General Physics PHY 1112

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General Physics (PHY 1112)
Lecture 39

• Modern Physics
• Nuclear and Particle Physics
• Nuclear Energy
• Elementary particles

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Lightning Review

• Last lecture
• Nuclear physics
• Nuclear reactions

Review Problem A beam of particles passes
undeflected through crossed electric and magnetic
fields. When the electric field is switched off,
the beam splits up in several beams. This
splitting is due to the particles in the beam
having different A. masses. B. velocities. C.
charges. D. some combination of the above E.
none of the above
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Processes of Nuclear Energy

• Fission
• A nucleus of large mass number splits into two
  smaller nuclei
• Fusion
• Two light nuclei fuse to form a heavier nucleus
• Large amounts of energy are released in either
  case

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Nuclear Fission

• A heavy nucleus splits into two smaller nuclei
• The total mass of the products is less than the
  original mass of the heavy nucleus
• First observed in 1939 by Otto Hahn and Fritz
  Strassman following basic studies by Fermi
• Lisa Meitner and Otto Frisch soon explained what
  had happened
• Fission of 235U by a slow (low energy) neutron
• 236U is an intermediate, short-lived state
• X and Y are called fission fragments
• Many combinations of X and Y satisfy the
  requirements of conservation of energy and charge

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Sequence of Events in Fission

• The 235U nucleus captures a thermal (slow-moving)
  neutron
• This capture results in the formation of 236U,
  and the excess energy of this nucleus causes it
  to undergo violent oscillations
• The 236U nucleus becomes highly elongated, and
  the force of repulsion between the protons tends
  to increase the distortion
• The nucleus splits into two fragments, emitting
  several neutrons in the process

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Energy in a Fission Process

• Binding energy for heavy nuclei is about 7.2 MeV
  per nucleon
• Binding energy for intermediate nuclei is about
  8.2 MeV per nucleon
• Therefore, the fission fragments have less mass
  than the nucleons in the original nuclei
• This decrease in mass per nucleon appears as
  released energy in the fission event
• An estimate of the energy released
• Assume a total of 240 nucleons
• Releases about 1 MeV per nucleon
• 8.2 MeV 7.2 MeV
• Total energy released is about 240 Mev
• This is very large compared to the amount of
  energy released in chemical processes

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In the first atomic bomb, the energy released was
equivalent to about 30 kilotons of TNT, where a
ton of TNT releases an energy of 4.0 109 J. The
amount of mass converted into energy in this
event is nearest to (a) 1 ?g, (b) 1 mg, (c)
1 g, (d) 1 kg, (e) 20 kilotons
QUICK Problem
(c). The total energy released was E (30 103
ton)(4.0 109 J/ton) 1.2 1014 J. The mass
equivalent of this quantity of energy is
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Chain Reaction

• Neutrons are emitted when 235U undergoes fission
• These neutrons are then available to trigger
  fission in other nuclei
• This process is called a chain reaction
• If uncontrolled, a violent explosion can occur
• The principle behind the nuclear bomb, where 1 g
  of U can release energy equal to about 20000 tons
  of TNT

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Nuclear Reactor

• A nuclear reactor is a system designed to
  maintain a self-sustained chain reaction
• The reproduction constant, K, is defined as the
  average number of neutrons from each fission
  event that will cause another fission event
• The maximum value of K from uranium fission is
  2.5
• In practice, K is less than this
• A self-sustained reaction has K 1

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Basic Reactor Design

• Fuel elements consist of enriched uranium
• The moderator material helps to slow down the
  neutrons
• The control rods absorb neutrons
• When K 1, the reactor is said to be critical
• The chain reaction is self-sustaining
• When K lt 1, the reactor is said to be subcritical
• The reaction dies out
• When K gt 1, the reactor is said to be
  supercritical
• A run-away chain reaction occurs

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• Elementary Particles

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1. The Big Question of Particle Physics
How did we get from here to here?
And what does it have to do with heavy
quarks?
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Time
Seems like
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• Just after the Big Bang
• symmetric Universe
• equal number of particles and antiparticles
• Now
• asymmetric Universe
• planets, stars, galaxies, Wayne State,

Note macroscopic laws of Nature do not
distinguish matter and antimatter
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A 10,000,000.00 Swedish Kronor questionWhere
did all the antimatter go?

