1. Electric Forces and Electric Fields
Phys2120/2160 Spring 2017
Dzyubenko
Chapter 15
Electric Forces and
Electric Fields
© 2003 Thomson – Brooks/Cole

3. A Bit of History Ancient Greeks
Observed and “identified” electric and magnetic phenomena as early as 700 BC
Found that amber, when rubbed, became electrified and attracted pieces of straw or feathers
Magnetic forces were discovered by observing magnetite attracting iron

4. A Bit More of History William Gilbert 1600 Charles Coulomb 1785
Found that electrification was not limited to amber
Charles Coulomb
Confirmed the inverse square relationship of electrical forces

5. History Final Hans Oersted 1819 Michael Faraday 1831
Compass needle deflects when placed near an electrical current
Michael Faraday
A wire moved near a magnet, an electric current is observed in the wire
James Clerk Maxwell
Formulated the laws of electromagnetism
= complete theory
Heinrich Hertz
Verified Maxwell’s equations

6. Properties of Electric Charges
Two types of charges exist: they are called positive and negative
Named by Benjamin Franklin
Unlike charges attract one another and Like charges repel

7. Properties of Electric Charges
Electric charge is always conserved:
Charge is not created, only exchanged
Objects become charged because negative charge (electrons) is transferred from one object to another

8. Properties of Electric Charges
Nature’s basic carrier of positive charge is the proton
Protons rarely move from one material to another because they are held firmly in the nucleus Exception: electrolytes
Nature’s basic carrier of negative charge is the electron
Gaining or losing electrons is how an object becomes charged (typically)

9. Properties of Electric Charges
Charge is quantized
All charge is a multiple of a fundamental unit of charge e: q=Ne
Quarks are the exception
Electrons have a charge of – e
Protons have a charge of +e

10. Elementary Particles:
Charges and Masses

11. Quick quiz
If you rub an inflated balloon against your hair, the two materials attract each other. Is the amount of charge present in the system of the balloon and your hair after rubbing
(a) less than,
(b) the same as, or
(c) more than the amount of charge present before rubbing?
(b) the same

12. Conductors
Conductors are materials in which electrons move freely through the material
Copper Cu, aluminum Al, and silver Ag are good conductors
When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the conductor

13. Insulators
Insulators are materials in which all electrons are bound to atoms and cannot move freely through the material
Glass and rubber are examples of insulators
When insulators are charged by rubbing, only the rubbed area becomes charged
There is almost no tendency for the charge to move into other regions of the material

14. Semiconductors
The characteristics of semiconductors are between those of insulators and conductors
Silicon Si and germanium Ge are examples of semiconductors
The electrical properties of semiconductors can be changed over many of orders of magnitude by the addition of controlled amounts of certain atoms to the materials (called doping)

15. Charging by Conduction
A charged object (the rod) is placed in contact with another object (the conducting 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

16. Charging by Induction
A negatively charged rubber rod is brought near an uncharged sphere
The charges in the sphere are redistributed

17. Charging by Induction
The region of the sphere nearest the negatively charged rod has an excess of positive charge because of the migration of electrons away from this location
A grounded conducting wire is connected to the sphere
Allows some of the electrons to move from the sphere to the ground

18. Charging by Induction
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

19. Charging Objects by Induction
In most neutral atoms or molecules, the center of positive charge coincides with the center of negative charge
In the presence of a charged object, these centers may separate slightly
This results in more positive charge on one side of the molecule than on the other side
This realignment of charge on the surface of an insulator is known as polarization

20. An Example of Polarization
A charged comb attracts neutral but polarized bits of paper

21. QUICK QUIZ 15.1
If a suspended object A is attracted to object B, which is charged, we can conclude that (a) object A is uncharged, (b) object A is charged, (c) object B is positively charged, or (d) object A may be either charged or uncharged.

22. QUICK QUIZ 15.1 ANSWER
(d). Object A could possess a net charge whose sign is opposite that of the excess charge on B. If object A is neutral, B would also attract it by creating an induced charge on the surface of A:.

