentum zero?
4.
In which picture(s) is the initial linear
momentum of the positron zero?
both at rest
C
both at rest
D
5.
In which picture(s) is the final linear
momentum zero in the vertical direction but not in
the horizontal direction?
6.
In which picture(s) is there an increase in
linear momentum to the left?
particles collide head on
with equal energy
E
particles collide head on
with equal energy
F
7.
In which picture(s) is there an increase in
linear momentum in the vertical direction?
8.
Which picture(s) represent an event which
could occur?
particles collide head on
with equal energy
G
particles collide head on
with equal energy
H
9.
In which picture(s) representing an event
that could occur will the total energy of the
photons be equal to the rest energy of the
electron plus positron?
10.
In which picture(s) representing an event
that could occur can the energy of the photons exceed the rest energy of the electron plus positron?
11. In which picture(s) is electric charge conserved?
Finish this assignment by completing questions: Question 50C Comprehension 'Creation from
annihilation' and Question 30S Short Answer 'Creation and annihilation'