Chapter 20 Electric Fields and Forces

Chapter 20 Electric Fields and Forces Chapter Goal: To develop a basic understanding of electric phenomena in terms of charges, forces, and fields. Slide 20-1

Chapter 20 Preview Looking Ahead: Charges and Coulomb s Law A comb rubbed through your hair attracts a thin stream of water. The charge model of electricity explains this force. You ll learn to use Coulomb s law to calculate the force between two charged particles. Slide 20-2

Chapter 20 Preview Looking Ahead: The Electric Field Charges create an electric field around them. In thunderclouds, the field can be strong enough to ionize air, causing lightning. You ll learn how to calculate the electric field for several important arrangements of charges. Slide 20-3

Chapter 20 Preview Looking Ahead Text: p. 632 Slide 20-4

Section 20.1 Charges and Forces

Experimenting with Charges The major tools in a modest laboratory for studying electricity include: A number of plastic and glass rods, each several inches long. These can be held in your hand or suspended by threads from a support. Pieces of wool and silk Small metal spheres, an inch or two in diameter, on wood stands Slide 20-6

Experimenting with Charges: Experiment 1 Take a plastic rod that has been undisturbed for a long period of time and hang it by a thread. Pick up another undisturbed plastic rod and bring it close to the hanging rod. Nothing happens to either rod. Slide 20-7

Experimenting with Charges: Experiment 1 Interpretation: There are no special electrical properties to these undisturbed rods. We say that they are neutral. Slide 20-8

Experimenting with Charges: Experiment 2 Vigorously rub both the hanging plastic rod and the handheld plastic rod with wool. Now the hanging rod moves away from the handheld rod when you bring the two close together. Rubbing two glass rods with silk produces the same result: The two rods repel each other. Slide 20-9

Experimenting with Charges: Experiment 2 Interpretation: Rubbing a rod somehow changes its properties so that forces now act between two such rods. We call this process of rubbing charging and say that the rubbed rod is charged, or that it has acquired a charge. Slide 20-10

Experimenting with Charges: Experiment 2 This experiment shows that there is a long-range repulsive force between two identical objects charged the same way. Slide 20-11

Experimenting with Charges The electric force is the force between charged objects. Gravity is also a long-range force, but it is always attractive. The electric force can be repulsive and attractive. Slide 20-12

Experimenting with Charges: Experiment 3 Bring a glass rod that has been rubbed with silk close to a hanging plastic rod that has been rubbed with wool. These two rods attract each other. Slide 20-13

Experimenting with Charges: Experiment 3 Interpretation: We can explain this experiment as well as Experiment 2 by assuming that there are two different kinds of charge that a material can acquire. We define the kind of charge acquired by a glass rod as positive charge, and that acquired by a plastic rod as negative charge. Then these two experiments can be summarized as like charges (positive/positive or negative/negative) exert repulsive forces on each other, while opposite charges (positive/negative) exert attractive forces on each other. Slide 20-14

Experimenting with Charges: Experiment 4 If the two rods are held farther from each other, the force between them decreases. The strength of the force is greater for rods that have been rubbed more vigorously. Slide 20-15

Experimenting with Charges: Experiment 4 Interpretation: Like the gravitational force, the electric force decreases with the distance between the charged objects. And, the greater the charge on the two objects, the greater the force between them. Slide 20-16

QuickCheck 20.1 Charged glass and plastic rods hang by threads. An object attracts the glass rod. If this object is then held near the plastic rod, it will A. Attract the plastic rod. B. Repel the plastic rod. C. Not affect the plastic rod. D. Either A or B. There s not enough information to tell. Slide 20-17

QuickCheck 20.2 A rod attracts a positively charged hanging ball. The rod is A. Positive. B. Negative. C. Neutral. D. Either A or C. E. Either B or C. Slide 20-19

QuickCheck 20.2 A rod attracts a positively charged hanging ball. The rod is A. Positive. B. Negative. C. Neutral. D. Either A or C. E. Either B or C. Slide 20-20

Visualizing Charge A charge diagram gives a schematic picture of the distribution of charge on an object. Slide 20-21

Visualizing Charge: Experiment 5 Start with a neutral, uncharged hanging plastic rod and a piece of wool. Rub the plastic rod with the wool, then hold the wool close to the rod. The rod is attracted to the wool. Interpretation: From Experiment 3 we know that the plastic rod has a negative charge. Because the wool attracts the rod, the wool must have a positive charge. Slide 20-22

