1. Electric Charge & Stuff
Welcome to PHY2054
Chapter 17
Electric Charge & Stuff

2. A Slide Like This Every Day
Today we begin chapter 17 – Electric Charge, Coulombs Law and the Electric Field
There will be NO QUIZ this week.
Clicker use will begin on Friday.
Bring your i-clicker to class every session
Bring a scientific calculator ($13.00 – COSTCO)
If you have your clicker with you, you may use it today.
There WILL be a Quiz next Friday

3. Probable First Observation Electricity

4. Probably never happened, but good story!
Idiot!
If lightening had actually traveled down the kite string, old Ben Franklin would have been toast!
Probably never happened, but good story!

5. A Quick Experiment

6. Allowable Predictions (Use your clicker if you have one.)
Rods will attract each other
Rods will repel each other
Nothing will happen
Something not listed above will happen

7. Experiment #1 Rods will attract each other Rods will repel each other
Pivot
motion
Rubber rod
Rods will attract each other
Rods will repel each other
Nothing will happen
Something not listed above will happen

8. Experiment #2 Rods will attract each other Rods will repel each other
Pivot
Rubber rubbed with
skin of dead rabbit
Rods will attract each other
Rods will repel each other
Nothing will happen
Something not listed above will happen

9. The charges on the two rods are ..
Since we treated both rods in the same way, they should be of the same type
……. different types
I have no idea what you are asking for.
Leave me alone … I’m napping!

10. If you rubbed the rods longer and/or harder, do you think the effect that you see would be
Stronger
Weaker
The same

11. If the two rods are brought closer together, the force acting between them will get …
Stronger
Weaker
The same

12. Definition of sorts
We DEFINE the “stuff” that we put on the rods by the rubbing process as CHARGE.
We will try to understand what charge is and how it behaves.
We add to the properties of materials:
Mass
Charge

13. Experiment #3 Rods will attract each other Rods will repel each other
Pivot
Glass rubbed with wool
Glass rubbed with wool
Rods will attract each other
Rods will repel each other
Nothing will happen
Something not listed above will happen

14. Experiment #4 Rods will attract each other Rods will repel each other
Pivot
Rubber rubbed with
skin of dead rabbit
Glass rubbed with wool
Rods will attract each other
Rods will repel each other
Nothing will happen
Something not listed above will happen

15. What’s Going On?
All of these effects involve rubbing two surfaces together.
Or pulling two surfaces apart.
Something has “happened “to each of these objects.
These objects have a new PROPERTY
Other properties are mass, color
We call this NEW PROPERTY .………. ………CHARGE.
There seems to be two types of charge.

16. We call these two types of charge
Positive
Negative
An object without either a (+) or (-) charge is referred to as being NEUTRAL.

17. Example - Tape

18. Separation

19. An Example
Volunteer Please

20. Effect of Charge

21. We have also observed that there must be TWO kinds of charge.
Call these two types
positive (+)
negative(-)
We “define” the charge that winds up on the rubber rod when rubbed by the dead cat to be NEGATIVE.
The charge on the glass rod or the dead cat is consequently defined as POSITIVE.

22. Old Ben screwed up more than once!!

23. From whence this charge???
Easily Removed - +

25. AXON

27. Signal propagation in neurons
Signal propagation in neurons. Neurons are components of the nervous system of the body that transmit signals as electrical impulses travel along their length. These impulses propagate when charge suddenly rushes into and then out of a part of the neutron called an axon. Measurements have shown that, during the inflow part of this cycle, approximately
enter the axon. How many coulombs of charge enter a 1.5 cm length of the axon during this process?
(sodium ions) per meter, each with charge +e

29. Materials Two kinds of materials: Insulators Conductors
Electrons and Protons are tightly bound to their positions. Hard to move them around.
Conductors
Electrons are easily removed and moved around.
Electrons are said to be MOBILE charges.
There are other kinds of materials that we will not discuss: semiconductors, semi-metals

30. What about a charged rod and a piece of wooden dowel??
Rods will attract each other
Rods will repel each other
Nothing will happen
Something not listed above will happen

31. Neutral (Conducting) Object - POLARIZATION
Positive charge attracts negative charges.
Rod becomes “polarized.
Negative end is closer to positive charge
Distance effect causes attraction.

32. Contact Transfer Q
Q/ Q/2

33. A Question
Two iron spheres contain excess charge, one positive and the other negative.
(a) Show how the charges are arranged on these spheres if they are very far from each other.
(b) If the spheres are now brought close to each other, but do not touch, sketch how the charges will be distributed on their surfaces.
(c) In part (b), show how the charges would be distributed if both spheres were negative.

