1. Electric Forces and Electric Fields

2. The primary particle that carries charge (and therefore can be lost or gained) in an atom is a/an:
Proton
Neutron
Electron

3. When a positively charged object comes close to an negatively charged object, the negative object will:
Be attracted
Be repelled
Do nothing

4. When a positively charged object comes close to an neutral object, the neutral object will:
Be attracted
Be repelled
Do nothing

5. Anytime an object at rest starts to move, what must be present to cause it to move?
Difference in electric charge
Electricity
A force
Matter

6. A Bit of History Ancient Greeks
Observed electric and magnetic phenomena as early as 700 BC
Found that amber, when rubbed, became “electrified” and attracted pieces of straw or feathers

7. Properties of Electric Charges
Two types of charges exist
They are called positive and negative
Arbitrarily named by Benjamin Franklin
Like charges repel and unlike charges attract one another

8. More Properties of Charge
Positive charge – protons
Negative charge – electrons
Gaining or losing electrons is how an object becomes charged; protons remain with the nucleus

9. A little review: What are some conservable quantities in physics? Mass
Momentum
Energy
Charge

10. Conservation of Charge
Electric charge is always conserved
Charge is not created, only exchanged
Objects become charged because negative charge is transferred from one object to another

11. More review: A force is …? Contact vs. Field forces?
Anything that produces acceleration or a change in motion.
Contact vs. Field forces?
Contact: require matter to be in contact (ex. friction)
Field: matter not required (ex. gravitational, electrical, magnetic)

12. Fig. 15.1, p. 467

13. Properties of Charge, final
Charge is quantized
All charge is a multiple of a fundamental unit of charge, symbolized by e
Electrons have a charge of –e
Protons have a charge of +e
The SI unit of charge is the Coulomb (C)
1 e = 1.6 x C

14. Fig. 15.T1, p. 472

15. Conductors
Conductors are materials in which the electric charges move freely
Copper, aluminum and silver are good conductors

16. Insulators
Insulators are materials in which electric charges do not move freely
Glass and rubber are examples of insulators
When insulators are charged by rubbing, only the rubbed area becomes charged
There is no tendency for the charge to move into other regions of the material as opposed to conductors

17. Three Methods of Charging
Friction
Conduction (or Contact)
Induction

18. Charging by Friction
The act of rubbing creates friction which removes or adds electrons to the objects involved in the friction.

19. Charging by Conduction
A charged object (the rod) is placed in contact with another object (the sphere)
Some electrons on the rod can move to the sphere
When the rod is removed, the sphere is left with a charge
The object being charged is always left with a charge having the same sign as the object doing the charging

20. Charging by Induction
A negatively charged rubber rod is brought near an uncharged sphere
The charges in the sphere are redistributed
Some of the electrons in the sphere are repelled from the electrons in the rod

21. Charging by Induction, final
The wire to ground is removed, the sphere is left with an excess of induced positive charge
The positive charge on the sphere is evenly distributed due to the repulsion between the positive charges
Charging by induction requires no contact with the object inducing the charge

22. Not enough information to tell
A charged rod is brought close to a neutral electroscope. When touched by the rod, the leaves both become positive and repel. The rod must have been …
Positively charged
Negatively charged
Not enough information to tell

23. Not enough information to tell
A neutral electroscope is charged by induction. When touched by the rod, the leaves both become negative and repel. The rod must have been …
Positively charged
Negatively charged
Not enough information to tell

24. Not enough information to tell
A postively charged rod is brought close to a neutral electroscope to charge it by induction. The top of the electroscope must be:
Positively charged
Negatively charged
Not enough information to tell

25. Coulomb’s Law Mathematically, kc is called the Coulomb Constant
kc = 8.99 x 109 N m2/C2

26. Coulomb’s Law Typical charges can be in the µC range
Remember, Coulombs must be used in the equation
Remember that force is a vector quantity
It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs (you must note if it is attractive or repulsive after the magnitude – 3 N attractive)

27. How is the magnitude of the charges proportional to the electric force between them?
Directly
Inversely
Exponentially

28. How is the square of the distance between two charges proportional to the electric force between them?
Directly
Inversely
Exponentially

29. Coulomb’s Law
It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs (you must note if it is

30. Things that make you go, “Hmmmm…”
A) The electric force is significantly stronger than the gravitational force. However, although we are attracted to Earth by gravity, we do not usually feel the effects of the electric force.
Explain why.

31. B) An ordinary nickel contains about 1024 electrons, all repelling one another.
Why don’t these electrons fly off the nickel?

32. C) When the distance between two negatively charged balloons is doubled, by what factor does the repulsive force between them change?

33. Electrical Force Compared to Gravitational Force
Both are inverse square laws
The mathematical form of both laws is the same
Electrical forces can be either attractive or repulsive
Gravitational forces are always attractive

34. Gravitational Force Electric Force Same Unable to tell
If all other variables are held constant, is the electric force or the gravitational force greater for two oppositely charged objects?
Gravitational Force
Electric Force
Same
Unable to tell

35. Compare Gravitational and Electric Force
Calculate the gravitational force as well as the electric force for an electron and a proton which are located 1 cm from each other.

36. Electrical Field
An electric field is said to exist in the region of space around a charged object
When another charged object enters this electric field, the field exerts a force on the second charged object

37. Electric Fields
The concept of a field is used to describe any quantity that has a value for all points in space.
You can think of the field as the way forces are transmitted between objects.
Charge creates an electric field that creates forces on other charges.

38. Gravitational Field
Mass creates a gravitational field that exerts forces on other masses.

39. Gravitational vs. Electric Fields
Gravitational forces are far weaker than electric forces.

40. Van de Graaff Generator
An electrostatic generator designed and built by Robert J. Van de Graaff in 1929
Charge is transferred to the dome by means of a rotating belt
Eventually an electrostatic discharge takes place

41. Electrical Field
An electric field is said to exist in the region of space around a charged object
When another charged object enters this electric field, the field exerts a force on the second charged object

42. Electric Field, cont.
A charged particle, with charge Q, produces an electric field in the region of space around it
A small test charge, qo, placed in the field, will experience a force

43. Electric Field Lines Electric field is a vector.
There are electric field lines that help visualize this field and were introduced by Michael Faraday

44. Electric Field Lines
Electric field lines are drawn to visualize electric field strength and direction.
Introduced by Michael Faraday

45. Electric Field Line Rules
Electric Field Lines are drawn pointing in the way that a positive point charge would move when placed by the charged object.
Field lines can never cross
Electric fields exist even in the
absence of a point charge.

46. Electric Field Line Rules
The direction of the electric field is in the same direction as the electric force on the point charge.
The relative strength of the electric field is proportional to the number of field lines in a given location.
Electric field lines accumulate as sharp points more than rounded objects.

47. Proton
A positive test charge would be repelled by the field + +

48. Electron
A positive test charge would be attracted by the field + -

49. Opposite charges attract

50.                                      
Like Charges repel

51. Electric Field Lines for a Dipole

52. Electric Field for Parallel Plate Capacitor

53. Summary of Electric Field Lines

54. Electric Potential Energy of a Charge
Wants to move when it has high PE
Point b
PE = max
KE = min
Point a
PE = min
KE = max

55. Electric Field Intensity Equation
E = F/q
F is the force in Newtons acting on the test charge q in coulombs.
Combined with Coulomb’s law,
E = kQ/r2

56. Electric Fields Between Charges – Superposition Principle
Find “E” for each
charge, add all
together.
ET=E1+E2+E3…