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Lesson 16 Electrostatics

Eleanor Roosevelt High School Chin-Sung Lin. Lesson 16 Electrostatics. Electrostatics. Electrostatics is the electricity at rest It involves electric charges, the force between them, and their behavior in the materials. History.

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Lesson 16 Electrostatics

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  1. Eleanor Roosevelt High School Chin-Sung Lin Lesson 16Electrostatics

  2. Electrostatics • Electrostatics is the electricity at rest • It involves electric charges, the force between them, and their behavior in the materials

  3. History • The word electricity comes from the Greek “elektron” which means “amber” • The “amber effect” is what we call static electricity

  4. History • Based on the experiment situations, there must be two types of charges. Benjamin Franklin arbitrarily call one positiveand another one negative • He also argued that when a certain amount of charge is produced on one body, an equal amount of the opposite charge is produced on the other body…

  5. Electric CHARGE

  6. Electric Charges

  7. Electric Charges

  8. Electric Charges

  9. Electric Charges • The fundamental rule of all electrical phenomena is Like charges repel opposite charges attract

  10. Thomson’s Cathode Ray Experiment • Discover electrons in a gas discharge cathode-ray tube, and prove that the charge of electrons are negative

  11. Millikan’s Oil Drop Experiment • The charge of oil drops are always the multiples of -1.60 x 10-19 C

  12. Millikan’s Oil Drop Experiment • Charge histogram in units of e of the 42.5 million drops measured during the 36 month experimental run

  13. Millikan’s Oil Drop Experiment • Summary as of January  2007 • Total mass throughput for all experiments- 351.4 milligrams of fluid • Total drops measured all experiments - 105.6 million

  14. Millikan’s Oil Drop Experiment • What conclusion can you draw from these data?

  15. Millikan’s Oil Drop Experiment Electric charges are quantized (discrete) Charges are always multiple of fixed charges No fractional charge particles was found Prove the existence of unit charge

  16. Electric Charges • The SI unit of charge is the coulomb (C) • The charge of an electron is -1.60 x 10-19 C • 1 C is the charge of 6.24 x 1018 electrons • Symbol: q, Q

  17. Electric Charges • By convention, electrons are negatively charged protons are positively charged neutrons have no charge • All electrons are identical • All protons are identical • All neutrons are identical • A proton’s charge is equal in magnitude to the negative charge of the electron

  18. Electric Charges

  19. CHARGE BY FRICTION

  20. Charge by Friction Why is the rod got charged simplily by rubbing it with fur?

  21. Charge by Friction What decides which will gain or loss electrons during friction?

  22. Charge by Friction What happens when electrons are excited to such an extent that they leave the atoms?

  23. Charge by Friction Why won’t protons move?

  24. Charge by Friction • Electrons are being transferred by friction when one material rubs against another • The one that gains electrons becomes negative. The one that lose electrons becomes positive • Different materials have different tendency to gain or lose electrons

  25. Triboelectric Series • The triboelectric series is a list that ranks various materials according to their tendency to gain or lose electrons • The tendency of a material to become positive or negative after triboelectric charging has nothing to do with the level of conductivity of the material • The greater the relative position, the larger the expected electric charge

  26. Triboelectric Series

  27. Triboelectric Series

  28. Triboelectric Series

  29. Charging by Friction

  30. CHARGE BY CONTACT

  31. Charge by Contact • Electrons can be transferred from one material to another by simply touching

  32. Charge by Contact • A positively charged metal ball with charge Q contacts with an identical electrically neutral metal ball • The charges equally redistribute to both balls • Separate these two balls and each one has charge Q/2

  33. Electric Charges Distribution • If the object is good conductor, the charges will spread to all part of its surface because the like charges repel each other. Net charge inside is zero

  34. Electric Charges Distribution • If we dump a bunch of electrons to the statue of liberty, and the statue is insulated from the ground, where will those electrons go?

  35. Electric Charges Distribution • On symmetric objects, charge distributes uniformly, on non-symmetric objects, charges stay at sharp corners

  36. Electric Charges Distribution • Why do electrons stay at sharp corners?

  37. Electric Charges Distribution • If the object is a poor conductor, charge cannot move, so it remains localized around the contact region

  38. LAW OF CONSERVATION OF CHARGE

  39. Law of Conservation of Charge • The net amount of electric charge produced in any process is zero • In an isolated system the total charge is conserved • Electrons are neither created nor destroyed but are simply transferred from one material to another

  40. Law of Conservation of Charge • An object that has unequal numbers of electrons and protons is electrically charged • In a neutral atom, there are as many electrons as protons, so there is no charge • A charged atom is called an ion. An imbalance comes about by adding or removing electrons

  41. CHARGE BY INDUCTION

  42. Charge by Induction • Why does the balloon stick to the wall?

  43. Charge by Induction • If we bring a charged object near a conducting surface, even without physical contact, electrons will move in the conducting surface

  44. Charge by Induction • The introduction of a charge into another body without contact being made • Temporary charging of this type produces Temporary Polarity

  45. Charge by Induction • Two neutral metal balls A & B are placed next to each other and contact each other • Approach a negatively charged rod to ball A and different charges will be induced on each ball

  46. Charge by Induction • Separate the two metal balls while the rod is still there • Remove the rod and the charges will be redistributed evenly on both balls

  47. AIM: Charge by InductionDoNow: • Started with a neutral ball with metal paint and a negatively charged rod, how can you charge the ball with positive charges by induction?

  48. Charge by Induction • Approach a negatively charged rod to a neutral ball with metal paint and different charges will be induced on each side of the ball

  49. Charge by Induction • While the rod is still there, ground the opposite side of the ball and electrons will flow to the ground

  50. Charge by Induction • A charged object is needed to charge an object by induction (without any contact) • The object being charged ends up with a charge which is the opposite of the object being used to charge it • A ground must be used to charge on the object. The ground allows for electron movement into or out of the object being charged

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