Download Study Notes Lesson 23 Atomic and Nuclear Physics

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Lesson 23 Atomic and Nuclear Physics
The Fundamental Forces
a.
Four Fundamental Forces: There are four fundamental forces in nature. The gravity is the
weakest force among the four. The strong (nuclear) force is the strongest short-range force that
holds the nucleon (protons and neutrons) together. The electromagnetic force is a very strong force
comparing to gravity. The weak force is another short-range nuclear force that is responsible for
the decay of some nuclear particles.
Force
Strength
Range
b.
Strong
1
10-15 m
Electromagnetic
1/137
∞
Weak
10-6
10-18 m
∞
In modern physics, particles are force carriers, because forces are brought about as a result of an
exchange of particles.
€
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Gravity
6x10-39
€
Classification of Matter
a.
b.
Criteria: particles are classified according to the types
of interactions they have with other particles. If the
force carrier particles (such as gluons, gravitons, etc.)
are excluded, all particles can be classified into two
groups – hadrons and leptons.
Hadron – a particle that interacts through all four
fundamental forces is called a hadron. The hadrons
can be subdivided into baryons and mesons according
to their baryon number. The baryon number is
defined as:
B=
where
N −N
q
except strong force
electrons,
positions,
neutrinos
all four forces
protons,
neutrons
no. of quark –
no. of antiquark = 3
(B =1)
no. of quark =
no. of antiquark (B = 0)
q
3
N is the number of quarks, and
q
N is the number of antiquarks.
Both of them are composed of more fundamental
€particles called quarks.
€
q
c. €Baryons – a baryons is an elementary particle that can
be transformed into a proton or neutron and some number of mesons and lighter particles. A
baryon is made of three quarks.
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d.
Mesons – a meson is a particle of intermediate mass. A meson is made of quark and antiquark.
e.
Lepton – a particle that interacts through gravitational, weak, and electromagnetic forces, but not
the strong nuclear force is called a lepton. A lepton is much lighter than a proton. Electrons,
positrons, and neutrinos are all leptons.
f.
Positrons – a positron is a particle whose mass is the same as electron’s, and whose charge is
equal in magnitude but opposite in sign to the electron’s.
g.
Neutrino – a neutrino is a neutral particle that has little mass but does possess both energy and
momentum.
The Antiparticle
a.
Antiparticle: an antiparticle is associated with each particle. An antiparticle is a particle having
mass, lifetime, and spin identical to the associated particle, but with charge of opposite sign (if
charged) and magnetic momentum reversed in sign.
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Physics Study Notes
b.
Lesson 23 Atomic and Nuclear Physics
Antiparticle examples: The antiparticle of proton (p) is called antiproton ( p ). The antiparticle of
electron (e) is called positron (e+). The antiparticle of neutron (n) is called antineutron ( n). The
antiparticle of neutrino (ve / v / v ) is called antineutrino ( v / v / v ).
µ
τ
e
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µ
τ
c.
€ composed of antiparticles.
Antimatter: The antimatter is a material consisting of atoms that are
€
d.
Annihilation: Annihilation is defined as "total destruction" of an object. The word is used to
€
denote the process that occurs when a subatomic particle collides with its associated antiparticle.
When a low-energy electron annihilates a low-energy positron (anti-electron), they can only
produce two or more gamma ray photons. However, high-energy particle colliders produce
annihilations where a wide variety of exotic heavy particles are created.
The Quark
a.
b.
Baryons and mesons both are composed of more
fundamental particles called quarks. Quarks have
charges of ± 31 e and ± 32 e , from which many of the
elementary particles may be built up. A quark can have
charge
The quarks are named up, down, charm, strange, top
€and bottom.
€ Every baryon is consisting of three quarks
and every meson is consisting of a quark and an
antiquark. An antiquark has electric charge, baryon
number, and strangeness opposite in sign to that of the
corresponding quark.
c.
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The proton is made of three quarks: uud (up, up,
down) and neutron is made of three quarks: udd (up,
down, down). When quarks combine to form baryons,
their charges add algebraically to a total of 0, +1, or -1.
Regents Exam Questions
a.
