Electric Charges and Fields

1. Electric Charges and Fields Pages 628 - 653

2. Introduction Electricity in one form or another underlies just about everything around you from the lightning in the sky to the spark when you touch someone to holding atoms together to form molecules.

5. Atoms Basic units of matter Made up of a nucleus surrounded by the shells or clouds containing electrons

6. Atomic Particles Electrons are located in the shells or clouds and have a negative charge. Protons are located in the nucleus and have a positive charge. Neutrons are located in the nucleus and have no charge.

7. Types of Materials There are 3 types of materials: Conductors, Insulators, and Semiconductors.

8. Conductors Conductors have electrons that are free to move from one atom to another within the material. Therefore, they DO have the ability to conduct electricity.

9. Insulators Insulators have electrons that are tightly bound to the nucleus and are not free to migrate. Therefore, they DO NOT have the ability to conduct electricity.

10. Semiconductors Semiconductors are poor conductors of electricity under most conditions, but under very specific conditions / environments have the ability to become much better conductors.

11. Electroscopes Electroscopes are devices that detect the presence of charge. Electroscopes come in 3 types: pith-ball, gold leaf, and moving vane.

12. Gold Leaf Electroscopes Gold leaf electroscopes show they are charged when their leaves diverge. This happens because both leaves have the same charge and therefore repel each other. Gold leaf electroscopes show that they are neutral when their leaves hang vertically.

14. Induction of a temporary charge in an Electroscope Bring a positively charged rod or object NEAR the knob of a neutral electroscope. The excess positive charges in the rod attract the electrons in the electroscope.

15. Cont�d The electrons in the electroscope migrate up to the knob which is closest to the positively charged rod. The knob now has an excess of electrons and it is temporarily negatively charged. This migration of electrons causes only protons to remain in the leaves. Both of the leaves now have an excess of positive charges and repel each other.

16. Cont�d Once the charged rod is removed, the electrons will flow back to their original positions and the leaves of the electroscope will once again fall back to their original vertical positions. Bring a negatively charged rod or object NEAR the knob of a neutral electroscope. The excess negative charges in the rod repel the electrons in the knob of the electroscope.

17. Cont�d The electrons in the knob of the electroscope migrate down to the leaves which are furthest from the negatively charged rod. The leaves now have an excess of electrons and are temporarily negatively charged. Both of the leaves now have an excess of negative charges and repel each other.

18. Cont�d This migration of electrons causes only protons to remain in the knob which now has a temporary positive charge. Once the charged rod is removed, the electrons will flow back to their original positions and the leaves of the electroscope will once again fall back to their original vertical positions.

19. Polarization of a Conductor Bring a charged rod NEAR a conductor. Electrons in the conductor will migrate toward a positively charged rod and away from a negatively charged rod.

20. Cont�d This migration of electrons will leave one end of the conductor with a net positive charge and the other end with a net negative charge creating 2 poles, one positive and the other negative. Once the charged rod is removed, the electrons will drift back to their original positions and the whole conductor will no longer have poles.

22. Polarization of an Insulator Bring a charged rod NEAR an insulator. In an insulator the electrons are NOT free to migrate so the whole atom rotates to form an orderly arrangement that creates poles within the insulator.

25. Polarization of Conductors and Insulators

26. Charging by Induction Bring a charged object NEAR a neutral conductor. �Ground� the object by connecting it with a conductor to a reservoir of charge such as the Earth. This allows electrons to flow either into or out of the conductor.

27. Cont�d Remove the �ground� connection. Remove the charged rod from the vicinity. The conductor now has a �permanent� charge that is opposite to the charge of the object brought near it.

31. Charging by Conduction TOUCH a charged object to a conductor / electroscope. If it was a negatively charged object, some of the excess electrons will spread throughout the conductor / electroscope, causing both objects to have the same negative charge.

32. Cont�d If it was a positively charged object, electrons from the conductor / electroscope will be transferred to the positively charged until both objects become equally positively charged.

33. Detecting Charge with a Charged Electroscope When a positively charged rod is brought near a positively charged electroscope the leaves will diverge further because the positively charged rod causes the few remaining electrons to flow up to the rod, causing the leaves to become even more positively charged.

