2. Static Electricity / Electrostatics Where have you experienced this? –Shocked a friend? (or yourself) –Clothes in the dryer stick together –Stroke a cat and see the hair standing up on end. –Rub a rubber balloon in somebody’s hair –Bolt of lightning

3. Electrons are migrants. They are attracted to the protons in the nucleus, but they can be persuaded to become attracted to protons in a different nucleus and join another electron shell if energy is added.

4. The First Law of Electrostatics Like charges repel; unlike charges attract. Neg PosNeg Pos

5. http://www.colorado.edu/physics/2000/waves_particles/wavpart2.html

6. Here’s the fun part! Charged objects attract neutral objects! The electrons in the neutral object are attracted (or repelled) by the electrons (or protons) in the charged object. They move accordingly, and the object becomes polarized.

7. The Quantity of Charge The quantity of charge (q) can be defined in terms of the number of electrons, but the Coulomb (C) works better The Coulomb: 1 C = 6.25 x 10 18 electrons Which means that the charge on a single electron is: 1 electron: e - = -1.6 x 10 -19 C

8. Units of Charge The coulomb (selected for use with electric currents) is actually a very large unit for static electricity. Thus, we often encounter a need to use the metric prefixes. 1  C = 1 x 10 -6 C 1 nC = 1 x 10 -9 C 1 pC = 1 x 10 -12 C

9. Ex: Determine the quantity and type of charge on an object which has 3.62 x 10 12 more protons than electrons. = +5.8 x 10^-7 Coulombs (rounded) 3.62 x 10 12 x 1C 6.25 x 10 18 e

10. Example 1. If 16 million electrons are removed from a neutral sphere, what is the charge on the sphere in coulombs? 1 electron: e - = -1.6 x 10 -19 C q = -2.56 x 10 -12 C Since electrons are removed, the charge remaining on the sphere will be positive. Final charge on sphere: q = +2.56 pC + + + + + +

11. Coulomb’s Law The force of attraction or repulsion between two point charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them. F r F F q q q’q’ q’q’ -+ --

12. Coulomb’s Law r Charge (Q) Coulombs (C) 1 C = 6.2421 x 10 18 e e = 1.602 x 10  9 C Distance (m) Coulomb’s constant (k) k = 8.988 x 10 9 N  m 2 /C 2 Force (N)

13. The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same. Coulomb’s Law

14. Example 2. A –5  C charge is placed 2 mm from a +3  C charge. Find the force between the two charges. - + 2 mm +3  C-5  C q q’q’ Draw and label givens on figure: r F F = 3.38 x 10 4 N; Attraction Note: Signs are used ONLY to determine force direction.

15. Problem-Solving Strategies 1. Read, draw, and label a sketch showing all given information in appropriate SI units. 2. Do not confuse sign of charge with sign of forces. Attraction/Repulsion determines the direction (or sign) of the force. 3. Resultant force is found by considering force due to each charge independently. Review practice on vectors, if necessary. 4. For forces in equilibrium:  F x = 0 =  F y = 0.

16. Example 3. A –6  C charge is placed 4 cm from a +9  C charge. What is the resultant force on a –5  C charge located midway between the first charges? - + 2 cm +9  C-6  C q1q1 q2q2 r2r2 2 cm - r1r1 1. Draw and label. q3q3 2. Draw forces. F2F2 F1F1 1 nC = 1 x 10 -9 C 3. Find resultant; right is positive. F 1 = 675 N F 2 = 1013 N

17. Example 3. (Cont.) Note that direction (sign) of forces are found from attraction- repulsion, not from + or – of charge. - + 2 cm +9  C-6  C q1q1 q2q2 r2r2 2 cm - r1r1 q3q3 F2F2 F1F1 F 1 = 675 N F 2 = 1013 N + The resultant force is sum of each independent force: F R = F 1 + F 2 = 675 N + 1013 N; F R = +1690 N

18. Example 4. Three charges, q 1 = +8  C, q 2 = +6  C and q 3 = -4  C are arranged as shown below. Find the resultant force on the –4  C charge due to the others. Draw free-body diagram. - 53.1 o -4  C q3q3 F1F1 F2F2 Note the directions of forces F 1 and F 2 on q 3 based on attraction/repulsion from q 1 and q 2. +- 4 cm 3 cm 5 cm 53.1 o +6  C -4  C +8  C q1q1 q2q2 q3q3 +

19. Example 4 (Cont.) Next we find the forces F 1 and F 2 from Coulomb’s law. Take data from the figure and use SI units. F 1 = 115 N, 53.1 o S of W F 2 = 240 N, West Thus, we need to find resultant of two forces: +- 4 cm 3 cm 5 cm 53.1 o +6  C -4  C +8  C q1q1 q2q2 q3q3 F2F2 F1F1 +

