Module 8 From the Universe to the Atom sfx copy

HSC Physics for NSW Study Guide Series

Baryons
Baryons contain three quarks (qqq). For example:

neutrons are made of one up and two down quarks (udd)
protons are made of two up quarks and one down quark (uud).

Mesons
Mesons are particles that contain one quark (q) and one antiquark ( q ) . For example:

the meson is the or the quark combination
the negative kaon is and the positive kaon is .

Leptons
Leptons include the electron, muon and neutrino and behave differently from mesons and baryons. They
are much less massive. The mass of the electron is almost 2000 times smaller than the mass of the proton.
The muon is just a heavier version of the electron, its mass being nine times smaller than that of the proton.
The neutrino has a very, very small mass.
The electron and muon interact with matter mainly through their electric charges; the neutrino being neutral,
has very little interaction with matter.

Model of an atom
We can represent an atom with the following diagram showing the nucleus containing protons and neutrons
which in turn contain quarks. Electron clouds surround the nucleus of the atom.
Note: the diagram is not to scale.

down quark

proton

electron
(a lepton)

nucleus

neutron

electron

up quark

Question:
Which atom is represented above?

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Bosons

For every type of fundamental force there is an exchange particle called a boson that carries the force.
There are four fundamental forces.

The gravitational force is a long-range force that acts on all masses in the universe. It is the weakest
of all the fundamental forces. The gravitational force is thought to be carried by a graviton but as yet
this has not been detected. This is an example of how we know that human understanding of matter is
still incomplete.

The electromagnetic force is a long-range force that acts on all charges in the universe. It holds atoms
and molecules together. The electromagnetic force is carried by a photon.

The nuclear strong force is a short-range force working at nuclear distances. It joins quarks together
to form protons and neutrons and also holds neutrons and protons together. The typical range is
10-15 m. The nuclear strong force is carried by gluons.

The weak nuclear force interacts with some particles to change them into other forms of particle. The
typical range for the weak force is 10-17 m. The nuclear weak force is carried by the W and Z particles.

The quark nature of beta decay
Beta decay provides evidence that protons and neutrons are made up of quarks, the quark nature of the
proton and neutron being used to explain beta decay.

Beta negative decay 10 → 11 + −01 +
In beta negative decay a down quark changes into an up quark with the emission of an electron and an
antineutrino.

d d
d uu

u

electron antineutrino

neutron proton

Beta positive decay 11 → 10 + +01 +
In beta positive decay an up quark changes into a down quark with the emission of a positron and a neutrino.
The positron is also referred to as a positive beta particle.

d d
u ud

u

positron neutrino

proton neutron

The antineutrino is an antimatter particle, the counterpart to the neutrino. Antineutrinos are produced in
beta negative decay while neutrinos are produced in beta positive decay.

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HSC Physics for NSW Study Guide Series
Antimatter matter interaction
For every type of matter particle there also exists a matching antimatter particle, or antiparticle.
Antiparticles look and behave just like their corresponding matter particles except they have opposite
charge. For example, an electron is negative whereas an antielectron, called a beta positive particle or
positron, is positive. Gravity affects matter and antimatter the same way because gravity is not a charged
property and a matter particle has the same mass as its antiparticle.
When a matter particle and antimatter particle meet, they annihilate producing energy. The relationship
E = mc2 can be used to find the energy released.

Sample problem 8.9
The deuterium nucleus consists of one proton and one neutron. What is the quark composition of the
deuterium nucleus?
Solution:
A proton contains two up quarks and one down quark and a neutron contains one up and two down quarks.
Adding these together we get three up quarks and three down quarks.
As a check if we add the quark charges, we get 3 x (+ 2 ) and 3 x (- 1 ) which gives a charge of + 1 e

33

which is correct for a deuterium nucleus.
Graph of isotope stability
Another interesting graph is the number of neutrons plotted against the number of protons. As atoms get
larger that is, they have more protons, then the coulombic repulsion increases. To counteract this more
neutrons are added so that the coulombic repulsion is unchanged but the strong nuclear force increases.
This is why the graph of stable nuclei is shown below with a ratio of neutrons to protons greater than one
which increases as the nuclei get larger.

130

neutron number

120
110
100

90 Stable nuclei

80
70
60

50 p = n

40
30
20
10

0

0 10 20 30 40 50 60 proton7n0umber 80 90

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Question sheet 8.12
Information on the sign and charge relative to the proton of six types of quarks and their corresponding
antiquarks are shown in the table below. Use this as required in the following questions.

Quark name Charge relative to Antiquark name Charge relative to

up (u) size of proton charge antiup size of proton charge
down (d) anticharm
strange (s) + 2 antistrange - 2
charm (c) anticharm
bottom (b) 3 antibottom 3
top (t)
- 1 antitop + 1

3 3

- 1 + 1
3 3

+ 2 - 2

3 3

- 1 + 1
3 3

+ 2 - 2
3 3

1. One type of meson consists of a down quark and an antiup quark. What is the charge on this meson?

2. A common way of describing beta decay is that a neutron in a radioactive nucleus becomes a proton,
with a beta particle and an antineutrino being ejected from the nucleus. How does the Standard Model
explain beta decay?

3. The LHC has an estimated cost of ten billion dollars to build and operate. Justify the expenditure on this
instrument. Note you may need more space to fully answer this question.

4. Draw a diagram of a helium-4 atom showing the constituents and the various fundamental forces
involved.

5. Lithium 7 was formed in small quantities in the Big Bang. What are the types and number of quarks
found in lithium 7?

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HSC Physics for NSW Study Guide Series
6. (a) Use Coulomb s law to calculate the repulsive force between two protons separated by a distance of

3.8 x 10-15 m in the nucleus.