• The Onion paradigm
• identify degrees of freedom
• see if the problem has a solution
• if not, dig deeper

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What are the right degrees of freedom?

• Fire
• Water
• Earth
• Air
• that is, according
• to the Greeks!

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What would be the modern picture?
Imagine that we have a very powerful microscope
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Modern understanding the onion picture
Atom
Lets see whats inside!
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Modern understanding the onion picture
Nucleus
Lets see whats inside!
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Modern understanding the onion picture
Protons and neutrons
Lets see whats inside!
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Modern understanding the onion picture
Collective name for particles containing 3 quarks
Mesons and baryons
Collective name for particles containing quark
and antiquark
Lets see whats inside!
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Modern understanding the onion picture
Collective name for particles containing 3 quarks
(such as proton and neutron)
Mesons and baryons
Collective name for particles containing quark
and antiquark
Lets see whats inside!
Note apparent excess of matter over antimatter
can be traced to excess of the number of baryons
over antibaryons. Thus our Big Problem is called
Problem of Baryon Asymmetry of the Universe.
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Modern understanding the onion picture
Quarks and gluons
Lets see whats inside!
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Modern understanding the onion picture
?
so the answer depends on the energy scale!
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same thing about the interactions
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Unification of forces
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The Standard Model of particle physics
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The Standard Model of Elementary Particle Physics

• Periodic table of matter
• Interactions electromagnetic, weak, strong,
  (gravity)
• Contains 26 parameters needs experimental
  input

Higgs particle
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Conditions for baryon asymmetry
Matter-antimatter imbalance in the Universe
A.D. Sakharov

• Baryon (and lepton) number - violating processes
• to generate asymmetry
• Universe that evolves out of thermal equilibrium
• to keep asymmetry from being
  washed out
• Matter interactions differ from antimatter
  interactions (Microscopic CP-violation)
• to keep asymmetry from being
  compensated in the anti-world

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Can Standard Model explain baryon asymmetry?

• does it have the right stuff?
• what are the conditions for the baryon
  asymmetry?
• does it have enough of the right stuff?

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Experimental methods
video
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Experimental methods
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Experimental Facilities I
Cornell University
SLAC
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Experimental Facilities II
KEK (Japan)
Fermilab (Batavia, IL)
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What do physics PhDs do?

• Science route
• Research in physics (national lab, research
  university)
• Teaching and research (college)
• Industry route
• Computing/engineering jobs in companies
• Finance industry (problem solving)
• Scientific Publishing route

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A couple of review problems and notes to remember
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Remember

• Electricity
• Electric field and electric potential are
  different things
• Moreover, field is a vector while the potential
  is a scalar
• Remember the difference between parallel and
  series connections
• Remember that formulas for capacitors and
  resistors are reversed
• Magnetism
• Use right hand rule properly
• Special relativity
• If the problem involves speeds close to the speed
  of light, use relativistic formulas for momentum,
  energy, addition of velocities
• In particular, KEmv2/2 is a NONRELATIVISTIC
  expression for KE
• Atomic and nuclear physics
• In a way of handling, nuclear reactions are very
  similar to chemical reactions

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Example Proton moving in uniform magnetic field

• A proton is moving in a circular orbit of radius
  14 cm in a uniform magnetic field of magnitude
  0.35 T, directed perpendicular to the velocity of
  the proton. Find the orbital speed of the proton.

Given r 0.14 m B 0.35 T m 1.67x10-27
kg q 1.6 x 10-19 C
Recall that the protons radius would be
Thus
Find v ?