1. Static Electricity Physics

2. 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.

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

4. Leyden Jar

5. 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.

6. Law of Conservation of Electrical Charge
The net charge of an isolated system remains constant.

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

8. Law of Charges
Like charges repel
Opposite charges attract

9. J J 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.

10. JJ Thomson with the CRT

11. Cathode Ray Tube

12. Cathode Ray

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

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

16. First We Have to Remember Matter

17. How It Relates to Charge
Charged versus Uncharged Particles
Positively Charged
Negatively Charged
Uncharged
Possesses more protons than electrons
Possesses more electrons than protons
Equal numbers of protons and electrons

18. It’s All About Electrons
Electrons within atoms can be persuaded to leave their own electron shells and become members of the electrons shells of other atoms of different materials.
In short, electrons are migrants - constantly on the move and always ready to try out a new atomic environment.

19. It’s All About Electrons
For electrons to make a move from the atoms of one material to the atoms of another material, there must be:
an energy source
a motive
and a low-resistance pathway.

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

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

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

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

24. Conductors
Materials that permit electrons to flow freely from atom to atom and molecule to molecule.
Due to the metal’s “sea of electrons”

25. Conductors
Outer electrons of the atoms in a metal are not anchored to the nuclei of particular atoms, but are free to roam in the material.
Materials through which electric charge can flow are called conductors.
Metals are good conductors for the motion of electric charges because their electrons are “loose.”

26. Insulators
Electrons in other materials—rubber and glass, for example—are tightly bound and remain with particular atoms.
They are not free to wander about to other atoms in the material.
These materials, known as insulators, are poor conductors of electricity.

27. Electrostatic Charging Methods
Friction
Conduction
Induction

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

29. Charging by Friction and Contact
We can stroke a cat’s fur and hear the crackle of sparks that are produced.
We can comb our hair in front of a mirror in a dark room and see as well as hear the sparks of electricity.
We can scuff our shoes across a rug and feel the tingle as we reach for the doorknob.
Electrons are being transferred by friction when one material rubs against another.

30. Charging by Friction and Contact
If you slide across a seat in an automobile, you are in danger of being charged by friction.

31. Triboelectric Series - Celluloid
+ Fur (rabbit) Glass Wool Fur (cat) Lead Silk Human skin, Aluminum Cotton Wood Amber Nickel, Copper, Brass, Gold Rubber Sulfur
- Celluloid

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.

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

34. Charging by Induction
When we touch the metal surface with a finger, charges that repel each other have a conducting path to a practically infinite reservoir for electric charge—the ground.
When we allow charges to move off (or onto) a conductor by touching it, we are grounding it.

35. Charging by Induction
Charging by induction occurs during thunderstorms.
The negatively charged bottoms of clouds induce a positive charge on the surface of Earth below.
Most lightning is an electrical discharge between oppositely charged parts of clouds.
The kind of lightning we are most familiar with is the electrical discharge between clouds and oppositely charged ground below.

36. Charging by Induction
If a rod is placed above a building and connected to the ground, the point of the rod collects electrons from the air.
This prevents a buildup of positive charge by induction.
The primary purpose of the lightning rod is to prevent a lightning discharge from occurring.
If lightning does strike, it may be attracted to the rod and short-circuited to the ground, sparing the building.

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

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

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

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

41. Charge Polarization
Charging by induction is not restricted to conductors. Charge polarization can occur in insulators that are near a charged object.
When a charged rod is brought near an insulator, there are no free electrons to migrate throughout the insulating material.
Instead, there is a rearrangement of the positions of charges within the atoms and molecules themselves.

42. The atom or molecule is said to be electrically polarized.
Charge Polarization
One side of the atom or molecule is induced to be slightly more positive (or negative) than the opposite side.
The atom or molecule is said to be electrically polarized.

43. Charge Polarization
When an external negative charge is brought closer from the left, the charges within a neutral atom or molecule rearrange.
All the atoms or molecules near the surface of the insulator become electrically polarized.

44. Examples of Charge Polarization
Polarization explains why electrically neutral bits of paper are attracted to a charged object, such as a charged comb.
Molecules are polarized in the paper, with the oppositely charged sides of molecules closest to the charged object.

45. Charge Polarization The bits of paper experience a net attraction.
Sometimes they will cling to the charged object and suddenly fly off.
Charging by contact has occurred; the paper bits have acquired the same sign of charge as the charged object and are then repelled.

46. Charge Polarization
A charged comb attracts an uncharged piece of paper because the force of attraction for the closer charge is greater than the force of repulsion for the farther charge.

47. Charge Polarization
Rub an inflated balloon on your hair and it becomes charged.
Place the balloon against the wall and it sticks.
The charge on the balloon induces an opposite surface charge on the wall. The charge on the balloon is slightly closer to the opposite induced charge than to the charge of the same sign.

48. Charge Polarization Electric Dipoles
Many molecules—H2O, for example—are electrically polarized in their normal states.
The distribution of electric charge is not perfectly even.
There is a little more negative charge on one side of the molecule than on the other.
Such molecules are said to be electric dipoles.

49. Polarization

50. In summary, objects are electrically charged in three ways.
Charge Polarization
In summary, objects are electrically charged in three ways.
By friction, when electrons are transferred by friction from one object to another.
By contact, when electrons are transferred from one object to another by direct contact without rubbing.
By induction, when electrons are caused to gather or disperse by the presence of nearby charge without physical contact.

51. Charge Polarization
If the object is an insulator, on the other hand, then a realignment of charge rather than a migration of charge occurs.
This is charge polarization, in which the surface near the charged object becomes oppositely charged.

52. Grounding
A ground is simply an object that serves as a seemingly infinite reservoir of electrons; the ground is capable of transferring electrons to or receiving electrons from a charged object in order to neutralize that object.

53. Part II
Coulomb’s Law

54. Coulomb’s Law
1785,Charles Augustin Coulomb (French scientist)
F = k q1q2
______ d2
k = 9 x 109 Nm2/C2
d (or r): distance between the charges.
q : charge of each object.

55. The Direction of the Electrical Force

56. Coulomb’s Law is an Inverse Square Law

57. The electrical force is one of the four fundamental forces.

58. Comparison with Gravitational Force
What are 3 differences between the electrical force and the gravitational force?

59. Comparison with Gravitational Force
What are 3 differences between the electrical force and the gravitational force?
1) Electrical forces are greater than gravitational forces.
2) Electrical forces can repel or attract. Gravitational forces are only attractive.
3) Gravitational forces apply to masses; electrical forces apply to charges.

60. Example 1
A charge of 2mC is 0.5 m from a charge of 3mC. Find the electric force.

61. Example 2
Three charges are positioned as shown. Find the force acting on the 2 C charge.
+1 C
0.5 m
+2 C
0.5 m C

62. 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