34. Ways to charge an object
Rubbing or bond breaking (same thing)
Transfer
Direct transfer
Polarization
Induction

35. Quickie: How many kinds of charge are there?
Two: +,-
Three: +,-.neutral

36. Induction

37. Polarize

38. Ground

39. Remove Ground

40. Positive !

41. Balloon Physics

42. Same as before: Polarization

43. What happens when two surfaces touch or rub?
Bonding!
What happens when two surfaces touch or rub?

44. The Triboelectric Series
When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower listed material becomes negatively charged. MORE POSITIVE rabbit's fur glass mica nylon wool cat's fur silk paper cotton wood acrylic cellophane tape polystyrene polyethylene rubber balloon saran wrap MORE NEGATIVE
No! No!
No! No!

45. SUMMARY: So far we have found?
There are TWO types of charge.
Positive
Negative
Like Charges Attract
Un-Like charges repel
The force between charges increases as they are brought closer together.
This charge separation results from chemical bonds which are severed.

46. Getting down to business:
We will discuss the inverse square law that describes how electric charges interact via forces.
This is COULOMB’S LAW
We will discuss the FACT that Coulomb's law is a VECTOR equation.
We will add forces acting on a charge from an assembly of other charges.
We will discuss the concept of FLUX and use it to “derive” a useful law for continuous distributions of charge with high symmetry. This is Gauss’s LAW

47. Forces Between Charges
The force between charges is along the line between them.
The direction depends on the type of charges-
Like Charges Repel
Unlike Charges Attract
The Magnitude of the force is proportional to the inverse square of the distance between the charges.

48. Coulomb’s Law
The magnitude F of the force that each of two point charges q1 and q2 a distance r apart exerts on the other is directly proportional to the product of charges (q1q2) and inversely proportional to the square of the distance between them (r2). The relationship is expressed symbolically as
This relationship is called Coulomb’s law.

49. Coulomb’s Law

50. The Unit of Charge is called
Coulomb’s Law
Force is a Vector !
The Unit of Charge is called
THE COULOMB
Smallest Charge: e ( a positive number)
1.6 x Coul.
electron charge = -e
Proton charge = +e

51. Force is a VECTOR!!

52. EXAMPLE Q = +12 nC. What is the magnitude of the force F on charge Q?

53. The Electric Field

54. Fields Imagine an object is placed at a particular point in space.
When placed there, the object experiences a force F.
We may not know WHY there is a force on the object, although we usually will.
Suppose further that if we double some property of the object (mass, charge, …) then the force is found to double as well.
Then the object is said to be in a force field.
The strength of the field (field strength) is defined as the ratio of the force to the property that we are dealing with.

55. Example – Gravitational Field.
Property is MASS (m).
Force is mg.
Field strength is defined as Force/Property

56. The Gravitational Field That We Live In.M m mg Mg

57. Properties of a FORCE FIELD
It is a property of the position in space.
There is a cause but that cause may not be known.
The force on an object is usually proportional to some property of an object which is placed into the field.

58. Mysterious Force F

59. Electric Field
If a charge Q is in an electric field E then it will experience a force F.
The Electric Field is defined as the force per unit charge at the point.
Electric fields are caused by charges and consequently we can use Coulombs law to calculate it.
For multiple charges, add the fields as VECTORS.

60. Two Charges

61. to know the Electric Field
Doing it Q
A Charge r F q
The spot where we want
to know the Electric Field

62. General-

63. Force  Field

64. The two S’s
Superposition
Symmetry

65. What is the electric field at the center of the square array?
Superposition and Symmetry

66. The FIELD DIAGRAM

67. Field Lines

69. NEW CONCEPT
FLUX
How Much Stuff Is There?

73. What is so important about FLUX??
OUTWARD Pointing Normal
CLOSED Surface DF

74. What is the TOTAL FLUX leaving a closed surface??

75. Gauss’s Law
Gaussian Surface

76. Gauss’s Law

77. Conducting Materials Conductors
Electrons are free to move.
In equilibrium, all charges are a rest.
If they are at rest, they aren’t moving!
If they aren’t moving, there is no net force on them.
If there is no net force on them, the electric field must be zero.
THE ELECTRIC FIELD INSIDE A CONDUCTOR IS ZERO!

78. More on Conductors
Charge cannot reside in the volume of a conductor because it would repel other charges in the volume which would move and constitute a current. This is not allowed.
Charge can’t “fall out” of a conductor.

79. Isolated Conductor Electric Field is ZERO in
the interior of a conductor.
Gauss’ law on surface shown
Also says that the enclosed
Charge must be ZERO.
Again, all charge on a
Conductor must reside on
The SURFACE.

80. Charged Conductors s E=0 E Charge Must reside on the SURFACE - - - - -
Very SMALL Gaussian Surface

81. Isolated (Charged) Conductor with a HOLE in it.
E=0 everywhere
inside the conductor.
So Q (total) =0 inside the hole
Including the surface.

82. A Spherical Conducting Shell with A Charge Inside.
A Thinker!

83. So much for Mr. Coulomb!