Edexcel GCE A2 Physics Unit 4 Topic 3 Particle physics_lesson notes

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


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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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Edexcel GCE A2 Physics Unit 4 Topic 3 Particle physics_lesson notes