1. The study of electric charges
Electrostatics
The study of electric charges

2. Introduction
Did you ever run a comb through your hair? What do you notice.
What causes the paper holes to jump onto the comb?
There are electrical forces that are in place due to the presence of charge on the comb.

3. The Atom An atom consists of various charged and uncharged particles.
The central region is called the nucleus.
Protons (+) and neutrons make up the nucleus.
Electrons (-) move around the nucleus in an orbital path.
Nucleus
Protons (+)
Neutrons
Electrons (-)

4. The Significance of Charge
As mentioned before, protons are positive and electrons are negative.
An atom with balanced charges is considered neutral.
The overall charge can be changed by adding or removing electrons. This makes the atom an ion.
Add e-
Take e-
Overall Charge:
(Neutral Atom)
(Positive Ion)
(Negative Ion)

5. Sample Problem (Atomic Charge)
An helium atom has a net electric charge of -8.0x10-19C.
Is it neutral or an ion?
Are there extra electrons or a shortage of them?
How many extra electrons are there?
Charge Per e-: -1.60*10-19C

6. Sample Problem
How many excess electrons are on a ball that has a charge of q = -4x10-17C?

7. Sample Problem (Atomic Charge)
An atom has a net electric charge of 4.8x10-19C.
Are there extra electrons or a shortage of them?
How many electrons short is this atom?
Draw this atom given it is Boron.

8. Electrostatic Demo’s
Tape
Electroscope
Pith Balls

9. Electric Forces Charges exert a force on other charges
Like Charges Repel
Opposites Attract

10. Actual Charge of Protons/Electrons
Recall, charge is measured in Coulombs (C).
Even though protons and electrons are very small, they still have charge.
Let us use q as a variable for charge.
Electron
Proton

11. How do atoms get charged?
Work can remove electrons from the atom.
Results in a positively charged atom
The free Electron can be transferred to another atom.
Results in negatively charged atoms

12. Separating Charge Charges are balanced in neutral objects.
Work must be done to separate charge (free electrons).
Once charge is separated, it can be used in experiments.

13. Separation of Charge Bring a charge rod near a neutral conductor
Un-like charges are attracted
Like charges are repelled

14. Charge by conduction A charge rod touches a neutral conductor
Like charges are repelled and uniformly distribute

15. Charge by Induction
The charges on the spheres redistribute to maximum separation
The charge object is removed
A charge object is placed near neutral conductors
Result: Two spheres charged by induction
Contact between the conducting sphere is broken
Separation of charge takes place A A A A B B B B

16. Charging by Polarization
Certain substances, such as the one below, have polar molecules. These molecules have opposite charges at each end.
Charging by polarization takes place when a charged object is brought near, realigning the molecules in the substance.
Magnification

17. Conductors and Insulators
Electrical Conductors are similar to Heat conductors.
Electrical Conductors allow charge to move easily.
Electrical Insulators do not allow charge to move easily

18. Conductors and Insulators
Electrical Conductors all electrons to move easily.
Metals
Graphite
Electrical Insulators do not allow electrons to move easily
Glass
Plastic
Rubber

19. Homework Ch
Page 464
1-8
11-12

20. Lightning Charging by induction occurs during thunderstorms
The negatively charged cloud induces a positive charge on the ground
Lightning: An electrical between the clouds and oppositely charged ground.

21. Grounding
The earth stores a seemingly infinite amount of charge, both kinds.
An object is grounded when it is connected to the earth or another large object.
Electrical devices often have a “ground,” which prevents unwanted charge buildup.
Grounding is also the principle behind lightning rods.
Outlet
Ground

22. Lightning Average Temperature 30,0000C
(roughly 5x as hot as the sun)
Typical charge (q) for lightning
10C to 25C
How many electrons is this?

23. Lightning is actually a discharge of static electricity.
Charge differences are developed from the friction of dust particles within the cloud.
When the concentration of charges becomes too great, an electrical discharge results

24. The Electroscope
An electroscope is a device that detects electrical charge in objects brought near.
Its metallic inner contents, which are usually neutral, have to be separated from surroundings by some type of insulator.
There are two metal leaves that hang inside.
When a charged object is brought near, the leaves separate.
Charge can also be stored in the electroscope by touching it with the rod.
Leaves

25. Coulomb’s Law
The electrostatic force one charged object exerts on an other
The force is related to the amount of charge
i.e more charge – more force
The force is proportional to 1/d2
i.e. the further apart the charges, the smaller the force

26. Coulomb’s Law Symbol Unit F Force N q1 Charge C q2 Charge C
d Distance m
K constant N m2 / C2

27. Ex. Coulomb’s Law WS 20.1 #4
Object A has a charge of 6x10-6C. Object B has a charge of 3x10-6C and is 0.03m away. Calculate the force on A.