23. Coulomb’s Law Charles Coulomb (1736-1806)
Measured the magnitudes of the electric forces between charged objects using the torsion balance, which he invented

24. Coulomb’s Law
The electric forces between charged spheres A and B causes the spheres to either attract or repel each other
The resulting motion cause the suspended fiber to twist
A measurement of the angle of the twist provides a quantitative measure of the electric force
We will use the term point charge to mean a particle of zero size that carries an electric charge

25. Coulomb’s Law, cont.
An electrical force between two point charges has the following properties:
It is inversely proportional to the square of the separation r between the particles and directed along the line joining them
It is proportional to the product of the charges |q1| and |q2|on the two particles
It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs

26. Coulomb’s Law, cont. Mathematically, ke is called the Coulomb Constant
The value of ke depends on the choice of units
The SI unit of charge is the Coulomb (C)
The Coulomb constant ke in SI
ke = 8.99 x 109 N m2/C2

27. Vector Nature of Electric Forces
Two point charges are separated by a distance r
The like charges produce a repulsive force between them
The force on q1 is equal in magnitude and opposite in direction to the force on q2
F12= - F21

28. Vector Nature of Forces
Two point charges are separated by a distance r
The unlike charges produce a attractive force between them
The force on q1 is equal in magnitude and opposite in direction to the force on q2

29. The Electric Force This is the second example of a field force
Gravity was the first
A field force: the force is exerted by one object on another object even though there is no physical contact between them
There are some important differences between electrical and gravitational forces

30. 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

31. QUICK QUIZ 15.2
Object A has a charge of +2 µC, and object B has a charge of +6 µC. Which statement is true: (a) FAB = –3FBA, (b) FAB = –FBA, or (c) 3FAB = –FBA

32. QUICK QUIZ 15.2 ANSWER
(b). By Newton’s third law, the two objects will exert forces having equal magnitudes but opposite directions on each other.

33. The Superposition Principle
The resultant force on any one charge equals the vector sum of the forces exerted by the other individual charges that are present.
Remember to add the forces vectorially

34. Superposition Principle Example
The force exerted by q1 on q3 is F13
The force exerted by q2 on q3 is F23
The total force exerted on q3 is the vector sum of F13 and F23

35. Electric Field Faraday developed an approach to discussing fields
An electric field is said to exist in the region of space around a charged object, the source charge
When another charged object – the test charge –enters this electric field, the field exerts a force on the second charged object

36. Electric Field
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

37. Electric Field Definition of the electric field:
The electric field vector E at a point in space is the electric force Fe acting on a positive test charge q0 placed at that point divided by the test charge
the units in SI: Newton per Coulomb (N/C=V/m)

38. Direction of Electric Field
The electric field produced by a negative charge is directed toward the charge
A positive test charge would be attracted to the negative source charge

39. Direction of Electric Field
The electric field produced by a positive charge is directed away from the charge
A positive test charge would be repelled from the positive source charge

40. More About a Test Charge and The Electric Field
The test charge is required to be a small charge
It can cause no rearrangement of the charges on the source charge
The electric field exists whether or not there is a test charge present
The Superposition Principle can be applied to the electric field if a group of charges is present

41. QUICK QUIZ 15.3
A test charge of +3 µC is at a point P where the electric field due to other charges is directed to the right and has a magnitude of 4  106 N/C. If the test charge is replaced with a –3 µC charge, the electric field at P (a) has the same magnitude but changes direction, (b) increases in magnitude and changes direction, (c) remains the same, or (d) decreases in magnitude and changes direction.

42. QUICK QUIZ 15.3 ANSWER
(c). The electric field at point P is due to charges other than the test charge. Thus, it is unchanged when the test charge is altered. However, the direction of the force this field exerts on the test change is reversed when the sign of the test charge is changed.

