1. Static Electricity

2. Part I History Charge and its Conservation
Conductors, Semiconductors, Insulators
Methods of Charging

3. History Electron means “amber” in Greek
Thales of Miletos 600 BC discovered properties by Greek.
He rubbed amber (mineral) with cat fur and attracted feathers.

4. Ben Franklin’s Kite Experiment (1740’s)

5. Leyden Jar

6. Benjamin Franklin
1740’s lightning experiment with kite, key and Leyden jar (stores static electricity).
Franklin developed the lightning rod.
Proposed conservation of charge.
Saw electricity as a flowing fluid and called the flow direction positive. (Conventional Current)

7. Law of Conservation of Electrical Charge
The net charge of an isolated system remains constant.
Neutral
Positive charge and negative charge

8. Example:
An object of +10 Coulombs (C) touched an identical object that was neutral. What is the charge of each object?

9. Types of Materials in terms of Electrical Conductivity
Conductors (metals)
Semiconductors (germanium, silicon)
Insulators (wood, glass, rubber)

10. Law of Charges
Like charges repel
Opposite charges attract

11. J(oseph) J(ohn) Thomson (1897, England)
He discovered the electron.
He found that the mass of the electron is about 1/1800 of the mass of a hydrogen atom.
He won the Nobel Prize (1906) for his discovery of the electron.

12. JJ Thomson with the CRT

13. Cathode Ray Tube

14. Cathode Ray

15. Electrical Charge
Symbol: q
Unit : Coulomb, C

16. Charge and Mass of the Electron
Charge of Electron:
1.6 x C (Coulombs)
Mass of Electron:
9.11 x kg.

18. Positively charged objects lost electrons.
Example: Rubbing a glass rod with silk.
Rod becomes + (loses electrons)
Silk becomes - (gains electrons). 

19. Negatively charged objects have gained electrons.
Example: rubbing a rubber rod with fur.
Rubber Rod: - charged
Fur: + charged

20. Note
Negatively charged objects have more mass than an identical neutral object, since each extra electron has a mass of 9.11 x kg.

21. Electrostatic Charging Methods
Friction
Conduction
Induction

22. Charging by Friction
The two objects wind up with opposite charges.

23. Triboelectric Series

24. Electroscopes are used to test the charge of an object.

25. Electroscope
No Charge- leaves hang straight down

26. Contact
Rod with negative charge

27. Contact
Rod with negative charge

28. Contact
Rod with negative charge
Transfers electrons

29. Contact Rod with negative charge Transfers electrons
Extra negative charge

30. Contact Rod with negative charge Transfers electrons
Extra negative charge Move leaves apart.

31. Contact
Remove rod leaves stay apart.

32. Charging by Conduction (direct contact)
The objects end up with the same type charge.
If the charges are equal in size, they share the charge equally.

34. Charging by Induction Objects ends up with opposite charge.
Involves grounding.

37. When a charged object is brought near the electroscope, its leaves spread apart.

38. When a charged object touches an electroscope, the electroscope is now charged.

39. What was the charge of the object that touched this electroscope?

40. Polarization / Induced Charge Separation

41. Induced Charge Separation
Rod with negative charge

42. Induced Charge Separation
Rod with negative charge
Pushes electrons in electroscope down

43. Induced Charge Separation
Rod with negative charge
Pushes electrons in electroscope down
Extra negative charge

44. Induced Charge Separation
Rod with negative charge
Pushes electrons in electroscope down
Extra negative charge
Leaves move apart

45. Induced Charge Separation
Remove rod everything returns

46. Part II
Coulomb’s Law

47. Coulomb’s Law 1785,Charles Augustin Coulomb (French scientist)
K = 9 x 109 Nm2/C2
d (or r): distance between the charges.
q : charge of each object.

48. The Direction of the Electrical Force

49. Coulomb’s Law is an Inverse Square Law

50. The electrical force is one of the four fundamental forces.
The Others:
Gravitational force
Strong nuclear force
Weak nuclear force

51. Comparison with Gravitational Force
What are 3 differences between the electrical force and the gravitational force?
Electric forces are stronger than gravitational forces
Electric forces can attract or repel, gravitational forces can only attract.
Electric forces deal with charges, gravitational forces deals with masses.

52. Example 1
A charge of + 2mC is 0.5 m from a charge of + 3mC. Find the electric force.
ANS: N

53. Note Positive sign indicates repulsion.
Negative sign indicates attraction.

54. Example 2
Three charges are positioned as shown. Find the force acting on the B charge.
qA = +1C, qB = +2C, qC = -3C and all charges are separated by 0.5 m.
ANS: 2.3 x o S of E A B C

55. Example 3
Two equal charges are located 1m from each other. The force acting between them is 2N. How many Coulombs is each charge?
Answer: 15μC