Introduction to Electric Forces and Fields: Understanding Charge Properties

1. Welcome to Physics 112 • Instructor: Mike Talbot • Web Page : delta.edu/mttalbot/physics112

2. Chapter 15 Electric Forces and Electric Fields

4. Introduction • Greek contributions (700 BC) • Friction generates electricity • Rub a piece of amber with wool 61

5. Introduction • Charged particles • What are they? • What colors are they?

6. Introduction • Coulomb’s Law • What does it say?

7. Introduction • Electric fields • What is a gravitational field? • What is an electric field?

8. Properties of Electric Charges • An electroscope may be used to demonstrate the existence of electrostatic forces. • It detects and identifies charges produced by such things as: • Plastic combs • Glass and silk • Hard rubber and wool • Balloons 62, 15.1

9. Properties of Electric Charges • Walking across a carpet in the winter can also generate static electricity.

10. Properties of Electric Charges • Dust accumulates on a TV screen because of the large static charge on the glass.

11. Properties of Electric Charges • What should you do if a power line falls on your car while you are in it?

12. Properties of Electric Charges • How many kinds of charge exist?

13. Properties of Electric Charges • How many kinds of charge exist? • Two

14. Properties of Electric Charges • Two kinds of charge (+ and -) • Named by Benjamin Franklin

15. Properties of Electric Charges • Law of Charges 155

16. Properties of Electric Charges • Law of Charges • At least two particles are involved • Like charges repel, opposites attract. • Motion may result

17. Properties of Electric Charges • Law of Charges • At least two particles are involved • Like charges repel, opposites attract. • Motion may result • Planetary model of the atom 125

19. Properties of Electric Charges • Are atoms neutral?

20. Properties of Electric Charges • What is meant by the conservation of charge?

21. Properties of Electric Charges • What is meant by the quantization of charge (e)

22. Properties of Electric Charges • Demonstrating the quantization of charge (e) • The Millikan Oil-Drop Experiment • Measured the elementary charge on an electron 15.21

24. Insulators and Conductors • Materials may be classified by their ability to conduct electricity.

25. Insulators and Conductors • Materials may be classified by their ability to conduct electricity. • Conductors (many free electrons) • Insulators (few free electrons)

26. Insulators and Conductors • Materials may be classified by their ability to conduct electricity. • Conductors (many free electrons) • Insulators (few free electrons) • Semiconductors

27. Insulators and Conductors • What are some examples of good conductors?

28. Insulators and Conductors • What are some examples of good insulators?

29. Insulators and Conductors • What is an example of asemiconductor? • Where are semiconductors used? 225

30. Charging by Friction

31. Charging by conduction 15.3

32. Charging by induction 15.4

33. Grounding 127

34. Insulators and Conductors • Polarization of charge

36. Coulomb’s Law • An electric force has three properties: 15.6

37. Coulomb’s Law • An electric force has three properties: • It is attractive or repulsive depending upon the sign of the charges.

38. Coulomb’s Law • An electric force has three properties: • It is attractive or repulsive depending upon the sign of the charges. • It is directly proportional to the product of the magnitudes of the charges (q1.q2).

39. Coulomb’s Law • An electric force has three properties: • It is attractive or repulsive depending upon the sign of the charges. • It is directly proportional to the product of the magnitudes of the charges (q1.q2). • It is inversely proportional to the square of the separation (r2).

40. Coulomb’s Law • Coulomb’s formula:

41. Coulomb’s Law • Definitions • Coulomb • The amount of charge that has passed a given point in one second when one ampere of current is flowing

42. Coulomb’s Law • Definitions • Ampere • One coulomb of charge passing a given point in one second • Analogy: Like water flowing through a pipe

43. Coulomb’s Law • Coulomb’s constant: ke = 8.9875 x 10 9 N.m2/C2

44. Coulomb’s Law • Charges and masses of particles • Table 15.1 (pg. 501)

45. Coulomb’s Law • Reminders: • Force is a vector quantity • r is the distance between centers • Newton’s Third Law applies • The Coulomb force is a field force just like …?

46. Coulomb’s Law • Reminders: • Force is a vector quantity. • r is the distance between centers. • Newton’s Third Law applies. • The Coulomb force is a field force just likegravity.

47. Coulomb’s Law • The two field force formulas are mathematically identical.

48. Coulomb’s Law • The two field force formulas are mathematically identical. F = k q1q2/r2

49. Coulomb’s Law • The two field force formulas are mathematically identical. F = k q1q2/r2 F = G m1m2/r2

50. Coulomb’s Law • Differences between electrical and gravitational forces • Gravity only attracts