Electrostatics

1. Electrostatics The Shocking Truth February 1/2, 2010

2. Unit Summary • Chapter 17 - Electrostatics • Electric charge • Electric force • Electric field strength • Lightning!

3. Today • Electric Charge • Conductors and insulators • Charging by • Contact • Induction • Polarization

4. Fun with balloons… • Anyone want to sacrifice your hairstyle for science? • What is going on?

5. Electrostatic charges • Electrostatic charges • Positive • Negative • Opposites attract • Like charges repel • No kidding!

6. Conservation of charge • You might recall from chemistry… • positive and negative charges • Some atoms LOVE electrons • Electronegativity • Different materials have different electronegativity • Stronger or weaker hold on electrons

7. What happens when… • You rub two objects together? • You put some energy into the system… • Electrons get excited! • May allow one material to pull electrons off of another!

8. Conservation of Charge • You can separate or join what you have, • but you can’t “create” charge. • How would you illustrate this?

9. Conductors and insulators • Why are electric cords coated in plastic? • What is electric wire made of? • Insulators • Materials that do not transfer charge easily • Conductors • Materials that transfer charge easily • Examples?

10. You’re grounded! • What does it mean? • The earth is a big bank. • Supply or receive large number of electrons • Grounding neutralizes the charge in an object • Through contact

11. Types of charge transfer • Contact • Friction/rubbing (slightly different) • Polarization • Induction

12. Contact • High concentration to low concentration • One object gains as much as the other looses • Insulators vs conductor • Which one will hold onto more of its charge

13. Polarization • Electrostatic forces are “field” forces • Contact is not necessary • If you put a positively charged object close to another object • It will repel the positive charges • And attract the negative charges • What about a neutral object? • What happens when you remove the charged object?

14. Grounding and Induction • What happens when you ground an object that has a polarized charge? • It becomes charged!

15. CFU • …

16. Lab • Read the questions on the back • Lucite and blancite • Wool and blancite • Silk and Lucite (or glass) • Ground the pith ball and the rods before each step • Be patient – use lots of charge

17. Electrostatics February 3/4, 2010 Sorry… no pun today.

18. Today • Millikan’s Experiment • Coulomb’s Law • Electrostatic Force • Finish Lab 1

19. Back to the lab! • 10 minutes • If you’re done – work on completing the in-class worksheet from last time.

20. R. Millikan (in 1909) • Oil drops?? • Measured the charge on the drops…

21. Millikan’s machine…

22. The point? • Charges are “quantitized” • Always a multiple of a very small number… • qe = 1.60 x 10-19 Coulombs (C) • Charge on an electron • And a proton…

23. So… • How many “free electrons” does an object with a charge of 5.00 coulombs have?

24. Yes, it’s a Force • First day – 2 x 2 and the charged plastic rod • Electrostatics are field forces • Do not require contact to be “felt” • What makes a force a force? • F = ma • When it causes a change in velocity of an object

25. Charles Augustin de Coulomb

26. Charles Coulomb • Two ideas: • #1 The force was proportional to the product of the two charges • If you double the charge on one object the force between them doubled

27. Charles Coulomb – idea #2 • #2 The force was inversely proportional to the distance squared • If he doubled the distance … • The force decreased by 4 times • This is called an “inverse square relationship”

28. Coulomb’s Law • F is proportional to (q1 x q2)/d2 • Or… F = k (q1 x q2)/d2 • Where k is a constant that makes the numbers “work” • k = 9.0 x 109 N m2/C2 • q is the symbol for charge • It’s units are Coulombs

29. So we can calculate… • F = k (q1 x q2)/d2 • 2 charges are 0.30 meters away from each other • Q1 has a charge of 12.0 x 10-9 C • Q2 has a charge of –18 x 10 -9 C • F12 = -2.2 x 10 -5 N • What does a negative force imply?

30. Calculating forces • 1 Coulomb is a large charge • 1x10-6 C 1x10-9 C or is more typical • μC or nC • You will need to be adept at using scientific notation • The sign matters! • Opposite sign – attracting • Same sign - repelling

31. Electrostatics February 5/9, 2010 “Ouch!! Do it again…” Anonymous

32. Today • Review Homework • More Electrostatic Force • Electrostatic Fields

33. 633 2. Charge is quantitized 3. 6.25 x 1019 electrons 636 -230 N a) -2.2 x 10-5 N b) 9.0 x 10-7 N a ) 16 N b) Attract c) 2.7 x 1013 e- d) 3.8 x 1013 e- 0.393 m Homework

34. What do you think? • Does it make a difference? • If you are standing barefoot in water… • or on rubber soled shoes? • Can you ground an insulator by touching one end? • Which can you induce a charge in? • An insulator or a conductor?

35. Review Coulomb’s Law • F = k (q1 x q2)/d2 • Bigger the charge (q) • Bigger the force • Larger the distance • Smaller the force • Inverse square

36. What happens…

37. What happens…

38. What happens…

39. Example • 3 objects in sitting in a line Q1 = 4.0 x 10-6 C at x = 0 m Q2 = 3.0 x 10-6 C at x = 1.2 m Q3 = -1.2 x 10-6 C at x = 2.0 m • Find the total force acting on Q2 • F12 = 0.075 N • F32 = 0.051 N • Fnet = 0.126 N

40. Remember… • If I put a charged object • between 2 like-charged objects • The forces push (or pull) against each other. • These forces can be balanced if the middle object is in the right spot…

41. -2 So, what do I mean? F12 = F23? +1 +3 Are these charges going to stay where they are?

42. Example… • Two “+” charges are 2.2 meters apart. Q1 = 2.0 x 10-6 C Q2 = 1.5 x 10-6 C • A third charge “-” is put between them Q3 = -1.2 x 10-6 C • Where does it need to be if it is in equilibrium?

43. New Tools - Lab • Electroscope • Electrophorus

47. Electrostatics February 10/11, 2010 Electric fields

48. Electroscope lab