Chapter 21 Electric Charge 21-2 - Electric Charge: In dry weather Spark B produce by o walking across certain types of carpet o Bringing B fingers near a metal doorknob o Bringing B fingers near a metal faucet o Bringing B fingers near a friend Multiple sparks B produce when o Pulling B a sweater from the body o Pulling B clothes from a dryer Sparks and the "static cling" of clothing Are illustrated in 9 Chapter 21 Electric Charge FIG. 21·1 Static cling, an electrical phenomenon that accompanies dry weather, causes these pieces of paper to stick to one another and to the plastic comb, and your clothing Electric charge B an intrinsic characteristic of the fundamental particles making up those objects Benjamin Franklin [1706 - 1790] arbitrarily B named the kind of electricity On Glass Positive Plastic negative Object B contains equal amounts of the two kinds of charge B electrically neutral B no net charge Object B contains no equal amounts of the two kinds of charge B charge imbalance B there is a net charge Charged objects interact by exerting forces on one another. . Charges B have the same electrical sign B repel each other 10 Chapter 21 Electric Charge Charges B have opposite electrical signs B attract each other. First B charge a glass rod by rubbing one end with silk (Fig. 21-2a) B the two rods repel each other; that is, each rod experiences a force directed away from the other rod. Second B charge a plastic rod by rubbing one end with fur (Fig. 21-2b), the two rods attract each other; that is, each rod experiences a force directed toward the other rod 11 Chapter 21 Electric Charge Attraction and repulsion between charged bodies have many industrial applications • Electrostatic paint spraying • powder coating, • fly-ash collection in chimneys • Nonimpact ink-jet printing, • Photocopying Figure 21-3 B shows a tiny carrier bead in a photocopying machine, covered with B particles of black powder called toner B stick to it B electrostatic forces. 12 Chapter 21 Electric Charge 21-3 Conductors and Insulators The properties of conductors and insulators are due to the structure and electrical nature of atoms Atoms consist of Negatively charged Positively charged Neutral particle electrons proton neutron All matter is electrical interaction between electrically charged particle Bohr atom Bohr atom B analogous to a solar system The electrical forces B taking the place of gravitational forces. Nucleus B Protons + Neutrons Electrons B outside the nucleus B large distance from it. Solar System 13 Chapter 21 Electric Charge Ionization The atom as a whole is electrically neutral e P N N P e N P e P e N N P neutral atom e N P e Negative ion Positive ion ; How conduction can eliminate excess charge on an object ? Do The following experiments 6 Experiment (1) rubbed Observation Hand Fur Metal Earth The rod does not charge 6 Experiment (2) rubbed Fur Hand Plastic Observation 14 Metal Chapter 21 Electric Charge The metal rod is charged and it attracted with the piece of fur 6 Experiment (3) The rod of plastic is replaced by the rod of glass rubbed Fur Hand Glass Metal Observation The metal rod is charged and it attracted with the piece of fur Conclusion (1) The metal, the human body, and the earth are conductors of electricity. Evidence: Permit electric charge to move from one to the other region of material Conclusion (2) The glass, plastic, and etc. are insulators. Evidence: Do not permit electric charge to move from one to the other region of material. Conclusion (3) ; In electricity 15 Chapter 21 Electric Charge Materials classified B according to the ability of charge to move through them. Conductors B charge can move rather freely metals Û copper Û human bodyÛ water. Insulators B charge cannot move freely Plastic Û glass Û chemically pure water Semiconductors B intermediate B conductors Û insulators silicon Û germanium Superconductors B charge to move without any hindrance materials B perfect conductors ; Demonstrates the mobility of charge in a conductor FIG.21-4 16 Chapter 21 Electric Charge ; In B Fig.21-4 © First A neutral copper rod B electrically isolated from its surroundings by being suspended on a nonconducting thread The conduction electrons B in the closer end of the copper rod B repelled by the negative charge on the plastic rod Some of the conduction electrons B move to the far end of the copper rod B leaving the near end with an unbalanced positive charge Negative charge B attracts the remaining positive charge on the near end of the copper rod B rotating the copper rod B bring that near end closer to the plastic rod. A copper rod B still neutral B have an induced charge B B some of its positive and negative charges Bseparated due to the presence of a nearby charge 17 Chapter 21 Electric Charge Second A positively charged glass rod B brought near one end of a neutral copper rod B conduction electrons in B copper rod B attracted to that end. End B becomes negatively charged Û the other end B positively charged ; Only conduction electrons B negative charges B moveB positive ions B fixed in place B an object becomes positively charged only through the removal of negative charges. 18