1. AP Physics III.A Electrostatics

2. 18.1 Origin of Electricity Just two types of charge Magnitude of charge of an electron is the same as that of a proton (±1.60 EE –19 C) SI unit for charge is a Coulomb Most objects are electrically neutral, i.e. they have the same number of electrons as protons

3. The Fundamental Charge (Robert Millikan and his oil drop experiment)

4. Ex. How many electrons are in one Coulomb of negative charge?

5. 18.2 Charged Objects and Electric Force

6. Law of Conservation of Charge – during any one process, net electrical charge of an isolated system remains constant.

7. Attractions and repulsions

8. 18.3 Conductors and Insulators

9. 18.4 Charging by Induction and Conduction (also known as, “I wish I had a decent electroscope”)

10. Charging by Conduction

11. Charging by Induction

12. Induced charge on an insulator

13. 18.5 (Charles De) Coulomb’s Law

14. “Hmm, this looks like something I’ve seen before”

15. Ex. An electron “orbits” the proton of a hydrogen atom at an average distance of 0.53 EE 10 -10 m. What is the force that the proton exerts on the electron? What is the velocity of the electron for a circular orbit?

16. Electric forces and vectors

17. Ex. Three Charges in a Line

18. Ex. Three Charges in a Plane

19. p. 552: 3-5, 7, 11, 15, 19, 21, 23, 79B7 4.3.4 EE -17 kg, m A > m B 79B7 a)FBD b)

20. 18.6 Electric Field

21. A mass in a gravitational field

22. Charges experience an electrostatic force due to the presence of other charges

23. Force per Coulomb is the definition of an electric field (“show me the formula”)

24. An electric field is a vector that has a direction that the force exerts on a positive test charge.

25. Some examples

26. Ex. Find the electric force on a proton placed in an electric field of 2.0 EE 4 N/C that is directed along the positive x-axis.

27. Electric fields are vectors. The net electric field at a point in space can be determined by considering the contributions of each charged object and adding them together as vectors.

28. Electric field produced by a point charge

29. Ex. Electric Field Between Two Point Charges. Two point charges are separated by a distance of 0.100 m. One has a charge of –25.0 μC and the other 50.0 μC. a) What is the magnitude and direction of the electric field at point P between them 0.020 m from the negative charge? b) If an electron is placed at rest at P, what is the magnitude and direction of its initial acceleration?

30. Symmetry and the electric field.

31. 18.7 Electric Field Lines

32. Field lines around positive and negative charges

33. Field lines between plates of a capacitor.

34. Field lines between two dipoles

35. Field lines between two identical charges

36. p. 553: 25, 29, 31, 35-37; 81B3 36.a) 182 N/Cb) 312 N/C 81B3 a)FBD b)E = 5800 N/C, F T = 0.058 N c)drawing

37. 19.1 Electric Potential Energy

38. Work done on a charge in a uniform electric field

39. Let’s clarify but not overemphasize the signs

40. 19.2 Electric Potential Difference

41. Let’s look at “gravitational potential” first

42. OK, now electric potential

43. So change in electric potential is...

44. Electric potential decreases or increases not because the field exerts any more or less force (the field is uniform – like gravity near the Earth’s surface). V changes because of distance. A charge released in the field, traveling a greater distance converts more of its U e to K (like dropping an object from a greater height).

45. Everyday examples

46. Potential (and therefore potential difference) is scalar (this will simplify some things).

47. Summary Electric potential energy – energy a charge has because of its potential in an electric field (so far the field is uniform) Electric potential – electric potential energy per unit charge Potential difference – change in electric potential

48. Another formula and an hilarious story about twin boll weevils.

49. Muy importante – the displacement of the charge is in the direction of the electric field.

50. Ex. In the figure shown, the work done on a 2.0 µ C charge by the electric field from A to B is 5.0 EE -5 J. What is the change in electric potential energy and the potential difference? A ·A · B ·B ·

51. Worth noting: a positive charge accelerates from a higher potential to lower potential. A negative charge accelerates from lower potential to higher potentials.

52. Conservation of Energy – yep, here it is again with electrical potential energy in the picture

53. Ex. A proton is released in an electric field with a magnitude of 8.0 EE 4 V/m directed along the positive x-axis. The proton undergoes a displacement of 0.50 m in the direction of the field. a) Find the potential difference. b) Find the change in electrical potential energy c) Find the speed if the proton starts from rest.

54. The electron-volt – the change in electrical potential energy as an electron moves through a potential difference of one volt

55. Ex. A particle with mass of 1.8 EE -5 kg and a charge of 3.0 EE - 5 C is released from rest at point A and accelerates horizontally to point B. The only force on the particle is the force from the electric field and the electric potential at A is 25 V greater than the potential at B. What is the velocity of the particle at B?

56. p. 581: 4, 6; p. 150: 36 4.a) 2.00 EE -14 J b) 2.00 EE -14 J 6.a) 1500 Vb) B is higher potential 36.2700 m

57. 19.2 Electric Potential Due to a Point Charge

58. Graphically – potential from a positive charge is positive and decreases to zero at infinity.

59. Potential from a negative charge is negative and increases towards zero at infinity.

60. Electric Potential for a Pair of Point Charges

61. Ex. A 5.0 µC charge is at the origin and a -2.0 µC charge is on the x-axis at (3.0, 0) m. a) If the electric potential is zero at infinity, find the total electric potential due to the charges at P, with coordinates (0, 4.0) m. b) How much work is required to bring a third charge of 4.0 µC from infinity to P?

62. Ex. How many places are there on the line below where the potential is zero? Where is (are) these locations? 2q2q-q-q

63. Ex. Potential energy for a group of charges

64. p. 582: 11-17; 87B2, 89B2 12.2.4 (let V A = V B ) 14.45 V 16. 0.37 m (let U 2 = 2U 1 ) 87B2 a)9 EE 4 V b)9 EE -2 V c)0.30 N d)8.0 EE 5 N/C (right) e)6 m/s (use con. of mom. and con. of E) 89B2 a)-2 microC b)3.6 N (right) c)-0.72 J d)0.16 m e) ?