A particle unaffected by an electric field could have a quark composition of (1) css (2) bbb (3) udc
(4) uud. (Q45, January 2008)
b.
What fundamental force holds quarks together to form particles such as protons and neutrons? (1)
electromagnetic force (2) gravitational force (3) strong force (4) weak force. (Q35, January 2007)
c.
What is the total number of quarks in a helium nucleus consisting of 2 protons and 2 neutrons? (1)
16 (2) 12 (3) 8 (4) 4. (Q51, January 2007)
d.
The charge of an antistrange quark is approximately (1) +5.33 ×10–20C (2) –5.33 ×10–20C (3)
+5.33 ×1020C (4) –5.33 ×1020C. (Q34, January 2007)
e.
A top quark has an approximate charge of (1) –1.07 ×10–19C (2) –2.40 ×10–19C (3) +1.07 ×10–19C
(4) +2.40 ×10–19C. (Q34, June 2006)
f.
A lithium atom consists of 3 protons, 4 neutrons, and 3 electrons. This atom contains a total of (1)
9 quarks and 7 leptons (2) 12 quarks and 6 leptons (3) 14 quarks and 3 leptons (4) 21 quarks and 3
leptons. (Q51, June 2006)
g.
The diagram below represents the sequence of events (steps 1 through 10) resulting in the
production of a D–meson and a D+meson. An electron and a positron (antielectron) collide (step
1), annihilate each other (step 2), and become energy (step 3). This energy produces an anticharm
quark and a charm quark (step 4), which then split apart (steps 5 through 7). As they split, a down
quark and an antidown quark are formed, leading to the final production of a D– meson and a
D+meson (steps 8 through 10).
Mr. Lin
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Physics Study Notes
Lesson 23 Atomic and Nuclear Physics
Which statement best describes the changes that occur in this sequence of events?
(1) Energy is converted into matter and then matter is converted into energy.
(2) Matter is converted into energy and then energy is converted into matter.
(3) Isolated quarks are being formed from baryons.
(4) Hadrons are being converted into leptons. (Q35, January 2008)
h.
A tau lepton decays into an electron, an electron antineutrino, and a tau neutrino, as represented in
the reaction below.
τ → e+v +v
e
τ
On the equation in your answer booklet, show how this reaction obeys the Law of Conservation of
Charge by indicating the amount of charge on each particle. (Q62, January 2008)
i.
€
If a proton were to combine with an antiproton, they would annihilate each other and become
energy. Calculate the amount of energy that would be released by this annihilation. (Q58, June
2006)
Answer Keys:
No.
Answer
Explanation
a
(1) css
An electrically neutral particle will be unaffected by an electric field. Only css
has the total charge equals to 0. (+2/3e) + (-1/3e) + (-1/3e) = 0
b
(3) strong force
The strong nuclear force is the force to hold protons and neutrons together.
c
(2) 12
Both protons and neutrons are baryons. Each baryon consists of 3 quarks.
Total number of quarks is (2 + 2) × 3 = 12
d
(1) +5.33 ×10–20C
The charge of strange quark is -1/3e, so, the charge for antistrange quark is
+1/3e = (1/3) ×1.6×10-19C = 5.33×10-20C.
e
(3) +1.07 ×10–19C
The charge of top quark is +2/3e, so, the charge for top quark is +2/3e = (2/3)
×1.6×10-19C = 1.07×10-19C.
f
(4) 21 quarks and 3
leptons
Both protons and neutrons are baryons. Each baryon consists of 3 quarks. So,
Total number of quarks is (3 + 4) × 3 = 21. Since electrons are leptons, the
total number of leptons is 3.
g
(2) Matter is converted
into energy and then
energy is converted
into matter.
An electron and a positron collided and annihilated to become energy, then the
energy further converted to mass.
h
(-1e) → (-1e) + (0e) +
(0e)
It obeys the Law of Conservation of Charge.
i
3.01 × 10 –10 J
E = mc 2, E = 2 × (1.67 × 10 –27 kg)(3.00 × 10 8 m/s) 2 = 3.01 × 10 –10 J
Mr. Lin
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