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1. Introduction How to solve the problems? Four types of interactions

Electricity, Magnetism and Light PHYSICS 208, Instructor: Olga Kocharovskaya Lectures 1,2 (Ch.21) Electric Charge and Electric Field. 1. Introduction How to solve the problems? Four types of interactions 2. Electric Properties of the matter Two types of electric charge

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1. Introduction How to solve the problems? Four types of interactions

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  1. Electricity, Magnetism and Light PHYSICS 208, Instructor:Olga KocharovskayaLectures 1,2 (Ch.21)Electric Charge and Electric Field 1. Introduction • How to solve the problems? • Four types of interactions 2. Electric Properties of the matter Two types of electric charge Conductors and Insulators Coulomb’s Law 3. Electric field

  2. How to solve problems? • Solve many before this one • Solve it yourself • Ask for hints but not for solution • Discuss it with your friends • Think about it before you go to sleep (you may find a solution in your dream) • Use symmetry of the problem • Use superposition principle • Do not surrender! • Forget it (not for exam!), then try again • Enjoy the solution!!!

  3. How to make sure your answer is correct? • Get a general (algebraic) solution • Check the units of the answer, make sure you have units consistency • Check limiting cases • Check an order of magnitude, is it REASONABLE? (the speed should not be greater then than that of the light in vacuum, a charge should not be smaller than that of electron, a distance is unlikely to be smaller than 10-15 m in our course, etc.) • Check the answer in the textbook

  4. Four types of interactions • Gravity (between massive bodies) Planetary systems, Galaxies,Space trajectories Weight:F=mg Earth

  5. e 2. Electromagnetic Interactions ( ELECTRICALLY CHARGED BODIES) Structure of ATOMS e

  6. Positive and negative Ions

  7. Structure of the Molecules e e e e H2 Na+Cl-

  8. Chemical reactions and biological processes Water is an excellent solvent due to the dipole character of its molecule

  9. Modern Technologies: internet, telecommunications, nanotechnolgies, CD, DVD, lasers, cell phones,…

  10. 3.Strong Interactions 4.Weak Interactions 27 km ring Hadrons (proton, neutron,…), colour charge Leptons (electron, muon, tau-lepton, neitrinos) lepton charge Interactions between elementary particles, using modern EM technologies Large Hadron Collider (LHC) Counter propagating proton beams accelerated to 7x1012 eV In search for a dark matter

  11. Electric Properties of the matter • Two types of charges: + and – (Ben Franklin,1740) glass silk plastic fur

  12. 2. Quatization of chargeQ=ne, n=1,2,3,…e is the minimum value of charge Particle mass charge electron 9.11×10-31kg -1.60×10-19 C (-e) proton 1.672×10-27kg +1.60×10-19C (+e) neutron 1.674×10-27kg 0 SI : [Q]=1C

  13. 3. Conservation of Charge: e e e e Na+Cl- NaCl q1+q2=0 q1=0 q2=0 Na+ q1=eCl- q2=-e

  14. 4. Three types of materials 1. Conductors (free electrons) Metalls, alloys, plasmas Induction 2. Insulators=Dielectrics (bounded electrons) Glass, plastic, paper Polarization 3. Semiconductors (number of free electrons strongly depends on external conditions such as temperature, electric field , pressure; under the usual conditions number of electrons is small)

  15. 5. Amber effect: Charged and neutral object always attract each other

  16. 6. Charging of neutral objects 1.By friction: q1=0, q2=0 q1=Q q2=-Q 2. By contact q1=Q q2=0 q1+q2=Q q1=Q/2 q2=Q/2 3. By induction

  17. Coulomb’s Law, 1786

  18. Coulomb’s Law For an ensemble of charges use a Superposition Principle:

  19. Example1. Compare the electric and gravity forces between an electron and a proton.

  20. Electric Field, Coulomb’s Law Newton’s Law Q M m r r q • “That one body may act upon another at a distance … • is to me so great an absurdity…” I. Newton • Michael Faraday (1791-1867) • Two steps: • Q creates electric field, • produces the force on q or

  21. Two steps in more details: the source and test charges 1. Q produces electric field at point P indepententely on the presence of charge q at this point: Q Source 2. produces a force on a test charge q: SI units of E: [E]=[F]/[q]=N/C

  22. of a point charge For charged bodies of finite size at r ∞

  23. -q +q E of a dipole: along x axis -a a 0 x x-a x+a

  24. E of a dipole: along y axis y q -q a -a

  25. Field of a line of charge (along the line) dq z

  26. E of a half of the ring of charge dq dEy dE dE Etot

  27. (near the disc it looks like an infinite plane)

  28. Two infinite planes

  29. Electric field lines • 2.Density of lines is proportional to |E| • 3. Originate on “+” and terminate on “-” charges • 4. Crossing of E lines is impossible • 5. Closed lines are impossible in ES • NB: in the general case • |E| is not const along E lines • Not the trajectories of the charged particles 1.Tangent is in direction of

  30. A positive vs. negative point charge

  31. of a dipole

  32. Infinite line of charge

  33. Two infinite planes Uniform E : the same direction and magnitude at each point N=const, S=const, E=const

  34. Motion in a uniform E Data: electron, , L,v0 Find: 1.trajectory; 2.vf - E e v0 F + vf parabola

  35. Electric dipole in a uniform E 1.Stable equilibrium 2.Unstable equilibrium

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