43. Problem Solving Strategy
Units
When using ke, charges must be in Coulombs, distances in meters and force in Newtons
If values are given in other units, they must be converted
Applying Coulomb’s Law to point charges
Use the superposition principle for more than two charges
Use Coulomb’s Law to find the individual forces
Directions of forces are found by noting that like charges repel and unlike charges attract

44. Problem Solving Strategies
Calculating Electric Fields of point charges
The Superposition Principle can be applied if more than one charge is present
Use the equation to find the electric field due to the individual charges
The direction is given by the direction of the force on a positive test charge

45. QUICK QUIZ 15.4
A Styrofoam ball covered with a conducting paint has a mass of 5.0 ´ 10-3 kg and has a charge of 4.0 C. What electric field directed upward will produce an electric force on the ball that will balance the weight of the ball? (a) 8.2 ´ 102 N/C (b) 1.2 ´ 104 N/C (c) 2.0 ´ 10-2 N/C (d) 5.1 ´ 106 N/C

46. QUICK QUIZ 15.4 ANSWER
(b). The magnitude of the upward electrical force must equal the weight of the ball. That is: qE = mg, so
= 1.2 × 104 N/C

47. QUICK QUIZ 15.6
A free electron and free proton are placed in an identical electric field. Which of the following statements are true? (a) Each particle experiences the same electric force and the same acceleration. (b) The electric force on the proton is greater in magnitude than the force on the electron but in the opposite direction. (c) The electric force on the proton is equal in magnitude to the force on the electron, but in the opposite direction. (d) The magnitude of the acceleration of the electron is greater than that of the proton. (e) Both particles experience the same acceleration.

48. QUICK QUIZ 15.6 ANSWER
(c) and (d). The electron and the proton have equal magnitude charges of opposite signs. The forces exerted on these particles by the electric field have equal magnitude and opposite directions. The electron experiences an acceleration of greater magnitude than does the proton because the electron’s mass is much smaller than that of the proton.

49. Electric Field Lines
A convenient aid for visualizing electric field patterns is to draw lines pointing in the direction of the field vector at any point
These are called electric field lines and were introduced by Michael Faraday

50. Electric Field Lines The field lines are related to the field by:
The electric field vector, E, is tangent to the electric field lines at each point. A direction is indicated by an arrow head
The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region

51. Electric Field Line Patterns
Point charge
The lines radiate equally in all directions
For a positive source charge, the lines will radiate outward

52. Electric Field Line Patterns
For a negative source charge, the lines will point inward

53. Electric Field Line Patterns
An electric dipole consists of two equal and opposite charges
The high density of lines between the charges indicates the strong electric field in this region

54. Electric Field Line Patterns
Two equal positive point charges
At a great distance from the charges, the field would be approximately that of a single charge of 2q
The bulging out of the field lines between the charges indicates the repulsion between the charges
The low field lines between the charges indicates a weak field in this region

55. Electric Field Patterns
Unequal and unlike point charges
Note that two lines leave the +2q charge for each line that terminates on -q

56. Rules for Drawing Electric Field Lines
The lines must begin on positive charges and terminate on negative charges
In the case of an excess of charge, some lines will begin or end infinitely far away
The number of lines drawn leaving a positive charge or ending on a negative charge is proportional to the magnitude of the charge
No two field lines can cross each other

57. QUICK QUIZ 15.7
Rank the magnitudes of the electric field at points A, B, and C in the figure below, largest magnitude first.

58. QUICK QUIZ 15.7 ANSWER
A, B, and C. The field is greatest at point A because this is where the field lines are closest together. The absence of lines at point C indicates that the electric field there is zero.

