Electrostatics

1. Electrostatics

2. Lessons from the Lab • Opposites attract, likes repel • Charged objects can attract neutral objects • Attraction is proportional to charge, distance

3. Positive charge results from removing electrons from a substance • Negative charge results from adding electrons to a substance • Conducting materials allow charge to flow freely • Insulating materials do not

4. Properties of “Charge” • Reflects relative number of electrons in a substance • Conserved • Units of Coulombs (C) • An electron has a charge of 1.6 x 10-19C of charge

5. Significant Charge Amounts • When we rub balloons on rabbit hair in the lab, we’re generating 10’s of mC

6. Which of the following will tell you without a doubt that an object is charged • 1) It attracts another object that has been rubbed with rabbit fur • 2) It repels another object that has been rubbed with fur • 3) It does not attract a neutral object

7. Three aluminum balls are suspended from the ceiling. All three are charged with various materials. It is found that 1 and 2 repel one another, and 2 and 3 repel one another. From this, we can conclude that: • 1) 1 and 3 carry the same charge • 2) 1 and 3 carry opposite charges • 3) all three carry the same charge • 4) one of the objects carries no charge • 5) we need more experiments to determine charge

8. Three aluminum balls are suspended from the ceiling. Two of the three are then charged with various materials. It is found that 1 and 2 attract one another, and 2 and 3 repel one another. From this, we can conclude that: • 1) 1 and 3 carry the same charge • 2) 1 and 3 carry opposite charges • 3) all three carry the same charge • 4) one of the objects carries no charge • 5) we need more experiments to determine charge

9. Which spheres experience the greatest attraction?

11. Methods of Charging

12. Charging by Friction • http://phet.colorado.edu/new/simulations/sims.php?sim=John_Travoltage

13. Charging by Induction • http://phet.colorado.edu/new/simulations/sims.php?sim=Balloons_and_Static_Electricity

14. Charge polarization • When the charges in a material arrange themselves in such a way that the material has + and – sides, the material is said to be polarized

15. Why does Induction Work? • Both positive and negative charges are still in a substance • Why is it attracted?

17. Charging by Contact/Conduction • The physical movement of charge from one object to another

18. What will happen to two neutral spheres below when you bring a + charged rod close by? • 1) A and B will become + • 2) A and B will become - • 3) The spheres will remain neutral • 4) A will become – and B will become + • 5) A will become + and B will become -

19. If you want A to remain – and B to remain +, what should you do? • 1) Remove the rod • 2) Separate the spheres and then remove the rod • 3) Remove the rod, then separate the spheres • 4) Touch the spheres with the rod

20. Lightning

23. We’ve talked about ways to generate attraction and repulsion with charged objects • Just to review, on what does the attraction depend?

24. Assuming you’ve got two charged objects, write a basic equation that describes the force of attraction between them (just worry about the magnitude)

26. Coulomb's Law • Coulomb’s Law gives us a way to calculate the force between two charged objects • FE = kq1q2/d2 • k is a constant = 8.99 x 109 N m2/C2

27. Similarities to Gravity • Recall how we calculated the gravitational force between objects: • FG = Gm1m2/d2 • G is a constant = 6.67 x 10-11 N m2/kg2 • Look familiar?

28. Different Constants • G = 6.67E-11, k = 8.99e9 • What does this tell us about the difference between gravitational forces and electrostatic forces?

29. Calculate the electrostatic force between a +6mC charge and a -5mC charge, located 2m apart

30. Calculate the electrostatic force between a proton in the nucleus of the atom (q = +1.60e-19C) and an electron (q = -1.60e-19C) located in an outer energy level (d = 3e-11m) • Calculate the electron’s acceleration

31. Levitation • I once heard a person ask, couldn’t you make a person float using charges? • Perhaps • Imagine a person (m = 70kg) gathered -10mC of charge by rubbing herself with rabbit fur • What charge would we need to lift her off the ground?

34. Examine the configuration below. Which charge would exert the greatest force on the -2 charge? • 1. +8 2. +4 • 3. +6 4. +20

35. Which list below ranks the charges in order of increasing force on the -2 charge? • 1. A, B, D, C • 2. A, C, B, D • 3. D, C, B, A • 4. C, A, B, D • 5. D, B, C, A

36. Which arrow represents the direction of the net force on the -2 charge?

37. Electric Fields • Like gravity, the electrostatic force is a non-contact force • To conceptually deal with this, we talk about electric fields • This is a region of space surrounding a charged particle that “carries” the electrostatic force

38. An electric field tells us the direction of the electrostatic force • It also gives us a sense of the force magnitude

39. Drawing the Field • Place a positive “test” charge near a charge, or charge configuration • Determine the direction of the net force acting on that positive charge • Draw an arrow in that direction (arrow length represents force magnitude) • Move the charge to another place and repeat

40. A single positive charge

42. Which diagram correctly illustrates the field surrounding a negative point charge?

44. Field Strength • For a single point charge, electric field strength a distance r from the charge: • E = kq/r2 • Units? • N/C

45. Calculate the electric field strength, 2m away from a 3mC charge

46. Force on a charge placed in an E field • F = qE • F = mg

47. Calculate the force on a 2mC charge, placed in an electric field of strength 500N/C

48. Calculate the acceleration of an electron (m = 9E-31kg, q = 1.6E-19C), placed in a field of strength 3E-7 N/C

49. A sphere (m = 2kg, q = 5mC) accelerates at 6m/s/s when placed in an electric field. Find the field strength

50. F = q1E = q1 (kq2/r2) • F = kq1q2/r2