Unit 14: Electrostatics. Units of Chapter 16 Static Electricity; Electric Charge and Its Conservation Electric Charge in the Atom Insulators and Conductors.
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<ul><li>Slide 1</li></ul><p>Unit 14: Electrostatics Slide 2 Units of Chapter 16 Static Electricity; Electric Charge and Its Conservation Electric Charge in the Atom Insulators and Conductors Induced Charge; the Electroscope Coulombs Law Solving Problems Involving Coulombs Law and Vectors The Electric Field Slide 3 Units of Chapter 16 Field Lines Electric Fields and Conductors Slide 4 Do Now 1.How do protons and electrons differ in their electrical charge? 2.Is an electron in a hydrogen atom the same as an electron in a uranium atom? 3.Which has more mass a proton or an electron? 4.In a normal atom, how many electrons are there compared to protons? Slide 5 Do Now 1.How do protons and electrons differ in their electrical charge? Same magnitude, but opposite charge. 2.Is an electron in a hydrogen atom the same as an electron in a uranium atom? Yes. 3.Which has more mass a proton or an electron? Proton 4.In a normal atom, how many electrons are there compared to protons? Same number, no net charge. Slide 6 Atomic Structure Slide 7 16.1 Static Electricity; Electric Charge and Its Conservation Objects can be charged by rubbing Slide 8 Triboelectric Series Friction can cause electrons to transfer from one material to another Different materials have a different degree of attraction for electrons The triboelectric series determines which materials have a greater attraction. When two materials are rubbed together, the one with the higher attraction will end up getting some of the electrons from the other material Slide 9 Triboelectric Series Human Hands (if very dry) Leather Rabbit Fur Glass Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel (neutral) Wood Amber Hard Rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (scotch tape) Polypropylene Vinyl (PVC) Silicon Teflon MORE POSITIVE MORE NEGATIVE If two materials are rubbed together, the one that is higher in the series will give up electrons and become more positive. Slide 10 Question If fur is rubbed on glass, will the glass become positively charged or negatively charged? Glass positive Fur -negative Slide 11 16.1 Static Electricity; Electric Charge and Its Conservation Charge comes in two types, positive and negative; like charges repel and opposite charges attract Slide 12 Interaction Between Charged and Neutral Objects Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object. Slide 13 16.1 Static Electricity; Electric Charge and Its Conservation Electric charge is conserved the arithmetic sum of the total charge cannot change in any interaction. Slide 14 16.2 Electric Charge in the Atom Atom: Nucleus (small, massive, positive charge) Electron cloud (large, very low density, negative charge) Slide 15 16.2 Electric Charge in the Atom Atom is electrically neutral. Rubbing charges objects by moving electrons from one to the other. Slide 16 16.2 Electric Charge in the Atom Polar molecule: neutral overall, but charge not evenly distributed Slide 17 16.3 Insulators and Conductors Conductor: Charge flows freely Metals Insulator: Almost no charge flows Most other materials Some materials are semiconductors. Slide 18 Slide 19 How Charge Is Transferred Objects can be charged by rubbing Metal objects can be charged by conduction Metal objects can be charged by induction Slide 20 16.4 Induced Charge; the Electroscope Metal objects can be charged by conduction: Slide 21 Charging by Induction When an object gets charged by induction, a charge is created by the influence of a charged object but not by contact with a charged object. The word induction means to influence without contact. If a rubber balloon is charged negatively (perhaps by rubbing it with animal fur) and brought near (without touching) the spheres, electrons within the two-sphere system will be induced to move away from the balloon. Slide 22 Charging By Induction With Negatively Charged Object In step iii, why is the charge on the right sphere almost uniformly distributed? Slide 23 Charging By Induction With Negatively Charged Object What was the source of positive charge that ended up on sphere B? Slide 24 Source of charge in induction In induction, the source of charge that is on the final object is not the result of movement from the charged object to the neutral object. Slide 25 Ground An infinite source or sink for charge Slide 26 16.4 Induced Charge; the Electroscope They can also be charged by induction: Slide 27 16.4 Induced Charge; the Electroscope The electroscope can be used for detecting charge: Slide 28 16.4 Induced Charge; the Electroscope The electroscope can be charged either by conduction or by induction. Slide 29 16.4 Induced Charge; the Electroscope The charged electroscope can then be used to determine the sign of an unknown charge. Slide 30 16.4 Induced Charge; the Electroscope Nonconductors wont become charged by conduction or induction, but will experience charge rearrangement. The atoms or molecules become polarized. : Slide 31 16.4 Induced Charge; the Electroscope Nonconductors wont become charged by conduction or induction, but will experience charge separation: Slide 32 Do Now True or False? Explain your reasoning. 