Static Electricity

1. Static Electricity

2. What does the term static mean? Not in motion Electricity? Involves electrons

3. Atomic model positively charged nucleus (protons)negatively charged electrons

4. Notation Outer PartElementary Chargeelectrons e- –1NucleusProtons p+ +1neutrons no 0Neutral objects have same # p+ & e-.Charged objects have net p+ or e-

5. When objects have excess or deficit of charge, can exert electrostatic force.

6. When objects have excess or deficit of charge, can exert electrostatic force.

7. Charged objects supply a force.Proof?

8. In solids,Charge transferred by e- only.How can we get positive charge object? Loss of e-.

9. Uncharged objects can feel electrostatic force too:by polarization

10. Polarization Atoms can be polarized by redistributing e-. Polarization is separation of charge not imbalance.

11. Charged balloon causes wall to become polarized.

12. Conservation Law applies to charge Although charge ( e-) can be transferred, charge cannot be created or destroyed. Sum of charges in system remains the same. For polarization the system is the balloon and the wall.

13. 2 types of materials. • Conductors – allow charges to move around – can be polarized. • Insulators – hold excess charge in place – hard to polarize.

14. Conductors – materials that allow e- to move freely often redistribute charge.Metals are good conductors.

15. Metal conductors distribute charge uniformly.

16. Insulators – charges do not move freely. Tend to stay concentrated in one spot on object.

17. What’s happening here?

18. Water Stream • Try at home.

19. Polarization produces only a surface charge.

20. Charging Objects: • 1. Friction – rub 2 neutral objects together. • Conduction - Contact with charged object. • Induction – by bringing charged object in vicinity of neutral conductor.

21. Friction Works for insulators and conductors. Do objects get same or opposite charge? Opposite!

22. Why you get a shock. • Charge yourself transfer e- either to or from your body to neutralize your charge. Always accompanied by E release.

23. Conduction: touch charged object to neutral object. Do objects get same or opposite charge? SAME!

24. Static Electricity 9:15 min. http://www.youtube.com/watch?v=A893_7FGMHY&feature=relmfu

25. Induction- – no touching of objects. Need to polarize & separate them.

26. Charging by inductionconductors only. A ground can serve as an infinite source or sink of e-. Earth, your hand, floor, wall.

27. Charge an Electroscope by Induction

28. Charging By Induction 9:30 Min. • http://www.youtube.com/watch?v=pJ36EtABLAk

29. Hwk Read Tx 17-1Answer pg 633 #1-2, 4-6pg 654 #1-10 not 3Type or write it all including questions.

30. How do we measure amount of charge? 1. Elementary Charge

31. Conductors will share elementary charges equally if they are allowed to touch. Example: 1. The elementary charge of each metal sphere below is shown. If they are allowed to touch, and are then separated, what will be the charge on each? +3 -6 -9

32. Total charge 3 – 6 – 9 = - 12 They will share the total charge so - 12/3 spheres = -4 charges each.

33. Determining Charge on electron.

34. 1909 Robert Millikan measured charge on e-

35. Millikan 1:15 http://www.youtube.com/watch?v=XMfYHag7Liw

36. Robert Millikan found charge is quantized. There is a smallest unit of charge.Charge can only exist in whole number integers of the charge on 1e-. Cannot have in between numbers.

37. Can an object have a charge of 3.53 x 10-19C? • No. • 3.53 x 10-19C ÷ 1.6 x 10-19 C = 2.2. • Charges must be whole number integrals of 1.6 x 10-19 C .

38. Units of charge = coulombs (C) Charge on e- is -1.6 x 10-19 C Charge on p+ is +1.6 x 10-19 Cor can consider fundamental units e- has charge –1 p+ has charge +1 Charge Units

39. It takes 6.25 x 1018 elementary charges (e- or p+) to carry 1 C of charge.Take the inverse of 1.6 x 10-19C.

40. 2. What would be the charge on an object with 2.2 x 1015 excess electrons? 3.52 x 10-4 C

41. 3. How many protons does it take to carry 0.001 C of charge? 6.25 x 1015 p+

42. 4. What is the total charge (in C) on 6.2 x 108 electrons? • 9.9 x 10 -11 C

43. 5. A metal sphere with an excess of 2 x 109 electrons is connected to a sphere with a deficit of 1 x 109 electrons. • What is the charge in Coulombs on each sphere before they’re connected? • What is the charge in Coulombs on each after they’ve been connected?

44. Electrostatic Force Charles Coulomb measured force exerted on one charged object by another.He used torsion balance.

45. Coulomb’s Torsion Balance

46. Coulomb’s Law Relates Force btw. 2 charged objects.Fe = kq1q2 r2k = constant 8.99 x 109 N m2/C2. q charge on obj in Coulombs (C)r is dist in meters.F is force (N)

47. Ex 1: An alpha particle is a nucleus with 2 protons and 2 neutrons. It is near a proton. What is the charge in Coulombs of each? They are separated by a distance of 3 nm. What is the force between them? Is the force repulsive or attractive?

48. nucl = 3.2 x 10-19 C. • p+ = 1.6 x 10-19 C. • F = 5.11 x 10-11 N

49. 2: Two protons are 0.025 m apart. Calculate: a) the gravitational attraction between them. B) the electrostatic force between them. C) what is the ratio between the forces. D) What do you think the sign + or – indicates for electrostatic force?

50. Hwk read text 17 -2 and pg 634 – 636 • Do pg 636 #1-4 and pg 654 #1, 2, 6, 10.