Electrostatics Chapter 23

1. Electrostatics Chapter 23 Week-1-2

2. What’s Happening • Clicker use will start on Friday (maybe). • Today we begin the study of charge with make-believe clickers. • There will NOT be a quiz this week. • There WILL be a quiz a week from Friday. • WebAssigns are now active.

3. Probable First Observation Electricity

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

5. A Quick Experiment

6. 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 we learn anything?

7. Allowable PredictionsIn the future we will use clickers for this. • Rods will attract each other • Rods will repel each other • Nothing will happen • Something not listed above will happen

8. 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

9. 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

10. 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

11. What is the effect of DISTANCE??

12. 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

13. 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.

14. Example - Tape

15. Separation

16. Another Example Volunteer Please

17. Effect of Charge

18. 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.

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

20. 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

21. 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

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

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

24. Induction

25. Polarize

26. Ground

27. Remove Ground

28. Positive !

29. Balloon Physics

30. Same as before: Polarization

31. From WebAssign • Three objects are brought close to each other, two at a time. When objects A and B are brought together, they attract. When objects B and C are brought together, they repel. From this, we conclude which of the following? (Select all that apply.) • [_] Objects A and C possess charges of the same sign. • [_] All three of the objects possess charges of the same sign. • [_] One of the objects is neutral. • [_] Objects A and C possess charges of opposite sign. • [_] We need to perform additional tests to determine information about the charges on the object

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

33. 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!

34. Summary - Rubbings

35. What have we 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.

36. Forces Between Charges

37. 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.

38. Remember Force is a VECTOR!!

39. 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

40. 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.

41. 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?

42. The Electric Field

43. 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.

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

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

47. This is WAR Ming the merciless this guy is MEAN! • You are fighting the enemy on the planet Mongo. • The evil emperor Ming’s forces are behind a strange green haze. • You aim your blaster and fire … but ……

48. Nothing Happens! The Green thing is a Force Field! The Force may not be with you ….

49. Side View The FORCE FIELD Force Big! |Force| o Position

50. Properties of a FORCE FIELD • It is a property of the position in space. • There is a cause but that cause may not be known. • The force on an object is usually proportional to some property of an object which is placed into the field.