Physics 2102Physics 2102Lecture 02: WED 14 JANLecture 02: WED 14 JANElectric Charge IIElectric Charge IIPhysics 2102Jonathan DowlingCharles-Augustin de Coulomb (1736-1806)Version: 1/11/09Electric Charges in SolidsElectric Charges in Solids• In Macroscopic Solids,Nuclei Often ArrangeThemselves Into a StiffRegular Pattern Called a“Lattice”.• Electrons Move AroundThis Lattice. Depending onHow They Move the SolidCan Be Classified by Its“Electrical Properties” Asan Insulator or aConductor.• In a Conductor, Electrons Move Around Freely,Forming a “Sea” of Electrons. This Is Why MetalsConduct Electricity.• Charges Can Be “Induced” (Moved Around) inConductors.Charges in SolidsCharges in SolidsBlue Background = Mobile ElectronsRed Circles = Static Positive Charge (Nuclei)--++Insulating SolidsInsulating Solids• In an Insulator, Each Electron Cloud Is TightlyBound to the Protons in a Nucleus. Wood, Glass,Rubber.• Note That the Electrons Are Not Free to MoveThroughout the Lattice, but the Electron CloudCan “Distort” Locally.+–How to Charge an ObjectHow to Charge an Object• An Object Can Be Given Some “Excess”Charge: Giving Electrons to It (We GiveIt Negative Charge) orTaking Electrons Away (We “Give” ItPositive Charge).• How Do We Do Charge an Object?Usually, Moving Charges From OneSurface to Another by Adhesion (Helpedby Friction), or by Contact With OtherCharged Objects.• If a Conductor, the Whole Electron SeaRedistributes Itself.• If an Insulator, the Electrons StayWhere They Are Put.ElectroscopeElectroscopehttp://www.physicsclassroom.com/mmedia/estatics/esn.htmlVan der Van der GraafGraafGeneratorGeneratorhttp://science.howstuffworks.com/vdg2.htmhttp://www.amasci.com/emotor/vdg.htmlConservation of ChargeConservation of ChargeYou connect thesetogether with a metalwire; what is the finalcharge distribution?? ?Total Amount of Charge in anIsolated System Is Fixed(“Conserved”)+1C -2CExample: 2 Identical MetalSpheres Have Charges+1C and –2C.–1/2 C –1/2 CQuantization of ChargeQuantization of Charge• Charge is always found in INTEGERmultiples of the charge on anelectron/proton.• Unit of charge: Coulomb (C) in SI units• Electron charge = –e = -1.6 x 10–19Coulombs• Proton charge = +e = +1.6 x 10–19 Coulombs• One cannot ISOLATE FRACTIONAL CHARGE(e.g. –1/2 e, +1/3 e, etc.)Ch. 21: SummaryCh. 21: Summary• Electric Charges Come With Two Signs: Positive andNegative.• Like Charges Repel, Opposite Charges Attract, With aMagnitude Calculated From Coulomb’s Law: F=kq1q2/r2• Atoms Have a Positive Nucleus and a Negative “Cloud”.• Electron Clouds Can Combine and Flow Freely inConductors; Are Stuck to the Nucleus in Insulators.•We Can Charge Objects by Transferring Charge, or byInduction.• Electrical Charge Is Conserved, and
View Full Document
Unlocking...