1.An object that is positively charged contains all protons and no electrons. False Positively charged objects have electrons; they simply possess more protons than electrons. 2. An object that is electrically neutral contains only neutrons. False Electrically neutral atoms simply possess the same number of electrons as protons. Slide 33 Do Now True or False? Explain your reasoning. 1.An object that is positively charged contains all protons and no electrons. 2. An object that is electrically neutral contains only neutrons. Slide 34 Coulombs Law The French physicist Charles Coulomb (1736 1806) investigated electric forces in the 1780s using a torsion balance. Slide 35 16.5 Coulombs Law Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them. Slide 36 Coulombs Law 1. If two point charges and are a distance r apart, the charges exert forces on each object of magnitude: These forces are an action/reaction pair, equal in magnitude but opposite in direction. Slide 37 16.5 Coulombs Law 2. The forces are along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same. Slide 38 16.5 Coulombs Law Unit of charge: coulomb, C The proportionality constant in Coulombs law is then: Charges produced by rubbing are typically around a microcoulomb: When we only need two significant figures: Slide 39 16.5 Coulombs Law Charge on the electron: Electric charge is quantized in units of the electron charge. This is the smallest charge in nature fundamental or elementary charge. The net charge of any object must be a multiple of that charge. Slide 40 16.5 Coulombs Law The proportionality constant k can also be written in terms of, the permittivity of free space: (16-2) Slide 41 16.5 Coulombs Law Coulombs law strictly applies only to point charges. Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors. Slide 42 16.6 Solving Problems Involving Coulombs Law and Vectors The net force on a charge is the vector sum of all the forces acting on it. Slide 43 16.6 Solving Problems Involving Coulombs Law and Vectors Vector addition review: Slide 44 Example 1 Suppose that two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. Determine the magnitude of the electrical force of repulsion between them. Given: Find: F - ? IQ I I= 1.00 C Q 2 I= 1.00 C r = 1.00 m Slide 45 Solve: The force of repulsion of two +1.00 Coulomb charges held 1.00 meter apart is 9 billion Newton. This is an incredibly large force that compares in magnitude to the weight of more than 2000 jetliners. Slide 46 Example 2 Determine the magnitude and direction of the electric force on the electron of a hydrogen atom exerted by the single proton that is the atoms nucleus. Assume the average distance between the revolving electron an the proton Given: Find: F-? Slide 47 Solution: Slide 48 Problem 1 A balloon with a charge of is held a distance of 0.10m from a second balloon having the same charge. Calculate the magnitude of the electrical force between the charges. Draw a diagram. Slide 49 Problem 2 Calculate the electrical force exerted between a 22-gram balloon with a charge of -2.6 C and a wool sweater with a charge of +3.8C; the separation distance is 75cm. (Note: ) Slide 50 Slide 51 Neutral vs. Charged Objects if an atom contains equal numbers of protons and electrons, the atom is described as being electrically neutral. if an atom has an unequal number of protons and electrons, then the atom is electrically charged and referred to as an ion. Any particle, whether an atom, molecule or ion, that contains less electrons than protons is said to be positively charged. Conversely, any particle that contains more electrons than protons is said to be negatively charged. Slide 52 Charged Objects as an Imbalance of Protons and Electrons Electrons can move to the electrons shells of other atoms of different materials. For electrons to make a move from the atoms of one material to the atoms of another material, there must be an energy source and a low-resistance pathway. rubbing your feet on carpet clothes tumble in the dryer Slide 53 Charged Objects as an Imbalance of Protons and Electrons The principle stated earlier for atoms can be applied to objects. Objects with more electrons than protons are charged negatively; objects with fewer electrons than protons are charged positively. Slide 54 True or False? 1.An object that is positively charged contains all protons and no electrons. False Positively charged objects have electrons; they simply possess more protons than electrons. 