20. Example 4 (Cont.) We find components of each force F 1 and F 2 (review vectors). 53.1 o - -4  C q3q3 F 1 = 115 N F 1y F 1x F 1x = -(115 N) Cos 53.1 o = - 69.2 N F 1y = -(115 N) Sin 53.1 o = - 92.1 N Now look at force F 2 : F 2x = -240 N; F 2y = 0 R x =  F x ; R y =  F y R x = – 69.2 N – 240 N = -309 N R y = -92.1 N – 0 = -92.1 N F 2 240 N R x = -309 N R y = -92.1 N

21. Example 4 (Cont.) Next find resultant R from components F x and F y. (review vectors). R x = -309 N R y = -92.1 N - -4  C q3q3 R y = -69.2 N R x = -309 N  R We now find resultant R,  : R = 322.4 N Thus, the magnitude of the electric force is: R = ( (309) 2 + (92.1) 2 ) 1/2 = 322.4

22. Example 4 (Cont.) The resultant force is 317 N. We now need to determine the angle or direction of this force. -69.2 N - -309 N  R  -92.1 N Or, the polar angle  is: Or, the polar angle  is:  = 180 0 + 73.4 0 = 253.4 0 The reference angle is: The reference angle is:   = 73.4 0 S of W Resultant Force: R = 322 N,  = 253.4 0 -92.1 N

23. Summary of Formulas: Like Charges Repel; Unlike Charges Attract. 1 electron: e - = -1.6 x 10 -19 C 1  C = 1 x 10 -6 C 1 nC = 1 x 10 -9 C 1 pC = 1 x 10 -12 C

24. Electric field of a point charge +Q r The Electric Field Electric Charge and Electric Field 16

25. A proton is released in a uniform electric field, and it experiences an electric force of 3.75 × 10  14 N toward the south. What are the magnitude and direction of the electric field? +south E F Electric Charge and Electric Field 16

26. What are the magnitude and direction of the electric field at a point midway between a +7.0  C and a  8.0  C charge 8.0 cm apart? Assume no other charges are nearby. + - d q1q1 q2q2 E1E1 E2E2 Electric Charge and Electric Field 16

27. A proton (m = 1.67 x 10  27 kg) is suspended at rest in a uniform electric field E. Take into account gravity at the Earth's surface, and determine E. + q E Electric Charge and Electric Field 16

28. The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge. The Electric Field Electric Charge and Electric Field 16

29. The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge. The Electric Field The electric field is stronger where the field lines are closer together. A BC E E Electric Charge and Electric Field 16

30. Electric field of two charges: The Electric Field Electric Charge and Electric Field 16

31. The electric field between two closely spaced, oppositely charged parallel plates is constant. The Electric Field Electric Charge and Electric Field 16

32. A +2 nC charge is placed at a distance r from a –8  C charge. If the charge experiences a force of 4000 N, what is the electric field intensity E at point P? Electric Field. - - - - - - - - -Q P First, we note that the direction of E is toward –Q (down). –8  C E ++q+q E 4000 N +2 n C r E = 2 x 10 12 N/C Downward Note: The field E would be the same for any charge placed at point P. It is a property of that space.

33. A constant E field of 40,000 N/C is maintained between the two parallel plates. What are the magnitude and direction of the force on an electron that passes horizontally between the plates.c E. F The E-field is downward, and the force on e - is up. F = 6.40 x 10 -15 N, Upward + + + + + + + + + - - - - - - - - - - e-e-e-e- - e-e-e-e- - e-e-e-e-

34. The E-Field at a distance r from a single charge Q + + + + + + + + Q. r P Consider a test charge +q placed at P a distance r from Q. The outward force on +q is: The electric field E is therefore: ++q+q F + + + + + + + + Q. r P E

35. What is the electric field intensity E at point P, a distance of 3 m from a negative charge of –8 nC?. r P -Q 3 m -8 nC E = ? First, find the magnitude: E = 8.00 N/C The direction is the same as the force on a positive charge if it were placed at the point P: toward –Q. E = 8.00 N, toward -Q

36. The Resultant Electric Field. The resultant field E in the vicinity of a number of point charges is equal to the vector sum of the fields due to each charge taken individually. Consider E for each charge. + -  q1q1 q2q2 q3q3 - A E1E1 E3E3 E2E2 ERER Vector Sum: E = E 1 + E 2 + E 3 Vector Sum: E = E 1 + E 2 + E 3 Directions are based on positive test charge. Magnitudes are from:

37. Example. Find the resultant field at point A due to the –3 nC charge and the +6 nC charge arranged as shown. + -  q1q1 q2q2 4 m 3 m 5 m -3 nC +6 nC E for each q is shown with direction given. E2E2 E1E1 A Signs of the charges are used only to find direction of E

38. Example. (Cont.)Find the resultant field at point A. The magnitudes are: + -  q1q1 q2q2 4 cm 3 cm 5 cm -3 nC +6 nC E2E2 E1E1 A E 1 = 3 N/C, NorthE 2 = 3.38 N, West E2E2 E1E1 Next, we find vector resultant E R ERER 