34. Cont�d When a negatively charged rod is brought near a negatively charged electroscope the leaves will diverge further because the negatively charged rod causes the electrons in the knob to flow down into the leaves, causing the leaves to become even more negatively charged. When the electroscope and charged object have the same charge, the leaves diverge further.

35. Cont�d When a positively charged rod is brought near a negatively charged electroscope the leaves will collapse back toward the vertical because some of the excess electrons in the leaves will be drawn back up into the knob causing the leaves to be less negatively charged.

36. Cont�d When a negatively charged rod is brought near a positively charged electroscope the leaves will collapse back toward the vertical because the few remaining electrons in the knob will be forced back down into the leaves making them less positively charged. When the electroscope and the charged object have opposite charges, the leaves will collapse back toward the vertical.

38. Static Electricity Static charges are charges at rest. There are 2 kinds: positive and negative. Like charges repel and unlike charges attract. Neutrals are attracted to all charged objects. Charges exert a force through a distance.

42. Static Electricity Electrostatics Lab

43. Coulomb The unit of charge in the metric system is the Coulomb (C). It is named after Charles Coulomb a French physicist.

44. Cont�d A coulomb is defined as the amount of charge found on 6.25*1018 electrons or protons. Therefore, the magnitude of the charge of a proton or electron is 1.6*10-19 C. The mass of a proton is 1.67*10-27 kg and the mass of the electron is 9.1*10-31 kg.

45. Coulomb�s Law F - Force (N) d - distance between charged particles (m) q1 - Charge of first particle (C) q2 - Charge of second particle (C) k - constant of proportionality - 9*109 Nm2/C2

46. A positive force indicates repulsion. A negative force indicates attraction.

47. Forces in a multiple charge system

48. Sample Problems - Coulomb�s Law Calculate the net charge on a substance consisting of 5*1014 electrons. How many electrons must be removed from a metal sphere to give the sphere a net positive charge of 4.8 �C?

49. Cont�d Two point charges of magnitude 3 �C and 5 �C are separated by a distance of 0.5 m. Find the electric force of repulsion between the two. A sphere with a charge of 4*10-5 C is attracted by a second sphere with a force of 350 N when the separation is 10 cm. Calculate the charge on the second sphere.

50. Cont�d Two identical point charges are 3 cm apart. Find the charge on each of them if the force of repulsion is 4*10-7 N. Three charged spheres are located along a metric ruler. The 3 �C charge is located at 0 cm, the 5 �C is located at the 5 cm mark, and the -15 �C charge is located at the 10 cm mark. Find the net force on the 5 �C charge and which direction will it move?

51. Homework Quiz 1 1. If an object has a deficit of 500 electrons, what is the charge on the object? 2. Two pith balls are separated by a distance of 20 cm. They experience an attractive force of 2 N. If one pith ball has a charge of 4 micro Coulombs, what is the charge on the second pith ball?

52. Electric Fields Electric fields have both magnitude and direction. Lines of force show the direction that a positive test charge would move within the field. Electric field strength or intensity is indicated by the spacing between the lines. The field is strong where the lines are close together and weaker where they are further apart.

53. Cont�d Field lines always begin on a positive charge and end on a negative charge. The number of field lines drawn around a charge is proportional to the magnitude of its charge. No two field lines ever cross.

54. Cont�d Field lines leave and arrive at surfaces perpendicular to the surface since that is the shortest path. Electric field surrounding a single positive charge Electric Field surrounding a single negative charge

61. Electric Field Intensity E - Electric Field Strength or Intensity (N/C) F - Force experienced by a test charge at that location (N) q - magnitude of the test charge placed at that location (C).

62. E � Electric Field strength or intensity (N/C) k � constant of proportionality (9*109) d �distance (m) q � magnitude of charge creating the field (C)

63. Sample Problems - Electric Field Intensity The force on a charge of -3*10-7 C is measured and found to be 0.24 N in a downward direction. What are the magnitude and direction of the electric field at this point? Calculate the magnitude and direction of the electric field at a point 50 cm directly above a charge of -2*10-6 C.

