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Elegant Universe, The: Einstein's Dream
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Classroom Activities
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Background
The building blocks of matter that have been experimentally verified
are the quarks and leptons described by the Standard Model. Since
the discovery of the electron in 1897, physicists have identified
some 200 subatomic particles, all of which are leptons or quarks or
a combination of quarks. In this activity, students will investigate
the "recipes" for constructing a proton and neutron from the quarks
described in the Standard Model.
Objective
To learn about some of the elementary particles in the Standard
Model by building a proton and neutron from quarks.
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copy of the "Particle Puzzle Pieces" student handout (PDF
or
HTML)
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Organize students into teams and distribute copies of the
"Particle Puzzle Pieces" student handout.
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Review with students the history and nature of the atom (for
sources of information, see
General Physics Resources). Then discuss with students the matter particles that make up
the Standard Model (see "Elementary Particles" and "Elementary
Antiparticles" below). Emphasize that all matter comprised of
Standard Model particles is made from first generation particles
(the instability of second and third generation particles causes
them to quickly decay into stable first generation particles).
Additionally, antimatter is rarely seen in the everyday world.
(See Activity Answer below for more
information.)
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Have students use the "Quark Chart" and "Quark Recipe Rules" on
their student handout to discover how to build a proton and a
neutron.
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Discuss students' results and answers to the questions on the
student handout. To supplement this activity, have students use
the "Atom Builder" to build a carbon atom out of elementary
particles. Find it at
www.pbs.org/aso/tryit/atom/
In Conclusion
Physicists have used particle accelerators and detectors to confirm
the existence of most of the elementary particles and antiparticles
predicted by the Standard Model. One particle that has been
theorized but not yet discovered is called the Higgs boson. This
particle is thought to be a force carrier particle linked with the
Higgs field, which might be the mechanism by which particles acquire
their mass. In the 1960s, the physicist Peter Higgs postulated the
existence of this field through which all particles are thought to
move. The Higgs boson is considered to be the final missing piece of
the Standard Model.
From the "Quark Recipe Rules," students will likely infer that they
should:
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only use 1st generation (up and down) quarks in their recipe.
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use more than one quark to build a proton and neutron.
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build a proton with a net integer charge of 1; and a neutron
with 0 charge.
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use the smallest number of quarks possible to meet the stated
criteria.
This information should help students discover through trial and
error the composition of quarks necessary to describe a proton and a
neutron: The proton should contain two up quarks and one down quark;
the neutron should contain one up quark and two down quarks.
Check to ensure that student recipes use the lowest number of quarks
possible—three. This concept is identical to that of the Least
Common Multiple in mathematics. To create a neutral atom, three
electrons would be needed in an atom containing three protons and
four neutrons.
You may want to note to students that while antimatter particles are
part of the basic building blocks in our universe, and have been
identified by particle detectors, they are not observed very much in
the everyday world. That's because when matter and antimatter meet,
they annihilate each other. The resulting energy, however, is not
lost; it can rematerialize as new particles and antiparticles.
Physicists theorize that at the time of the big bang, matter and
antimatter were created in identical amounts. So why didn't the
matter and antimatter annihilate each other and end the universe as
we know it? Part of the answer may be that an asymmetry in the weak
force occasionally converts antimatter into matter. But some
physicists believe that this effect accounts for only some of the
imbalance. New theories predict additional sources for asymmetry for
which physicists continue to search.
One of the few places where matter and antimatter occur outside of a
particle accelerator is in the medical imaging technique known as
Positron Emission Tomography (PET).
In PET, positrons (the antimatter partner of electrons) are created
by the decay of radioactive nuclei. The process works by first
attaching a radioactive element to a natural body substance (glucose
is commonly used) and injecting it into a patient. After the
targeted area absorbs the substance, the radioactive nuclei undergo
beta plus decay and the positrons that are created collide almost
immediately with the electrons they encounter. The mass of both
particles is converted into two gamma rays that travel outward and
away from each other in exact opposite directions.
Gamma ray detectors that surround the patient register and measure
these events. After algorithms are applied to the data, an image is
constructed that shows areas where radioactivity is concentrated.
These areas indicate signs of metabolic activity, giving clues to
where tumors are or providing information about physiologic function
to help diagnose disease.
See the full set of
String Theory Resources
"The Elegant Universe" activities align with the following National
Science Education Standards.
Grades 9-12
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Science Standard B: Physical Science
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Structure of Atoms:
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Matter is made of minute particles called atoms, and atoms are
composed of even smaller components. These components have
measurable properties, such as mass and electrical charge. Each
atom has a positively charged nucleus surrounded by negatively
charged electrons. The electric force between the nucleus and
electrons holds the atom together.
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The nuclear forces that hold the nucleus of an atom together, at
nuclear distances, are usually stronger than the electric forces
that would make it fly apart.
Structure and Properties of Matter:
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