Visualizing Charge When a rod is rubbed by wool, not only does the plastic rod acquire a negative charge, but the wool acquires a positive charge. A neutral object is not something with no charge. A neutral object contains equal amounts of positive and negative charge. Slide 20-23

Visualizing Charge An object becomes positively charged if the amount of positive charge on it exceeds the amount of negative charge. Similarly, an object is negatively charged when it has more negative charge on it than positive charge. Slide 20-24

Visualizing Charge Slide 20-25

Visualizing Charge Slide 20-26

Visualizing Charge Slide 20-27

Visualizing Charge The law of conservation of charge states that charge is neither created nor destroyed, only transferred from one place to another. If a certain amount of positive charge is seen somewhere, an equal amount of negative charge must appear elsewhere so that the net charge does not change. Slide 20-28

Visualizing Charge Text: p. 635 Slide 20-29

Visualizing Charge Text: p. 635 Slide 20-30

Insulators and Conductors: Experiment 6 Charge a plastic rod by rubbing it with wool. Touch a neutral metal sphere with the rubbed area of the rod. The metal sphere then repels a charged, hanging plastic rod. The metal sphere appears to have acquired a charge of the same sign as the plastic rod. Slide 20-31

Insulators and Conductors: Experiment 7 Place two metal spheres close together with a plastic rod connecting them. Charge a second plastic rod, by rubbing, and touch it to one of the metal spheres. Afterward, the metal sphere that was touched repels a charged, hanging plastic rod. The other metal sphere does not. Slide 20-32

Insulators and Conductors: Experiment 8 Repeat Experiment 7 with a metal rod connecting the two metal spheres. Touch one metal sphere with a charged plastic rod. Afterward, both metal spheres repel a charged, hanging plastic rod. Slide 20-33

Insulators and Conductors Charge can be transferred from one object to another only when the objects touch. Discharging is removing a charge from an object, which you can do by touching it. Slide 20-34

Insulators and Conductors Conductors are those materials through or along which charge easily moves. Insulators are materials in which charge is immobile. Glass and plastics are insulators, metal is a conductor. Slide 20-35

Insulators and Conductors Text: p. 636 Slide 20-36

Insulators and Conductors Electrostatic equilibrium is the condition in which the charges on an isolated conductor are in static equilibrium with the charges at rest. A conductor is in electrostatic equilibrium with the exception of the brief interval during which charges are adjusting. The movement of charge is extremely fast, so the interval is very brief. Slide 20-37

QuickCheck 20.3 Consider two objects A and B. Object A has a net charge while B is uncharged. Based on this information, it must be true that A. A is a conductor, B is an insulator. B. A is an insulator, B is a conductor. C. A and B are both insulators. D. A and B are both conductor. E. There s not enough information to tell. Slide 20-38

Polarization Charge polarization is the slight separation of the positive and negative charges in a neutral object when a charged object is brought near. Slide 20-40

Polarization The negative charges at the top of the sphere are more strongly attracted to the rod than the distant positive charges are repelled, so there is a net attractive force. Slide 20-41

Polarization The polarization force arises because the charges in a neutral object are slightly separated, not because the objects are oppositely charged. The polarization force between a charged object and a neutral one is always attractive. Slide 20-42

QuickCheck 20.4 Metal spheres 1 and 2 are touching. Both are initially neutral. a. The charged rod is brought near. b. The charged rod is then removed. c. The spheres are separated. Afterward, the charges on the sphere are: A. Q 1 is + and Q 2 is + B. Q 1 is + and Q 2 is C. Q 1 is and Q 2 is + D. Q 1 is and Q 2 is E. Q 1 is 0 and Q 2 is 0 Slide 20-43

QuickCheck 20.5 Metal spheres 1 and 2 are touching. Both are initially neutral. a. The charged rod is brought near. b. The spheres are separated. c. The charged rod is then removed. Afterward, the charges on the sphere are: A. Q 1 is + and Q 2 is + B. Q 1 is + and Q 2 is C. Q 1 is and Q 2 is + D. Q 1 is and Q 2 is E. Q 1 is 0 and Q 2 is 0 Slide 20-45

