Physics 2102 Jonathan Dowling Physics 2102 Lecture 02 WED 14 JAN Electric Charge II Version 1 11 09 Charles Augustin de Coulomb 1736 1806 Electric Charges in Solids In Macroscopic Solids Nuclei Often Arrange Themselves Into a Stiff Regular Pattern Called a Lattice Electrons Move Around This Lattice Depending on How They Move the Solid Can Be Classified by Its Electrical Properties As an Insulator or a Conductor Charges in Solids In a Conductor Electrons Move Around Freely Forming a Sea of Electrons This Is Why Metals Conduct Electricity Charges Can Be Induced Moved Around in Conductors Blue Background Mobile Electrons Red Circles Static Positive Charge Nuclei Insulating Solids In an Insulator Each Electron Cloud Is Tightly Bound to the Protons in a Nucleus Wood Glass Rubber Note That the Electrons Are Not Free to Move Throughout the Lattice but the Electron Cloud Can Distort Locally How to Charge an Object An Object Can Be Given Some Excess Charge Giving Electrons to It We Give It Negative Charge or Taking Electrons Away We Give It Positive Charge How Do We Do Charge an Object Usually Moving Charges From One Surface to Another by Adhesion Helped by Friction or by Contact With Other Charged Objects If a Conductor the Whole Electron Sea Redistributes Itself If an Insulator the Electrons Stay Where They Are Put Electroscope http www physicsclassroom com mmedia estatics esn html Van der Graaf Generator http science howstuffworks com vdg2 htm http www amasci com emotor vdg html Conservation of Charge Total Amount of Charge in an Isolated System Is Fixed Conserved Example 2 Identical Metal Spheres Have Charges 1C and 2C You connect these together with a metal wire what is the final charge distribution 1C 2C 1 2 C 1 2 C Quantization of Charge Charge is always found in INTEGER multiples of the charge on an electron proton Unit of charge Coulomb C in SI units Electron charge e 1 6 x 10 19 Coulombs 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 Summary Electric Charges Come With Two Signs Positive and Negative Like Charges Repel Opposite Charges Attract With a Magnitude Calculated From Coulomb s Law F kq1q2 r2 Atoms Have a Positive Nucleus and a Negative Cloud Electron Clouds Can Combine and Flow Freely in Conductors Are Stuck to the Nucleus in Insulators We Can Charge Objects by Transferring Charge or by Induction Electrical Charge Is Conserved and Quantized
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