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Particles and Quantum Phenomena

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Luke Fessey

on 12 December 2012

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Transcript of Particles and Quantum Phenomena

Particles and Quantum Phenomena - Matter and Radiation Matter and Radiation - Inside the Atom Matter and Radiation - Stable and Unstable Nuclei Matter and Radiation - Stable and Unstable Nuclei Matter and Radiation - Particles and Antiparticles 1911 - Rutherford's experiment revels concentrated central positive charge

Electrons orbit positive nucleus because of electrostatic force

Electrons are 1/1800th mass of proton/neutron (nearly identical masses)

Proton has equal and opposite charge to electron Strong Nuclear Force holds protons and neutrons together

Overcomes electrostatic repulsion between protons

Very short range - 1fm (0.000 000 000 000 001m)

Electrostatic force has infinite range but decreases in strength with range Three types of radioctive decay: Antimatter - Identical to matter but has opposite quantum properties. There is an antimatter counterpart for all matter

Predicted by Paul Dirac in 1928

Based on Einstein's

Dirac predicted matter and antimatter would annihilate to release rest energy

Also predicted pair production; a photon with sufficient energy would convert its energy to mass producing a particle and its antiparticle which would move away Matter and Radiation - Inside the Atom Matter and Radiation - Inside the Atom Specific Charge = Charge (C)/ Mass (kg) Isotope: Atoms with the same number of protons and different number of neutrons

Isotope notation: Alpha - Helium nuclei are emitted Beta - Consists of fast-moving electrons, happens when neutron changes into a proton and an antineutrino is emitted Gamma - EM radiation emitted by an unstable nucleus. It is effective at penetrating materials but not at ionising them. It has no mass or charge. It is emitted by a nucleus with excess energy after beta or alpha emission Matter and Radiation - Stable and Unstable Nuclei It was observed that beta particles were emitted with varying kinetic energies but which left some energy lost unaccounted for

It was predicted that some energy would be lost through antineutrinos and neutrinos

Discovered 20 years later because of interaction with cadium nuclei in a large water tank Matter and Radiation - Photons Wavelength = c/f EM Spectrum: EM wave consists of electric and magnetic wave vibrating at right angles to each other and to the wave direction. They are in phase (peak together) Matter and Radiation - Photons EM waves are emitted when a charged particle loses energy (when an electron slows/ changes direction or moves to a lower energy orbital shell)

Emitted in random direction as packet of EM waves (photon)

Photon theory established in 1905 as Einstein's explanation of photoelectricity

Photon Energy = hf

h = Planck's Constant = 0.000000000000000000000000000000000663 Js

A laser is a constant flow of same frequency photons

Power of beam = nhf Matter and Radiation - Particles and Antiparticles Energy of particles is often measured in electron volts (the energy transfreed when an electron accelerates through 1 volt)

First antiparticle detected was positron by Carl Anderson in 1932

Observed a particle in a cloud chamber that deflected in the opposite direction to an electron Minimum energy of each photon produced in annihilation = h x f(min)

Minimum energy of photon needed in pair production = h x f(min) Pair Production: Annihilation: Matter and Radiation - How Particles Interact Richard Fenyman stated that EM force between charged objects is due to exchange of virtual photons

Can't be detected directly because intercepting them would stop the force

He represented these interactions with Feynman Diagrams

WNF causes nuclear decay

Does not affect stable nuclei so must be weaker than SNF

Bosons for WNF are W+, W-, and Z

These bosons: have a non-zero rest mass; have a range >0.001fm; can be charged (W+ or W-) Matter and Radiation - How Particles Interact W bosons transfer charge in decay as seen below where W bosons remove charge from the initial particle Matter and Radiation - How Particles Interact Sometimes a proton can interact with an inner shell electron

The proton interacts via the Weak Force and becomes a neutron

Can also occur if a proton collides with an electron at very high velocity Particles and Quantum Phenomena - Quarks and Leptons Quarks and Leptons - The Particle Zoo Quarks and Leptons - The Particle Zoo Cosmic rays (protons and small nuclei) collide with atoms in the atmosphere

Creates short-lived particles and antiparticles not found on Earth eg: muons, kaons, and pions Hidekei Yukawa predicted bosons existing for SNF

Predicted range pf >0.000000000000001m and mass between that of proton and electron

Hidekei named them mesons

A year later Carl Anderson discovered evidence of possible meson but it didn't decay quickly enough and was found to be a muon

Cecil Powell discovered first meson - pions - in high altitude experiment with photographic emulsion exposed to cosmic rays Quarks and Leptons - The Particle Zoo Kaons discovered less than a year after pions

Kaons: short lived; produced in strong interaction; decay through weak interaction but can decay to pions

Kaons and other particles were hence termed 'strange particles'

These new particles can be created in accelerators by colliding protons Quarks and Leptons - Particle Sorting Particles are sorted into matter and antimatter

Matter is then separated into hadrons (interact through strong force) and leptons (can't interact through strong force)