39. Example. (Cont.)Find the resultant field at point A using vector mathematics. E 1 = 3 N, West E 2 = 3.38 N, North Find vector resultant E R E2E2 E1E1 ERER  = 41.6 0 N of W; or  = 138.4 0 Resultant Field: E R = 4.25 N; 138.4 0  3.00N 3.38N

40. Electric Potential & Electric Potential Energy

41. Review: Work and Energy Work is defined as the product of displacement d and a parallel applied force F. Work = Fd; Units: 1 J = 1 N m Potential Energy U is defined as the ability to do work by virtue of position or condition. (Joules) Kinetic Energy K is defined as the ability to do work by virtue of motion (velocity). (Also in joules)

42. Electrical Work and Energy An external force F moves +q from A to B against the field force qE. Work = Fd = (qE)d At level B, the potential energy U is: U = qEd (Electrical) The E-field does negative work; External force does positive work. The external force F against the E-field increases the potential energy. If released the field gives work back. B + + - - A + +q d qE E FeFe

43. m h g F = mg W = mgh  PE =  W Electric Potential Energy and Potential Difference Electric Potential 17

44. m h g F = mg q E d F = qE W = mgh W = qEd a b  PE =  W  PE = W Electric Potential Energy and Potential Difference Electric Potential 17 W = Fd

45. E q x a b Electric Potential Energy and Potential Difference Electric Potential 17

46. Electric Potential Potential + + + + + + + + Q. r Electric potential is another property of space allowing us to predict the P.E. of any charge q at a point. Electric Potential: joules per coulomb(J/C) The units are: joules per coulomb (J/C) For example, if the potential is 400 J/C at point P, a –2 nC charge at that point would have P.E. : U = qV = (-2 x 10 -9 C)(400 J/C); U = -800 nJ P

47. Problem 17-04 An electron acquires 7.45 x 10  17 J of kinetic energy when it is accelerated by an electric field from plate A to plate B. What is the potential difference and which plate is at the higher potential? AB E Plate B has a positive charge and is at a higher potential. Electric Potential 17

48. E q x x =1.00 cm q = 1.60 x 10  19 C E = 2000 N/C m = 9.1 x 10  31 kg Find the speed of the charge at the lower plate. Find the potential through which the charge moves (ΔV). m Electric Potential Energy and Potential Difference Electric Potential 17

49. E q x x =1.00 cm q = 1.60 x 10  19 C E = 2000 N/C m = 9.1 x 10  31 kg Find the acceleration of the charge. Find the force on the charge as it moves. m Electric Potential Energy and Potential Difference Electric Potential 17

50. Work is required to move two point charge closer together. FF r This work is converted to potential energy. This electric potential energy of two point charges: Electric Potential Energy and Potential Difference Electric Potential 17

51. Problem 17-20 An electron starts from rest 32.5 cm from a fixed point charge with Q =  0.125  C How fast will the electron be moving when it is 50m away? Electric Potential 17

52. Relation between Electric Potential and Electric Field d E V 10 V0 V 5 V2.5 V 7.5 V Let E = 100 N/C d = 10 cm Using potentials instead of fields can make solving problems much easier – potential is a scalar quantity, whereas the field is a vector. Electric Potential 17

53. Equipotential Lines Electric Potential 17

54. One electron volt (eV) is the energy gained by an electron moving through a potential difference of one volt. The Electron Volt, a Unit of Energy Electric Potential 17

55. The electric potential due to a point charge Electric Potential Due to Point Charges VV +r rr VV rr Electric Potential 17

56. Problem 17-18 (a) What is the electric potential a distance of 2.5 x 10  15 m away from a proton? (b) What is the electric potential energy of a system that consists of two protons 2.5 x 10  15 m apart? Electric Potential 17

57. Potential For Multiple Charges The Electric Potential V in the vicinity of a number of charges is equal to the algebraic sum of the potentials due to each charge. + -  Q1Q1 Q2Q2 Q3Q3 - A r1r1 r3r3 r2r2 Potential is + or – based on sign of the charges Q.

58. Example 5: Two charges Q 1 = +3 nC and Q 2 = -5 nC are separated by 8 cm. Calculate the electric potential at point A. +  Q 2 = -5 nC - Q1Q1 +3 nC  6 cm 2 cm A B V A = 450 V – 2250 V; V A = -1800 V

59. A capacitor consists of two conductors that are close but not touching. A capacitor has the ability to store electric charge and stores energy in an electric field. Capacitance +Q QQ Battery V Capacitance Electric Potential 17

60. Problem 17-36 The charge on a capacitor increases by 18  C when the voltage across it increases from 97 V to 121 V. What is the capacitance of the capacitor? Electric Potential 17

61. A charged capacitor stores electric energy; the energy stored is equal to the work done to charge the capacitor. Storage of Electric Energy Battery +Q QQ V C Energy density of an electric field Electric Potential 17