2. An object that is electrically neutral contains only neutrons. False Electrically neutral atoms simply possess the same number of electrons as protons. Slide 55 Do Now Suppose you rub a glass rod with a silk cloth and a second glass rod with rabbit fur. The silk cloth will acquire a __________ (+, -) charge; the rabbit fur will acquire a __________ (+, -) charge. The rabbit fur and the silk cloth will then be observed to ______________________ (attract, repel, not interact with) each other. Slide 56 Three Ways to Charge an Object 1.Friction (by rubbing) 2. Conduction(with contact) 3. Induction(without contact) Slide 57 Ground An infinite source or sink for charge Charge always distributes itself evenly around a conducting sphere We can think of ground as a conductor that is so large that it can always accept more charge (or provide more charge). Symbol Slide 58 Charging by Conduction When charging something by contact it is important to note the following properties The objects must actually touch and transfer some electrons. The objects become charged alike. The original charged object becomes less charged because it actually lost some charge. Slide 59 The Electroscope The electroscope can be used for detecting charge: Slide 60 Electroscope Can be Charged by Induction Slide 61 Electroscope Can be Charged by Conduction Once the contact of the rod to the electroscope is made, the electrons move from the electroscope to the rod. The electroscope is positively charged. Slide 62 Nonconductors Nonconductors wont become charged by conduction or induction, but will experience charge rearrangement. The atoms or molecules become polarized. Slide 63 Do Now Object A is rubbed with object B. Object C is rubbed with object D. Objects A and D are observed to repel each other. Object B is observed to repel a negatively charged balloon. This is conclusive evidence that object A acquired a __________ (+, -) charge. object B acquired a __________ (+, -) charge. object C acquired a __________ (+, -) charge. object D acquired a __________ (+, -) charge. Slide 64 Law of Universal Gravitation Analogy Coulombs Law Law of Universal Gravitation Slide 65 Questions: 1.Two charged objects have a repulsive force of.080 N. If the charge of one of the objects is doubled, then what is the new force? 2. Two charged objects have a repulsive force of.080 N. If the distance separating the objects is doubled, then what is the new force? Slide 66 Do Now Object A is rubbed with object B. Object C is rubbed with object D. Objects A and D are observed to repel each other. Object B is observed to repel a negatively charged balloon. This is conclusive evidence that object A acquired a __________ (+, -) charge. object B acquired a __________ (+, -) charge. object C acquired a __________ (+, -) charge. object D acquired a __________ (+, -) charge. Slide 67 Six Flags Trip 1.Meet in the auditorium after Pd.2. 2.1PM Check in at buffet in Old Country Picnic Grove. 3.5 PM Meet at fountain. 4.Cell phone. 5.Sunscreen. Slide 68 Slide 69 Slide 70 Slide 71 Slide 72 Slide 73 Slide 74 Do Now Find the force exerted on the test charge. Indicate the direction of that force (Hint: Calculate individual forces on the test charge and add them as vectors.) q A = +2nC q test =-1Cq B =+3nC 1m 1m Slide 75 Solution to the left to the right Slide 76 Gravitational Force Objects near surface of Earth Gravitational force always directed towards the center of Earth. r Force Slide 77 Gravitational Force Force depends on mass of object. Gravitational force always directed towards center of earth. Even when there is no mass nearby the earth, we can still talk about a gravitational field near the earth- pointing towards earth. Slide 78 Electric Field Space around Earth and every mass is filled with gravitational field. Space around every electric charge is filled with electric field. Slide 79 Strength of Electric Field An electric field is a vector. It has both magnitude and direction. Its magnitude can be measured by its effect on a small positive test charge q placed in it. The greater the force acting on the charge, the stronger the electric field. E strength of electric field Units N/C Slide 80 16.7 The Electric Field The electric field is the force on a small (point) charge, divided by the charge: If you know the direction and magnitude of the electric field, you can determine the direction of the force. Negatively charged particles will have opposite direction of force. Slide 81 The Electric Field General Expression for point charge: q- test charge Q source charge Slide 82 Gravitational Field Analogy Strength of electric field: Strength of gravitational field: F=mg Slide 83 Example 1 Jack pulls his wool sweater over his head, which charges his cotton t-shirt as the sweater rubs against it. a) What is the magnitude and directi...</p>
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