QuickCheck 20.6 Based on the last experiment, where two spheres were charged by induction, we can conclude that A. Only the charges move. B. Only the + charges move. C. Both the + and charges move. D. We can draw no conclusion about which charges move. Slide 20-47

QuickCheck 20.7 Identical metal spheres are initially charged as shown. Spheres P and Q are touched together and then separated. Then spheres Q and R are touched together and separated. Afterward the charge on sphere R is A. 1 nc or less B. 0.5 nc C. 0 nc D. +0.5 nc E. +1.0 nc or more Slide 20-49

Section 20.2 Charges, Atoms, and Molecules

Charges, Atoms, and Molecules An atom has a dense, positively charged nucleus, containing positively charged protons and neutral neutrons. The nucleus is surrounded by the much-less-massive orbiting negatively charged electrons that form an electron cloud. Charge, like mass, is an inherent property of electrons and protons. Slide 20-52

An Atomic View of Charging Electrons and protons are the basic charges in ordinary matter. There are no other sources of charge. An object is charged if it has an unequal number of protons and electrons. Most macroscopic objects have an equal number of protons and electrons. Such objects are electrically neutral. Slide 20-53

An Atomic View of Charging Objects gain a positive charge not by gaining protons, but by losing electrons. Protons are extremely tightly bound within the nucleus, but electrons are bound much more loosely. Slide 20-54

An Atomic View of Charging Ionization is the process of removing an electron from the electron cloud of an atom. [Insert Figure 20.8] Slide 20-55

An Atomic View of Charging An atom that is missing an electron is called a positive ion. Atoms that can accommodate an extra electron become negative ions. Slide 20-56

An Atomic View of Charging Molecular ions can be created when one of the bonds in a large molecule is broken. Slide 20-57

Charge Conservation Charge is represented by the symbol q. The SI unit is a coulomb (C). The fundamental charge (e) is the magnitude of the charge of an electron or proton: e = 1.60 10 19 C Slide 20-58

Charge Conservation Slide 20-59

Insulators and Conductors The electrons in an insulator are tightly bound to the positive nuclei and are not free to move around. Charging an insulator may leave a patch of molecular ions on the surface, but the patches are immobile. Slide 20-60

Insulators and Conductors In metals, the outer electrons (valence electrons) are weakly bound to the nuclei. They are detached from their parent nuclei and are free to wander through the entire solid, creating a sea of electrons permeating an array of positively charged ion cores. Slide 20-61

Electric Dipoles An electric dipole is two equal but opposite charges with a separation between them. Slide 20-62

Electric Dipoles When the polarization is caused by an external charge, the atom has become an induced electric dipole. Because the negative end of the dipole is slightly closer to the positive charge in this figure, the attractive force on the negative end exceeds the repulsive force on the positive end. There is a net force toward the external charge. Slide 20-63

Hydrogen Bonding Some molecules have an asymmetry in their charge distribution that makes them permanent electric dipoles. In a water molecule, bonding between the hydrogen and oxygen atoms results in an unequal sharing of charge that leaves the hydrogen atoms with a small positive charge and the oxygen atom with a negative charge. Slide 20-64

Hydrogen Bonding A hydrogen bond is the weak bond between the hydrogen atom of one molecule of water and the negative oxygen atom in the second molecule. These weak bonds give water its stickiness responsible for properties such as expansion on freezing. Slide 20-65

Hydrogen Bonding Hydrogen bonds are extremely important in biological systems. The nucleotides, the four molecules guanine, thymine, adenine, and cytosine, on one strand of a DNA helix form hydrogen bonds with the nucleotides on the opposite strand. The nucleotides bond only in certain pairs. This preferential bonding is due to hydrogen bonds. The positive hydrogen atoms on one nucleotide attract the negative oxygen or nitrogen atoms on another. Slide 20-66

Hydrogen Bonding Slide 20-67

Section 20.3 Coulomb s Law

Coulomb s Law Text: p. 642 Slide 20-69

Coulomb s Law Coulomb s law describes the force between two charged particles. Slide 20-70

Coulomb s Law Coulomb s law looks much like Newton s gravity except the charge q can be positive or negative, so the force can be attractive or repulsive. The direction of the force is determined by the second part of Coulomb s law. Slide 20-71