28. Ex. Coulomb’s Law WS 20.1 #5
The distance that separates electrons in a typical atom is 1.5x10-10m. What is the electrostatic force between them?

29. Electro-static Applications
Electrostatic filter

30. Electro-static Applications
Electrostatic painting

31. Electrostatic filters
Scan pictures from p.416

32. Practice Problems
WS 20.1
#’s 2, 3, 6

33. Ex. Coulomb’s Law WS 20.2 #1
Three charges are aligned as shown. Calculate the force on q2 due to q1.
Electric Charge Positions
Scale: 1 Square = 0.05m q1 q3 q2

34. Ex. Coulomb’s Law WS 20.2 #1
Three charges are aligned as shown. Calculate the net force on q2.
Electric Charge Positions
Scale: 1 Square = 0.05m q1 q3 q2

35. Ex. Coulomb’s Law WS 20.2 #1
Three charges are aligned as shown. Calculate the force on q2 due to q3.
Electric Charge Positions
Scale: 1 Square = 0.05m q1 q3 q2

36. Practice Problems
WS 20.2
#’s 3

37. Coulomb’s Law in 2-D To find Fnet with 3 or more charges
Calculate each Force vector.
It helps to have a grid system on which to work.
Use vector addition to find the resultant Fnet q2 q3 q1
F13
F14
F12
FNet q4

38. Coulomb’s Law in 2-D (cont.)
Find the net force acting on q1.
First find the distance between q1 and the others.
Use the Pythagorean Theorem to find these distances.
1st Triangle
2nd Triangle q2 q3 C A B C B A
The variable C from each triangle’s hypotenuse is the variable d used in the Coulomb’s Law equation. q1

39. Coulomb’s Law in 2-D (cont.)
Charge (C) q1
3.0 X 10-4 q2
-2.6 X 10-5 q3
7.2 X 10-6
Attraction q2 q3
F12 q1
F13
Repulsion

40. Coulomb’s Law in 2-D Sample
Determine the direction of each of the forces prior to vector addition.
F12
F13 q2 q3
5.4N
hyp
opp
adj
hyp
adj
opp q1
Quad II Adjust
Quad III Adjust
0.972N

41. Coulomb’s Law in 2-D Sample
The remaining task is to use analytical vector addition.
Mag
Ang X Y Q
F12
5.4N
146.3°
-4.49
3.00 II
F13
0.972N
206.6°
-0.87
-0.44
III
FNet
5.94N
154.5°
-5.36
2.56
Quad II Adjust

42. And in it there’s no lightning
Conclusion
Electrostatics, the study of the forces between charges at rest.
Hospital
And in it there’s no lightning
I had a dream…

43. Practice Problems
WS 20.2
#’s 2

44. Coulomb’s Law in 2-D Sample
Three charges are aligned as shown. Find the net force on q1.
Electric Charge Positions
Scale: 1 Square = 0.5 cm q1 q3 q2
Mag
Ang X Y Q
F12
5.4N
146.3°
-4.49
3.00 II
F13
0.972N
20.6°
-0.87
-0.44
III
FNet
5.94N
154.5°
-5.36
2.56

46. Static Discharge Pic’s

47. End Ch 20

48. Example: Charge Distribution
What is the total charge of three conducting spheres with charges of 6q,-1q, and 0q A 6q B
-1q C 0q

49. Example: Charge Distribution cont.
What is the final charge distribution if sphere A and B touch? C 0q A
2.5q A 6q B
-1q B
2.5q

50. Example: Charge Distribution cont.
What is the final charge distribution if sphere B and C touch? A
2.5q B
2.5q B
1.25q C 0q C
1.25q

51. Example: Charge Distribution cont.
What is the total charge of three conducting spheres C
1.25q A
2.5q B
1.25q

52. Example: Charge Distribution cont.
Sphere B is twice as large as sphere A, what will be the charge distribution after they touch? A 5q B 5q

53. Charge by Conduction Bring a charge rod near a neutral conductor
Un-like charges are attracted
Like charges are repelled

54. A 5q B
-1q C 0q

57. q3 q1
Scale: 1 square =0.1cm q2

58. q1 q3 q2