Hadrons are further divided into baryons (3 quarks) and mesons (quark - antiquark pair) Quarks and Leptons - Leptons at Work Leptons can interact with anti-leptons to produce hadrons

Neutrinos travel at nearly the speed of light but interact very little

Different and corresponding neutrinos produced by different lepton interactions

Leptons cannot be broken down into non leptons - they are fundamental Quarks and Leptons - Leptons at Work In lepton-hadron interactions neutrinos/ antineutrinos can change into/ from a corresponding charged lepton

Muons decay into muon neutrinos, producing an electron and an electron antineutrino to conserve charge and lepton number Quarks and Leptons - Quarks and Antiquarks Kaons called 'V particles' when discovered because of v-shaped tracks they made in cloud chambers

Were called 'strange particles' because they only decayed to pions and protons

Certain interactions seemed allowed but did not happen suggesting an unknown quantum property

Strangeness was introduced to explain why these interactions did not happen and was always conserved in strong interactions Quarks and Leptons - Quarks and Antiquarks Properties of hadrons ( eg. charge, rest mass, strangeness) explained by assuming they are composed of smaller particles (quarks)

Three types of quark used to explain these quantum properties: up, down, and strange Quarks and Leptons - Quarks and Antiquarks In radioactive decay, quarks change to affect changes of the particles they make up

In beta- decay a neutron changes to a proton, this is because a down quark changes to an up quark

In beta+ decay a proton changes into a neutron, this is because an up quark changes to a down quark Quarks and Leptons - Conservation Rules Energy and charge are conserved in all interactions in science

In decays and particle - antiparticle interactions these properties must be conserved: lepton number, strangeness (except in weak interactions), and baryon number

Lepton number must also be conserved within different families of lepton Particles and Quantum Phenomena - Quantum Phenomena Quantum Phenomena - Photoelectricity It's the emission of electrons from a metal surface when the surface is illuminated by light with a frequency above the metal's threshold frequency

Discovered when Hertz observed strength of sparks in his spark gap increased when UV light was used instead of radio waves

Couldn't be explained by wave theory - threshold frequency had no explanation, and it couldn't explain why photoelectric emission occurred without delay Quantum Phenomena - Photoelectricity Einstein suggests photon theory of light in 1905

Assumed light was composed of packets of energy (photons)

Energy of photon = frequency x Planck constant

When light is incident on metal surface, an electron in the metal can absorb (only) one photon, gaining energy equal to that of the photon

Work function is the minimum energy needed for an electron to escape the metal surface

Hence KE max of emitted electron = (h x f) - work function

Emission can occur from a surface of zero potential, thus fmin = work function/ h Quantum Phenomena - Collisions of Electrons With Atoms Ionisation is the process of changing a neutral atom into a charged one by adding or removing electrons

Ionisation energy is the energy required to remove the outermost electron from an atom

Ionisation energies vary according to the atom, as do excitation energies

Excitation by collision is when an electron from an inner shell in an atom absorbs the energy of an electron that collides with the atom, and moves to a higher energy level but is not removed Quantum Phenomena - Energy Levels in Atoms Electrons can only orbit a nucleus within specific orbits (shells) each with its own constant energy value

Lowest energy level is called the ground state

Electrons can absorb energy from photons but only of certain frequencies because there are specific energy values for each electron orbit

Energy of a photon must exactly equal energy needed to jump to a shell (excitation by photons) or it will not be absorbed Quantum Phenomena - Energy Levels in Atoms De-excitation is what causes low pressure gases to glow when electric current is passed through them

Electrons absorb energy from excitation by collision but they do not retain it for long

When an electron is excited it leaves a vacancy in a lower energy level shell which is filled by an electron dropping down an energy level, this causes it to emit energy as a photon

The emitted photons must be of certain frequencies as the electron must lose energy equal to the energy difference between its energy shell and the shell it is dropping to

Fluorescence is the de-excitation of electrons and the emission of photons in the visible spectrum of light Quantum Phenomena - Energy Levels and Spectra Line spectrums are produced as a result of excitation and de-excitation of electrons in different elements

The photons released when an electron de-excites have unique frequencies because the jumps between energy levels are unique to each element

When emitted light from an element is analysed, the identity of the element can be determined by which frequency photons are emitted

The wavelength of the photons = c/f Quantum Phenomena - Wave Particle Duality Einstein's photon model suggested that light could behave as a particle in addition to behaving like a wave

Light acts as a wave when it diffracts, which only occurs with waves

Light acts as particles in the photoelectric effect which is what lead to the initial confusion over why energy was transferred to the metal's electrons instantaneously

De Broglie suggested in 1923 that matter particles also had a dual wave-particle nature. He stated that a matter's wave-like behaviour was characterised by its de Broglie wavelength which is related to its momentum (p)

The equation for this is: wavelength = h/p Quantum Phenomena - Wave Particle Duality 3 years after de Broglie's hypothesis it was proven that electrons can diffract, confirming that they could behave as waves

An electron in an atom has a fixed value of energy according to which energy shell it is in. This is because its de Broglie wavelength must fit the size and shape of the cell
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