Using Coulomb s Law Coulomb s law is a force law, and forces are vectors. Electric forces, like other forces, can be superimposed. The net electric force on charge j due to all other charges is the sum of the individuals forces due to each charge: Slide 20-72

Using Coulomb s Law Text: p. 643 Slide 20-73

Using Coulomb s Law Text: p. 643 Slide 20-74

QuickCheck 20.8 The charge of sphere 2 is twice that of sphere 1. Which vector below shows the force of 2 on 1? A. B. C. D. E. Slide 20-75

QuickCheck 20.8 The charge of sphere 2 is twice that of sphere 1. Which vector below shows the force of 2 on 1? A. B. C. D. E. Newton s third law Slide 20-76

QuickCheck 20.9 The charge of sphere 2 is twice that of sphere 1. Which vector below shows the force of 1 on 2 if the distance between the spheres is reduced to r/2? A. B. C. D. None of the above. Slide 20-77

QuickCheck 20.10 Which of the three right-hand charges experiences the largest force? A. q B. 2q C. 4q D. q and 2q are tied E. q and 4q are tied Slide 20-79

QuickCheck 20.10 Which of the three right-hand charges experiences the largest force? A. q B. 2q C. 4q D. q and 2q are tied E. q and 4q are tied Slide 20-80

QuickCheck 20.11 In each of the following cases, an identical small, positive charge is placed at the black dot. In which case is the force on the small charge the largest? Slide 20-81

QuickCheck 20.11 In each of the following cases, an identical small, positive charge is placed at the black dot. In which case is the force on the small charge the largest? C Slide 20-82

QuickCheck 20.12 In each of the following cases, an identical small, positive charge is placed at the black dot. In which case is the force on the small charge the largest? (All charges shown are of equal magnitude.) Slide 20-83

QuickCheck 20.15 The direction of the force on charge q is A. Up B. Down C. Left D. Right E. The force on q is zero Slide 20-85

QuickCheck 20.15 The direction of the force on charge q is A. Up B. Down C. Left D. Right E. The force on q is zero Q is slightly closer than +Q. Slide 20-86

Example 20.3 Adding electric forces in one dimension Two +10 nc charged particles are 2.0 cm apart on the x- axis. What is the net force on a +1.0 nc charge midway between them? What is the net force if the charged particle on the right is replaced by a +10 nc charge? Slide 20-87

Example 20.3 Adding electric forces in one dimension (cont.) The force due to q 1 is There is an equal force due to q 2, so the net force on the 1.0 nc charge is F net = (1.8 10 3 N, to the right). Slide 20-88

Example Problem Point charge A has a charge of 1.0 nc, and point charge B has a charge of 4.0 nc. They are separated by 1.0 cm. What are the magnitude and direction of the electric forces on charges A and B? Slide 20-89

QuickCheck 20.13 Which is the direction of the net force on the charge at the lower left? E. None of these. Slide 20-90

QuickCheck 20.13 Which is the direction of the net force on the charge at the lower left? B. E. None of these. Slide 20-91

QuickCheck 20.14 Which is the direction of the net force on the charge at the top? E. None of these. Slide 20-92

QuickCheck 20.14 Which is the direction of the net force on the charge at the top? D. E. None of these. Slide 20-93

Example 20.5 Comparing electric and gravitational forces A small plastic sphere is charged to 10 nc. It is held 1.0 cm above a small glass bead at rest on a table. The bead has a mass of 15 mg and a charge of +10 nc. Will the glass bead leap up to the plastic sphere? Slide 20-94

Example 20.5 Comparing electric and gravitational forces (cont.) SOLVE Using the values provided, we have F 1 on 2 exceeds the bead s weight by a factor of 60, so the glass bead will leap upward. Slide 20-95

Example Problem Two 0.10 g honeybees each acquire a charge of +23 pc as they fly back to their hive. As they approach the hive entrance, they are 1.0 cm apart. What is the magnitude of the repulsive force between the two bees? How does this force compare with their weight? Slide 20-96

Example Problem A housefly walking across a clean surface can accumulate a significant positive or negative charge. In one experiment, the largest positive charge observed was +73 pc. A typical housefly has a mass of 12 mg. What magnitude and direction of an electric field would be necessary to levitate a housefly with the maximum charge? Could such a field exist in air? Slide 20-97