59. Conductors in Electrostatic Equilibrium
A good electrical conductor contains electrons that are not bound to any atom and therefore are free to move about within the material
When no net motion of charge occurs within a conductor, the conductor is said to be in electrostatic equilibrium

60. Property 1
The electric field is zero everywhere inside the conducting material
Consider if this were not true
if there were an electric field inside the conductor, the free charge there would move and there would be a flow of charge
If there were a movement of charge, the conductor would not be in equilibrium

61. Property 2
Any excess charge on an isolated conductor resides entirely on its surface
A direct result of the 1/r2 repulsion between like charges in Coulomb’s Law
If some excess of charge could be placed inside the conductor, the repulsive forces would push them as far apart as possible, causing them to migrate to the surface

62. Property 3
The electric field just outside a charged conductor is perpendicular to the conductor’s surface
Consider what would happen it this was not true
The component along the surface would cause the charge to move
It would not be in equilibrium

63. Property 4
On an irregularly shaped conductor, the charge accumulates at locations where the radius of curvature of the surface is smallest (that is, at sharp points)

64. Property 4 Any excess charge moves to its surface
The charges move apart until an equilibrium is achieved
The amount of charge per unit area is less at the flat end
The forces from the charges at the sharp end produce a larger resultant force away from the surface

65. Experiments to Verify Properties of Charges
Millikan Oil-Drop Experiment
Measured the elementary charge, e
Found every drop had a charge equal to some integer multiple of the elementary charge

66. Millikan Oil-Drop Experiment

67. 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

68. Electric Flux
Consider an electric field: uniform in magnitude and direction
The product of the magnitude of the electric field E and a surface area A perpendicular to the field is called the electric flux E
units of E : (Nm2/C)
Electric flux E is proportional to the number of electric field lines penetrating the surface

69. Electric Flux If the area A is not perpendicular to the field lines?
Consider an area A which normal is at an angle  to the uniform electric field.
Area A' is a projection of area A onto a plane oriented perpendicular to the field: A' =A cos  .
The flux through area A is equal the flux through area A' :

70. Electric Flux
The flux through the surface of fixed area has the maximum value when the surface is perpendicular to the field:  = 0º
The flux is zero when the surface is parallel to the field:  = 90º

71. The Flux Through a Closed Surface
A closed surface divides space into an inside and an outside region, so that one cannot move from one region to another without crossing the surface
The net flux through the surface is proportional to the net number of lines leaving the surface = the number leaving the surface minus the number entering the surface
Normal vector of the surface is outward
The net flux can be positive or negative

72. Flux Through a Cube Uniform electric field E = (E,0,0)
The flux through the surfaces 3, 4, 5, and 6 equal zero =>
The net flux over all six faces is

73. Gauss’s Law
Describes a general relationship between the net electric flux through a closed surface (a gaussian surface) and the charge enclosed by the surface

74. Gauss’ Law
Gauss’ Law states that the electric flux through any closed surface is equal to the net charge Q inside the surface divided by εo
εo is the permittivity of free space and equals x C2/Nm2
The area in Φ is an imaginary surface, a Gaussian surface, it does not have to coincide with the surface of a physical object

75. QUICK QUIZ 15.8
For a surface through which the net flux is zero, the following four statements could be true. Which of the statements must be true? (a) There are no charges inside the surface. (b) The net charge inside the surface is zero. (c) The electric field is zero everywhere on the surface. (d) The number of electric field lines entering the surface equals the number leaving the surface.

76. QUICK QUIZ 15.8 ANSWER
Statements (b) and (d) are true and follow from Gauss’s law. Statement (a) is not necessarily true because Gauss’s law says that the net flux through any closed surface equals the net charge inside the surface divided by ε0. For example, a positive and a negative charge could be inside the surface. Statement (c) is not necessarily true. Although the net flux through the surface is zero, the electric field in that region may not be zero.

77. Electric Field of a Charged Thin Spherical Shell
The calculation of the field outside the shell is identical to that of a point charge
The electric field inside the shell is zero

78. Electric Field of a Nonconducting Plane Sheet of Charge
Use a cylindrical Gaussian surface
The flux through the ends is EA, there is no field through the curved part of the surface
The total charge is Q = σA
Note, the field is uniform