Electrostatics Chapter 23 - PowerPoint PPT Presentation

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Electrostatics Chapter 23

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  1. Electrostatics Chapter 23

  2. Most classes will begin with a slide similar to the following.

  3. What’s Happening • Clicker use will start on Friday (maybe). We will use them today informally. • There will NOT be a quiz this week. • There WILL be a quiz a week from Friday. • WebAssigns should be active shortly. Keep an eye out for it. • It might be already there !!!

  4. Probable First Observation Electricity

  5. Idiot! If lightening had actually traveled down the kite string, old Ben Franklin would have been toast! Probably never happened, but good story!

  6. A Quick Experiment

  7. Experimental Procedure Pivot • The sequence of Experiments • Identify the two rods • Treat each rod • Bring one rod near to the other • PREDICT WHAT WILL HAPPEN • VOTE ON POSSIBILITIES • Observe what happens • Did you learn anything? What?

  8. Allowable Predictions(Use your clicker if you have one.) • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

  9. motion Rubber rod Pivot Rubber rod Experiment #1 • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

  10. Rubber rubbed with skin of dead rabbit Pivot Rubber rubbed with skin of dead rabbit Experiment #2 • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

  11. The charges on the two rods are .. • Since we treated both rods in the same way, they should be of the same type • ……. different types • I have no idea what you are asking for. • Leave me alone … I’m napping!

  12. If you rubbed the rods longer and/or harder, do you think the effect that you see would be • Stronger • Weaker • The same

  13. If the two rods are brought closer together, the force acting between them will get … • Stronger • Weaker • The same

  14. Definition of sorts We DEFINE the “stuff” that we put on the rods by the rubbing process as CHARGE. We will try to understand what charge is and how it behaves. We add to the properties of materials: Mass Charge

  15. Pivot Experiment #3 Glass rubbed with wool Glass rubbed with wool • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

  16. Pivot Experiment #4 Rubber rubbed with skin of dead rabbit Glass rubbed with wool • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

  17. What’s Going On? • All of these effects involve rubbing two surfaces together. • Or pulling two surfaces apart. • Something has “happened “to each of these objects. • These objects have a new PROPERTY • Other properties are mass, color • We call this NEW PROPERTY .………. ………CHARGE. • There seems to be two types of charge.

  18. We call these two types of charge • Positive • Negative An object without either a (+) or (-) charge is referred to as being NEUTRAL.

  19. Example - Tape

  20. Separation

  21. An Example Volunteer Please

  22. Effect of Charge

  23. We have also observed that there must be TWO kinds of charge. • Call these two types • positive (+) • negative(-) • We “define” the charge that winds up on the rubber rod when rubbed by the dead cat to be NEGATIVE. • The charge on the glass rod or the dead cat is consequently defined as POSITIVE.

  24. Old Ben screwed up more than once!! ++++++++++--------- ----+++---++---+-++-

  25. From whence this charge??? Easily Removed - +

  26. Materials • Two kinds of materials: • Insulators • Electrons and Protons are tightly bound to their positions. Hard to move them around. • Conductors • Electrons are easily removed and moved around. • Electrons are said to be MOBILE charges. • There are other kinds of materials that we will not discuss: semiconductors, semi-metals

  27. Pivot Experiment #5 Rubber rubbed with skin of dead rabbit Metal Rod • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

  28. What about a charged rod and a piece of wood?? • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

  29. Ways to charge an object • Rubbing or bond breaking (same thing) • Transfer • Direct transfer • Polarization • Induction

  30. Neutral Object - POLARIZATION Positive charge attracts negative charges. Rod becomes “polarized. Negative end is closer to positive charge Distance effect causes attraction.

  31. Induction

  32. Polarize

  33. Ground

  34. Remove Ground

  35. Positive !

  36. Balloon Physics

  37. Same as before: Polarization

  38. What happens when two surfaces touch or rub? Bonding!

  39. The Triboelectric Series When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged.MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk paper cotton wood acrylic cellophane tape polystyrene polyethylene rubber balloon saran wrap MORE NEGATIVE No! No! No! No!

  40. So far we have found? • There are TWO types of charge. • Positive • Negative • Like Charges Attract • Un-Like charges repel • The force between charges increases as they are brought closer together. • This charge separation results from chemical bonds which are severed.

  41. Forces Between Charges

  42. Coulomb’s Law – Force between charges • The force between two charges is proportional to the product of the two charges and inversely proportional to the square of the distance between them. • The force acts along the line connecting the two charges.

  43. Remember Force is a VECTOR!!

  44. Coulomb’s Law Force is a Vector ! The Unit of Charge is called THE COULOMB Smallest Charge: e ( a positive number) 1.6 x 10-19 Coul. electron charge = -e Proton charge = +e

  45. Three point charges are located at the corners of an equilateral triangle as shown in Figure P23.7. Calculate the resultant electric force on the 7.00-μC charge.

  46. Two small beads having positive charges 3q and q are fixed at the opposite ends of a horizontal, insulating rod, extending from the origin to the point x = d. As shown in Figure P23.10, a third small charged bead is free to slide on the rod. At what position is the third bead in equilibrium? Can it be in stable equilibrium?

  47. The Electric Field

  48. Fields • Imagine an object is placed at a particular point in space. • When placed there, the object experiences a force F. • We may not know WHY there is a force on the object, although we usually will. • Suppose further that if we double some property of the object (mass, charge, …) then the force is found to double as well. • Then the object is said to be in a force field. • The strength of the field (field strength) is defined as the ratio of the force to the property that we are dealing with.

  49. Example – Gravitational Field. • Property is MASS (m). • Force is mg. • Field strength is defined as Force/Property

  50. The Gravitational Field That We Live In. M m mg Mg