edexcel gce a2 physics unit 4 topic 3 particle physics_lesson notes
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7/29/2019 Edexcel GCE A2 Physics Unit 4 Topic 3 Particle physics_lesson notes
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0. Introduction / Whats the story?1. Structure of the atom2. Probing deeper / cosmic rays
and particle accelerators
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0. Introduction / Whats the
story?
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In search of giants with BrianCoxj.mp/WgjY6h>>
http://j.mp/WgjY6hhttp://j.mp/WgjY6h -
7/29/2019 Edexcel GCE A2 Physics Unit 4 Topic 3 Particle physics_lesson notes
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Fundamental particles
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What holds the nucleus together?
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Quarks
Quarks have an attribute arbitrarily named colour that feels the strongforce. Virtual gluons carry colour between quarks so that the quarkcolours are continuously changing. All observed particles are colourless sin a baryon there must be three continuously changing but different colouand in a meson a quark and an antiquark so that a colour and its anti-colour again give a colourless particle.Colourlessness orcontainment means that isolated quarks and gluons
should never be observed.
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Although pentaquarks[]and glueballs[confined gluonmight seem possible they have not been reliably observed.
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~10-10m
~10-14
~10-15
m
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The strong force acts onlyover very short distances
that separate the quarks in
a nucleon or in the adjacenucleons of a nucleus.
Strong force range~10-15m
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The four fundamental forces
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Virtual particles mediate the fundamental forces
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1. Structure of the atom
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Rutherford scattering
High energy alpha particlesfrom a natural radioisotope
bombard a gold foil [only a f
atoms thick].
Most particles pass through
gold with little or no deflectioVery small number undergo
huge deflections [>90O]
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Rutherford scattering [cont.]
Provides strong evidence fonuclear atom [1911].
Electrons had been discove
by JJ Thompson [1896].
The neutron was confirmed
Chadwick [1932].
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2. Probing deeper / cosmic rays and particaccelerators
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Electron ray gun
Electronsboiled off thecathode and acceleratedtoward the anode. Can obe explained in terms of
negatively chargedelectrons.
Moving es can be deflecby E- and/orB- field.
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Electron diffraction
Electrons behave as waveDe Broglie
=
Electrons diffract as they pthough solids revealing the
atomic/molecular spacing a
arrangements.
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Linac [linear accelerator]
Charged particles
accelerated in E-fielin gaps between driftubes.
Polarity of tubes mu
alternate. Why?
Tubes get longer. W
= =
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Relativistic effects
At high speeds relativistic effects are significant and must betaken into account.
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Cyclotron
E-field accelerates charged
particles across gap betweedees.
B-field provides centripetal
acceleration
= =
2
=
=
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Cosmic rays
Cosmic rays contain particles that were not just protons,neutrons or electrons. Soon there was a zooof so-called
fundamental particles.
This is somewhat like the situation in the 19th century when t
number of elements in the periodic table was >80. Rather tha
having >80 different atoms surely there was something morefundamental than the atom.
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The standard model
Each generation of particles have
the same attributes [or quantumnumbers] except increasing mass
The t and bquarks are only creatin extremely high energy events a
were predicted for reasons ofsymmetry, i.e. so that the quarks
have three generations as do theleptons.
Each particle has a corresponding
antiparticle.
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The large hadron collider [LHC] / CERN
E-fields increase speed of charged
particles. B-fields accelerate particlkeep them moving in a circle of
constant r.
In collisions
= E2
Colliding beams of particles moving
opposite directions transfer more
energy to create particles than a sin
beam hitting a stationary target. Wh
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Conservation laws
In all interactions
charge
mass-energy
momentum
colour
baryon number
lepton number
conserved
In the weak interaction quaflavour is not conserved.
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Non SI units / convenience
1 electron-volt [eV] is the work done to a particle with a chaof 1e [1.6x10-19C] passing through a pd of 1V.
E = QV = 1.6x10-19C x 1V = 1.6x10-19J
1keV = 103 x 1.6x10-19J = 1.6x10-16J
1MeV = 106 x 1.6x10-19J = 1.6x10-13J
1GeV = 109 x 1.6x10-19J = 1.6x10-10J
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