DOC PREVIEW
UW-Madison PHYSICS 107 - Particles and fields

This preview shows page 1-2 out of 5 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 5 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 5 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 5 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

1Wed. Apr. 25, 2007 Phy107 Lect. 351Particles 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 fieldToday: more fields andmore particles!Essay due next week Wed., May 2Wed. Apr. 25, 2007 Phy107 Lect. 352• Modern view is thatparticles, fields, and forcesare intertwined.• The electromagnetic fieldis quantized.• It can be in many differentenergy states– Ground state (lowest energy)– First excited state(next highest energy)Wed. Apr. 25, 2007 Phy107 Lect. 353Interactions between chargesThe positively charged rod attracts negativecharges to the top of the electroscope.This leaves positive charges on the leaves.The like-charges on the leaves repel each other.attractive force between positive andnegative charges.repulsive force between two positiveor two negative chargeWhy did the electrons flow?Wed. Apr. 25, 2007 Phy107 Lect. 354Force 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 particleWed. Apr. 25, 2007 Phy107 Lect. 355Electric field lines• Faraday invented the idea of field linesfollowing 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.Wed. Apr. 25, 2007 Phy107 Lect. 356Magnetism: Permanent magnets• North Pole and South Pole• This is the elementarymagnetic particle• Called magnetic dipole(North poleand south pole)• There are no magnetic‘charges’NSNSNSSNLikesattractUnlikesrepel2Wed. Apr. 25, 2007 Phy107 Lect. 357Field lines of a magnet• Field lines indicatedirection of force• Density indicatesstrength of force• Similar toelectrostatic force,but force is felt bymagnetic dipoleWed. Apr. 25, 2007 Phy107 Lect. 358Electromagnetic wave• Oscillating electric and magnetic fields• Present without associated charges or currents• Fields exist on their own.Wed. Apr. 25, 2007 Phy107 Lect. 359Particles and fields• “Field Theory” says that everything is a field• Even particles.• Particles are quanta of a corresponding field.• What does this mean?• Think about photons.– One photon means the electromagnetic field has(Planck’s const)x(frequency) = hf of energy.– Two photons means 2hf of energy.Wed. Apr. 25, 2007 Phy107 Lect. 3510Quanta of the EM field• Possible energies for green light (λ=500 nm)E=hfE=2hfE=3hfE=4hf– One quantum of energy:one photon– Two quanta of energytwo photons– etc• A photon is an excitationof the EM field.•Quantum mechanically, brightness can only bechanged in steps, with energy differences of hf.Wed. Apr. 25, 2007 Phy107 Lect. 3511QuestionConsider the field for red photons (620 nmwavelength). How much energy is requiredto excite the field from the 3rd quantumstate to the 5th quantum state?A. 1 eVB. 2 eVC. 4 eVD. 200 eVE. 4 MeV3rd quantum state: 3 photons5th quantum state: 5 photons—requires 2 photons—Each has energy hc/λ=2eVWed. Apr. 25, 2007 Phy107 Lect. 3512How is EM (photon) field excited?• Charged particle can excite the EM field.– A photon is producedelectron electronphoton• Around a charged particle,photons continually appear anddisappear.Here the photon field is excited by an electron, thenreabsorbs the energy and the photon disappears3Wed. Apr. 25, 2007 Phy107 Lect. 3513Other 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.Wed. Apr. 25, 2007 Phy107 Lect. 3514Electrons 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 -Feynman studied the idea that all QEDprocesses reduce Feynman diagram.electron electronphotonEmission of a photonQED: First component of theStandard Model of particle physics.Wed. Apr. 25, 2007 Phy107 Lect. 3515Uncertainty 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.Wed. Apr. 25, 2007 Phy107 Lect. 3516Energy 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!Wed. Apr. 25, 2007 Phy107 Lect. 3517QuestionThis Feynman diagram shows twoelectrons and a photon.Here, electron 1A. Changes energy onlyB. Changes momentum onlyC. Neither energy nor mom. changedD. Changes both energy and mom.E. This interaction cannot occurElectron 1Electron 2photonλ1λ1λ2λ2Wed. Apr. 25, 2007 Phy107 Lect. 3518Quantum Electrodynamics: QED• This is the Coulomb interaction.• Normal electromagnetic force comes about fromexchange of photons.Electromagneticrepulsion via emissionof a photonelectronelectronphoton• Attraction a bit moredifficult to visualize.time4Wed. Apr. 25, 2007 Phy107 Lect. 3519Interactions between particles• The modern view of forcesis in terms of particle exchange.• These are ‘virtual’ particles of the fieldscreated by the particle


View Full Document

UW-Madison PHYSICS 107 - Particles and fields

Documents in this Course
Exam 2

Exam 2

7 pages

Lect 01

Lect 01

5 pages

Exam 1

Exam 1

7 pages

Exam 3

Exam 3

48 pages

Exam 2

Exam 2

6 pages

Load more
Download Particles and fields
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Particles and fields and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Particles and fields 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?