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Transcript

How Particles Interact Particle Physics Lesson 5 Today’s Objectives Describe how forces are caused by particle exchange. State that for the electromagnetic force, virtual photons are the exchange particles Draw Feynman Diagrams for the following interactions: Neutron-neutrino interaction Proton-antineutrino interaction β− decay β+ decay Electron capture Homework Know what exchange particles do PPQ Learn the four fundamental forces of nature and their exchange particles. Learn the Feynman diagrams for the following interactions: Neutron-neutrino interaction Proton-antineutrino interaction β− decay β+ decay Electron capture How Hard Am I Trying? Effort Descriptor Estimated Result Example Result (Target Grade = B) Complete extra questions out of the book. (about Target Grade + 1 1 hour independent study per hour in class.) A Complete all homework questions, seeking help when you are stuck or don’t understand something at workshop or on Wikispaces. B Target Grade Complete all homework leaving questions blank Target Grade – 1 when you can’t do them, asking questions in class. C Complete homework half the time, got stuck but couldn’t get help. Passively listening in class. Target Grade – 2 D Generally don’t complete homework but try your best to learn it all just before exams. Target Grade – 3 E What homework? Oh I lost the sheet! Target Grade – 4 U What does the Evidence show? I have about half the homework – which I have marked – it was done pretty well target grade performance. Does this mean half the class is on track to get a Target Grade -3 or -4? Solve the Following charge flowing into a junction = charge flowing out Video In Search of Giants (12 of 15) QED - The Jewel of Physics.wmv Exchange Particles What causes forces? Newton’s 2nd Law states that force is equal to the rate of change of momentum. Momentum is calculated by multiplying the mass and the velocity of a particle. Richard Feynman (1918 – 1988) came up with the idea that the forces are transmitted by exchange particles. Feynman Diagram Is a pictorial representation of what is going on. First devised by American particle physicist Richard Feynman (1918 – 1988). The forces between electrically charged particles are thought to be transmitted by photons, which are emitted and absorbed by the particles. Weak Nuclear Force Which force is involved when a neutron changes to a proton or proton neutron? Can’t be electromagnetic because neutrons have no charge. Must be weaker than the strong force otherwise it would affect stable nuclei. Therefore it is called the weak nuclear force. Weak Nuclear Force II Due to the exchange of particles called W bosons. They are unlike photons in that they have:Non zero rest mass A very short range (less than 0.001 fm!) Are positively or negatively charged. Four Fundamental Forces Type of Interaction Strong Exchange Particle/ Particles Affected Gauge Boson Gluon Hadrons only Electromagnetic Photon Weak W+,W−,Z0 Charged particles only All types Gravity Graviton? All types Feynman diagrams They were developed by Richard Feynman to decribe the interactions in quantum electrodynamics (QED). The diagrams are used to describe a variety of particle interactions. Virtual particles are represented by wavy or broken lines and have no arrows. Graphical ways to represent exchange forces, time is upward but the lines don’t represent the paths the particles take. Neutron-Neutrino Interaction Proton-Antineutrino Interaction − β decay + β decay Electron Capture Exchange Particles Exchange particles do the following:Transfer momentum Transfer Energy Transfer Charge (in the case of W+ and Wbosons.) Summary The are four fundamental forces of nature: gravity, electromagnetic, strong & weak. Each force is thought to have a corresponding exchange particle that transfer the momentum. Feynman diagrams are a way of summarising the interactions.