Electric Charge & Electric Fields

1. Electric Charge & Electric Fields

2. Objects with excess + or - charge give rise to electric force.

3. Acquisition of ChargeSolid objects charged by e- transfer.+charge results from loss of e--charge fr gain of e-.In liquids or gasses + and - ions are free to move about.

4. Neutral objects can be polarized Charges which are free to move are redistributed.

5. Charged objects can induce polarization. Balloon is attracted to positive wall surface.

6. Outer PartElementary Chargeselectrons e- –1NucleusProtons p+ +1neutrons no 0 Quantity of Charge in Atoms 6

7. Conservation of Charge Although e- can be transferred, charge cannot be created or destroyed. Sum of charges in system remains constant.

8. Two types of materials. • Conductors – allow charges to move around (metals). • Insulators – hold excess charge in place.

9. When metal or conductor receives excess charge it distributes charge.

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

11. Polarization occurs easily in conductor.

12. Charging Objects: Friction – rub 2 objects. Conduction (contact) – touch charged to uncharged. Induction – use polarization & grounding.

13. Conductors can be charged or discharged by grounding

14. The Charge on electron.

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

16. Coulomb’s Torsion Balance Coulomb measured the twist in the thread as the charged pith balls were brought close together. The twist was noted on the scale at the top. The twist was the force on the charges.

17. 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

18. 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.

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

20. The constant k can be written as: k = 1/4peo. Where eo is the permittivity of free space in a vacuum (air)= 8.85 x 10-12 N m2/C2. 20

21. Ex 1: State Coulomb’s Law in words. • The amount of force between 2 point charges is directly proportional to the amount of charge and • Inversely proportional to the square of the distance between their centers.

22. 2. If two charges q1 and q2 have a force F between them, what is the new force if the charge on q1 is tripled, q2 is quadrupled and the distance is doubled? • (3 x 4)/22. • 12/4 • 3F

23. Hwk Rd Kerr 6.2.1 – 6.2.4do pg 162 #1 – 3, 7, 9,12, 14, 17 .

24. Electric Fieldregion of space around charged object where a charge feels an electrostatic force.

25. Electric Fields-Charge alters space around it. Charged objects feel a force. Either repulsion or attraction.

26. Electric Field (E) defined as:The force per unit charge at a point in space on a small +test charge..E = F/q.E = Electric Field (N/C)F is force on test charge (N).q is amt of charge on test charge (C).

27. Remember gravitational field? Region of space where mass feels a force. g = force/unit mass on a small mass N/kg or m/s2.

28. 3. Calculate the E field strength 0.4 m away from a charge +20 mC. E = F/q F = kQ1q2 r2. Sub in for F kQ1q2 where q = q2. r2 q E = kQ1 Memorize it. r2

29. E = kQ1 r2 Fill in the numbers. E = 9x109(20x10-6)C (0.4)2 1.1 x 106 N/C.

30. Field Lines represent electric fields. Electric field lines show the force that a small positive test charge feels in a field created by a much larger charge. They represent the strength and direction of the field.

31. Sketch vectors to show force magnitude & direction on a + test charge at each point. +

32. Field around positive object. Lines start on + charge, end on –charge.

33. The denser the field lines are, the stronger the field. Stronger field near charge.

34. Field lines start on + and end on – charge.

35. Field Between Parallel Plates How would the strength of the field vary if a charge moves from the + to the – plate?

36. Fields have strength and direction - vector.Density of lines shows strength.Direction shown as arrows.Direction is determined by a + test charge.Lines start on pos end on neg.Electric Field lines don’t touch or cross.

37. Electric field due to more than one charge. Field is stronger near the larger charge. Density of lines show the increased strength. Strength at a point is the vector sum of field strengths.

38. Electrostatic Equilibrium Fields produced by more that a single charge will have spots where the forces on a charge in the field will be balanced. F net = 0.

39. Concentration of Lines show strength

40. E field inside a conductor is zero! • Why? • If an E field existed inside a conductor, the E field would exert a force on all e- that were present. This Fnet would accelerate the e- until equilibrium were reached and E goes to zero.

41. 4. Show that the electric field around a point charge E = kQ r2. Where Q is the charge producing the field.

42. Superposition To find the force on a charge q, due to the presence of more than one other charge, you must use vector addition.

43. Ex: Two 10 mC charges separated by 30 cm. What is the field strength 10 cm to the right of A? 10 cm 20 cm • A 10 mC 10 mC • B

44. Charges (3) of +1 C are located at the corners of a 45o right triangle. What is the resultant force on the charge located at the 90o angle. Find the direction too. 1C 1.3 x 1010 N 45o below horz 1m 1C 1C F2 1m F1

45. 1909 Millikan measured charge on e- Drops suspended when Fg = Fe. 45 45

46. Work & Energy

47. Charges in an E field can have PEelc. It takes work to bring charges near a repelling charge causing charge to gain PE.Charges that are attracted by opposite charges are said to “fall” toward them losing PE. Work is done by the field.

48. The convention: • If the charge gains PE then work is positive. • If it loses PE work is negative.

49. Where will a charge feel no force from electric field? + Infinity!

50. Electric Potential It requires energy to bring +q fr. infinity to point P in E field. They repel. Each point in a field has electric potential, like a height. +q P Must put a force on +q & push it charge thru a distance.