(b) Explain why a nucleus with such a large repulsive force is stable?

7. Consider the following statement made about particle accelerators:
Di co e ing ne a icle b colliding o high eed o on oge he in he La ge Had on Collide

is like firing two cricket balls together at high speed and discovering that they produce several footballs
moving off in different directions at high speed. Comment on this statement.

8. In beta positive decay a positron is emitted along with a neutrino. Explain in terms of quarks the changes
that must happen.

9. Give an example of a particle which is a lepton. .
..
10.Work out the charge on a meson made up of an:
(a) ̅ quark combination
(b) ̅ quark combination
(c) What does the superscript 0 represent in this case?

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Inquiry question: How is it known that human understanding of matter is still incomplete?

Now that you have covered this section you should be better able to answer the inquiry question with your new
understanding of classical physics and why it cannot explain the properties of the atom.

Summary
Write a summary of deep inside the atom, review your summary with others in the class, update as
necessary.

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HSC Physics for NSW Study Guide Series
Student evaluation of Module 8 - From the Universe to the Atom
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Depth studies - possibilities for From the Universe to the Atom
Your teacher may decide to do a depth study from this module. Depth studies are designed to provide
opportunities for students to:

consolidate their learning - and/or
develop competence and confidence in relation to their knowledge and skills - and/or
foster creativity by allowing students to apply their knowledge and skills to new situations.
Possibilities include:
An in-depth investigation of the last bullet point which says investigate the operation and role of particle
accelerators in obtaining evidence that tests and/or validates aspects of theories, including the Standard
Model of matter. This would have to be in more depth that students would normally undertake.
or
An excursion to the OPAL reactor in Sydney with additional research on the various facilities they have,
for example, the Bragg Institute. This depends on the location of the school and time available.
Summarise below any suggestions you have for a depth study.

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From the Universe to the Atom - revision questions
1. Light from the sun is passed through a spectroscope to produce a visible spectrum. The spectrum shows

some dark lines one of which is shown in each of the three spectra.

distant galaxy

nearby galaxy

the sun

400 500 600 700 (nm)

(a) Explain how these dark lines in the spectrum of sunlight are produced.

(b) One of the lines is due to hydrogen. The position of this hydrogen line in the visible spectrum is
shown for a distant galaxy, a nearby galaxy and the sun. Why is the position of the line different
for each of the spectra?

2. Describe, giving details of any experiments, how each of the following scientists contributed to our
knowledge of atomic structure.
(a) J. J. Thomson
(b) Ernest Rutherford
(c) Niels Bohr
(d) Louis de Broglie
(e) James Chadwick
(f) Erwin Schrödinger

3. Alpha, beta and gamma are different types of radiation. What are the similarities and/or differences
between them?

4. The Balmer series consists of single wavelengths of light. Explain how these single wavelengths of light
provide evidence for the existence of states with separate energies in hydrogen atoms.

5. A sample of a radioactive isotope contains 6.8 × 1023 active undecayed nuclei. The half-life of the
isotope is 6 hours. Calculate:
(i) the number of active nuclei of the isotope remaining after 18 hours
(ii) the number of active nuclei of the isotope remaining after 32 hours
(iii) Calculate the decay constant .

6. Calculate the wavelengths of the four visible spectral lines of the hydrogen spectrum.

7. Calculate the de Broglie wavelength of an electron moving with a speed of 5.4 x 106 m s-1.

8. A star s mass is one of the most significant factors affecting the life of the star. Describe and explain
how mass affects the life of a star.

9. Photons with enough energy can, on interacting with matter, produce an electron-positron pair. What is
the minimum frequency of a photon that could produce an electron-positron pair?

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HSC Physics for NSW Study Guide Series
10.The diagram below shows equipment used to measure the charge to mass ratio of a cathode ray.

(a) Label on the diagram the direction of magnetic field to deflect the cathode rays up.
(b) Explain the significance if the force created by the magnetic field balances the force created by the

electric field.
(c) Derive the equations Thomson used to calculate the charge to mass ratio of the cathode ray.
11.The drawing below shows some of the lines in the visible emission line spectrum of helium.

389 447 502 588 668 / nm
(a) Explain what is meant by the term emission line spectrum.
(b) Helium light is passed through a slit and analysed using a diffraction grating with 1.25 × 106 lines

m-1. Calculate the angle between the first order images of the 447 nm and 588 nm lines. You may
need to review module 7 - The Nature of Light.
(c) In observing the helium emission spectrum of a star, the 588 nm line is measured to be 590 nm.
Explain how this difference occurs.
12.A student makes the following statement. Quantum theory - I don t understand it. I don t really know
what it is. I believe that classical physics can explain everything. Comment on this statement.
13.The model of the atom has evolved from Rutherford's model to the Standard Model. Outline the Standard
Model that describes how the nuclear atom is constructed, include in your answer the interactions
involved to create the nucleus and atoms.
14.What difference, if any, would you expect alien astronomers in another galaxy to make when measuring
the age of the universe and the red shift of galaxies?
15.Alpha, beta and gamma radiation are passed through a magnetic field. Using a diagram show the path
of the radiation through the magnetic field.

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16.Sketch a HR diagram showing the main groups and the main nuclear reactions that take place in each
group. Remember to label your axis clearly.

17.Outline how you would protect yourself from the typical radioactive sources found in schools.

18.Outline the evidence that lead to the discovery of the expansion of the universe by Edwin Hubble.

19.Explain how the large hardon collider and accelerators before it have been used to provide evidence for
the Standard Model of matter.

20.Analyse the contribution of Schrödinger to the current model of the atom.

21.Why is the number of neutrons greater than the number of protons in stable nuclei with a mass number
greater than about 40, and why is this effect more noticeable for the heaviest nuclei?

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