64. Electric Potential PE - Electric Potential Energy (J) d - distance between charged particles (m) q1 - Charge of first particle (Coulomb, C) q2 - Charge of second particle (Coulomb, C) k - constant of proportionality 9*109 Nm2/C2

65. Electric Potential Electric potential is the ratio of electric potential energy to charge. Electric potential is measured in Joules per Coulomb otherwise known as a Volt. If two objects are attracted to each other and they are further separated, their PE increases but if they come closer together their PE decreases.

66. Cont�d If two objects are repelled by each other and they are further separated, their PE will decrease but if they come closer together their PE increases.

67. Potential Difference or Voltage As with gravitational potential energy, your frame of reference determines the �amount� of PE, we will only talk about changes in potential energy. The change in potential energy is called potential difference.

68. Cont�d If work is required to move a charge from one point to another, then there is a potential difference between the two locations. If NO work is required to move the charge from one place to another, then there is no potential difference between the two locations.

69. Cont�d V - voltage or potential difference (V) W - work done (J) q � magnitude of the charge (C)

70. Sample Problems - Potential Difference What is the potential difference between two points if 200 J of energy is required to move 40 C from one point to another? How much energy is required to move an electron between the two terminals of a large X-ray tube whose potential difference is 4 million Volts?

71. Cont�d An electron leaves the heated cathode of a radio or TV vacuum tube with negligible initial velocity, and is accelerated through an applied potential difference of 500V. What is the velocity of the electron as it approaches the electrode?

72. Equipotential Lines or Surfaces Equipotential lines or surfaces consist of all points around a charged object that have the same electric potential. No work is done moving along an equipotential line or surface.

73. Cont�d These lines or surfaces are always perpendicular to the lines of force at the point where the equipotential line or surface crosses the line of force. Equipotential lines / surfaces do not cross since the same point cannot have two different potentials.

74. Equipotential Lines

75. Parallel Plates of Charge A constant electric force and field can be made by placing two large, flat conducting plates parallel to each other and charging them oppositely. The electric field between them is constant except at the edges of the plates.

76. Cont�d If an electron is placed near the negatively charged plate, it will experience a constant force as it moves across the open space toward the positively charged plate. The electric field between the two plates is said to be directed from the positive plate to the negative plate.

79. Potential Difference between two parallel plates V - Voltage or Potential Difference between the plates (V) E - Electric Field Intensity (N/C) d - distance between the plates (m)

80. Sample Problems - Parallel Plates A 12 V battery is connected between two parallel metal plates separated by 0.3 cm. Find the strength of the electric field.

81. Cont�d A proton is released form rest in a uniform electric field of magnitude 8000 V/m. The proton undergoes a displacement of 0.5 m in the direction of the field. Find the change in the electric potential of the proton as a result of this displacement. Find the speed of the proton after it has been moved 0.5 m, starting from rest.

82. Electrostatic Equilibrium All systems want to be at equilibrium, when the energy of the system is a minimum. Charges in a conductor will move until the electric potential is the same everywhere on the conductor.

83. Distribution of Charges on a Conductor in Electrostatic Equilibrium The electric field is zero everywhere inside the conductor. Any excess charge on an isolated conductor will reside on its surface. The electric field just outside the conductor�s surface is perpendicular to the surface. On irregularly shaped conductors, charge accumulates at sharp points.

84. Electrical Discharges For an irregularly shaped conductor, electrical discharge is most likely to occur from the pointed regions since charge builds up there.

85. Millikan�s Experiment Robert Millikan was able to find the charge of an electron by suspending a drop of oil between two parallel plates. When the oil drop was suspended, the force of weight of the object had to be equal to the force exerted by the electric field.

86. Cont�d V - potential difference between plates (V) m - mass of particle (kg) g - acceleration due to gravity 9.8 m/s2 q - charge on particle (C) d - distance between plates (m)

87. m= mass in kg g = acceleration due to gravity E = electric field intensity or strength (N/C) q = charge in Coulombs

88. Sample Problems - Millikan�s Experiment A negatively charged oil drop weighing 1.59*10-14 N is balanced in the electric field between the two oppositely charged plates in a Millikan apparatus. The difference of potential between the plates is 100 V and the distance between them is 0.0050 m. What is the field strength between the plates? What is the charge on the oil drop? How many excess electrons are on the drop?