1Phy107 Fall 20061Exam Results•Exam:–Exam scoresposted onLearn@UW• No homeworkdue next weekF D C BC B AB APhy107 Fall 20062Particles and fields• We have talked about several particles– Electron, photon, proton, neutron, quark• Many particles have internal constituents– Not fundamental: proton and neutron• We have talked about various forces– Electromagnetic, strong, weak, and gravity• And some fields…– Electric field–Magnetic field• Modern view is that particles, forces, andfields are intertwined - and all quantizedPhy107 Fall 20063Force between chargesOpposite charges attractLike charges repel.• Other than the polarity, they interact much likemasses interact gravitationally.• Force is along the line joining the particles.Gravitational force: FG=GM1M2/ r2G=6.7x10-11 Nm2/kg2Electrostatic force: FE = k Q1 Q2 /r2k = 9x109 Nm2/C2+—Force on positive particledue to negative particlePhy107 Fall 20064Electric field lines• Faraday invented the idea of the Electricfield and field lines following the force tovisualize the electric field.Field lines emanate frompositive charge and terminateon negative charge.Local electric field is samedirection as field lines.Force is parallel or antiparallelto field lines.Charged particle will movealong these field lines.Phy107 Fall 20065Quanta of the EM field• What about quantum mechanics? What would thattell us about electric, magnetic… fields?– Field strength should be quantized– Quantization small, not noticeable at large field strengthor large times– However, for small strength or over a very short timemight be noticeable– Example: an electron flying be another electron veryquickly - Only time to have one quanta of repulsion occur• Quanta of the field– Need to name the thing that conveys the repulsion– What particle is mixed up in electricity and magnetism:The photon!Phy107 Fall 20066Other particles and fields• Electromagnetic field spread out over space.– Stronger near the the source of the electric/magneticcharge - weaker farther away.• Electromagnetic radiation, the photon, is the quantaof the field.• Describe electron particles as fields:– Makes sense - the electron was spread out around thehydrogen atom.– Wasn’t in one place - had locations it was more or lessprobable to be. Stronger and weaker like theelectromagnetic field.• Electron is the quanta of the electron field.2Phy107 Fall 20067How is EM (photon) field excited?• Charged particle can excite the EM field.• Around a charged particle, photons continuallyappear and disappear.Represented by a‘Feynman diagram’.Electron can excite the EMfield, creating a photonelectron electronphotonPhy107 Fall 20068Electrons and Photons:Quantum Electrodynamics: QED• QED is the relativistic quantum theory ofelectrons and photons, easily generalized toinclude other charged particles.• to photon emission or absorption which maybe represented by a simple diagram - aFeynman studied the idea that all QEDprocesses reduce Feynman diagram.electron electronphotonEmission of a photonQED: First component of theStandard Model of particle physics.Phy107 Fall 20069Quantum Electrodynamics: QED• If another charged particle is near, it can absorbthat photon.• Normal electromagnetic force comes about fromexchange of photons.Electromagneticrepulsion via emissionof a photonelectronelectronphoton• Attraction a bit moredifficult to visualize.timePhy107 Fall 200610Uncertainty principle• The uncertainty principle is important forunderstanding interaction in quantum field theory.• We talked about an uncertainty principle, thatmomentum and position cannot be simultaneouslydetermined.• There is an equivalent relation in the time andenergy domain.– Einstein's relation that space and time or momentumand mass/energy are similar.Phy107 Fall 200611Energy uncertainty• To make a very short pulse in time,need to combine a range of frequencies.• Frequency related to quantum energy by E=hf.• Heisenberg uncertainty relation can also bestated(Energy uncertainty)x(time uncertainty)~ (Planck’s constant)In other words, if a particle of energy Eonly exists for a time less than h/E,it doesn’t require any energy to create it!Phy107 Fall 200612Interactions between particles• The modern view of forcesis in terms of particle exchange.• These are ‘virtual’ particles of the fieldscreated by the particle charges.This shows Coulombrepulsion between twoelectrons. It is described asthe exchange of a photon.Momentum is uncertainover the short time: Couldbe negative: attraction3Phy107 Fall 200613Forces and particles‘Classical’ collision Interaction by particle exchangePhy107 Fall 200614Interactions betweenchargesThe like-charges on the leaves repel eachother.This repulsion is the Coulomb forceModern view of Coulombrepulsion between twoelectrons. It is described as theexchange of a photon.Phy107 Fall 200615Electrons and Photons• Non virtual interactions possible also:Photon is a real particle that is seen beforeor after the interaction• Photon could be absorbed by the electron:Photoelectric effect• Could be emitted by the electron:Decay from an excited state.• Still a QED interaction• Diagrams rotatedelectron electronphotonelectronelectronphotonPhy107 Fall 200616QED: Rotated Diagrams• Can rotate other diagramselectronelectronphotonWhat is an electron going backward in time?electronelectronphoton??????TimePhy107 Fall 200617Antiparticles• Several physicists had an explanation.• Antimatter!• There is a particle with exact same mass aselectron, but with a positive charge.• It is called the positron.• All particles have an antiparticle.• We’ve seen this particle before. Nuclear betadecay with a positive electron - positron.Phy107 Fall 200618Pair production, annihilation• Electron and positron can ‘annihilate’to form two photons.• Photon can ‘disappear’to form electron-positron pair.• Relativity: Mass and energy are the same– Go from electron mass to electromagnetic/photon energy4Phy107 Fall 200619Seeing antiparticles• Photons shotinto a tank ofliquid hydrogenin a magneticfield.• Electrons andpositrons bendin oppositedirections and,losing energyto ionization,spiral to rest.Phy107 Fall 200620Annihilation questionIf you annihilate an electronand a positron what energywavelength/type ofphotons(two) are made.Electron mass: 0.5 MeV/c2A. 2.5m radio waveB. 2.5um infraredC. 2.5pm x-rayPhy107
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