Chapter 15First Observations – GreeksBenjamin FranklinProperties of Electric ChargesMore Properties of ChargeProperties of Charge, finalConductorsInsulatorsSemiconductorsCharging by ConductionCharging by InductionCharging by Induction, 2Charging by Induction, 3Charging by Induction, finalPolarizationExamples of PolarizationCoulomb’s LawCoulomb’s Law, cont.Characteristics of ParticlesCharles CoulombVector Nature of Electric ForcesVector Nature of Forces, cont.Electrical Forces are Field ForcesElectrical Force Compared to Gravitational ForceThe Superposition PrincipleSuperposition Principle ExampleElectrical FieldElectric Field, cont.Electric FieldDirection of Electric FieldDirection of Electric Field, contMore About a Test Charge and The Electric FieldProblem Solving StrategyProblem Solving Strategy, contProblem Solving Strategy, Electric FieldsElectric Field LinesElectric Field Lines, cont.Electric Field Line PatternsSlide 39Slide 40Slide 41Electric Field PatternsRules for Drawing Electric Field LinesConductors in Electrostatic EquilibriumProperty 1Property 2Property 3Property 4Property 4, cont.Experiments to Verify Properties of ChargesVan de Graaff GeneratorElectric FluxElectric Flux, cont.Gauss’ LawElectric Field of a Charged Thin Spherical ShellElectric Field of a Nonconducting Plane Sheet of ChargeElectric Field of a Nonconducting Plane Sheet of Charge, cont.Parallel Plate CapacitorChapter 15Electric Forces andElectric FieldsFirst Observations – Greeks Observed electric and magnetic phenomena as early as 700 BCFound that amber, when rubbed, became electrified and attracted pieces of straw or feathersAlso discovered magnetic forces by observing magnetite attracting ironBenjamin Franklin1706 – 1790Printer, author, founding father, inventor, diplomatPhysical Scientist1740’s work on electricity changed unrelated observations into coherent scienceProperties of Electric ChargesTwo types of charges existThey are called positive and negativeNamed by Benjamin FranklinLike charges repel and unlike charges attract one anotherNature’s basic carrier of positive charge is the protonProtons do not move from one material to another because they are held firmly in the nucleusMore Properties of ChargeNature’s basic carrier of negative charge is the electronGaining or losing electrons is how an object becomes chargedElectric charge is always conservedCharge is not created, only exchangedObjects become charged because negative charge is transferred from one object to anotherProperties of Charge, finalCharge is quantizedAll charge is a multiple of a fundamental unit of charge, symbolized by eQuarks are the exceptionElectrons have a charge of –eProtons have a charge of +eThe SI unit of charge is the Coulomb (C)e = 1.6 x 10-19 CConductorsConductors are materials in which the electric charges move freely in response to an electric forceCopper, aluminum and silver are good conductorsWhen a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the materialInsulatorsInsulators are materials in which electric charges do not move freelyGlass and rubber are examples of insulatorsWhen insulators are charged by rubbing, only the rubbed area becomes chargedThere is no tendency for the charge to move into other regions of the materialSemiconductorsThe characteristics of semiconductors are between those of insulators and conductorsSilicon and germanium are examples of semiconductorsCharging by ConductionA charged object (the rod) is placed in contact with another object (the 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 chargingCharging by InductionWhen an object is connected to a conducting wire or pipe buried in the earth, it is said to be groundedA negatively charged rubber rod is brought near an uncharged sphereCharging by Induction, 2The charges in the sphere are redistributedSome of the electrons in the sphere are repelled from the electrons in the rodCharging by Induction, 3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 groundCharging by Induction, final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 chargePolarization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 polarizationExamples of PolarizationThe charged object (on the left) induces charge on the surface of the insulatorA charged comb attracts bits of paper due to polarization of the paperCoulomb’s LawCoulomb shows that an electrical force has the following properties:It is along the line joining the two particles and inversely proportional to the square of the separation distance, r, between them It is proportional to the product of the magnitudes 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 signsCoulomb’s Law, cont.Mathematically,ke is called the Coulomb Constantke = 8.9875 x 109 N m2/C2Typical charges can be in the µC rangeRemember, Coulombs must be used in the equationRemember that force is a vector quantityApplies only to point charges221erqqkF Characteristics of ParticlesCharles Coulomb1736 – 1806Studied electrostatics and magnetismInvestigated strengths of materialsIdentified forces acting on beamsVector 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