# ch22 - CHAPTER 22 The Electric Field I Discrete Charge...

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CHAPTER 22 The Electric Field I: Discrete Charge Distributions 1* If the sign convention for charge were changed so that the charge on the electron were positive and the charge on the proton were negative, would Coulomb's law still be written the same? Yes 2 ∙∙ Discuss the similarities and differences in the properties of electric charge and gravitational mass. Similarities: The force between charges and masses vary as 1/ r 2 . Differences: There are positive and negative charges but only positive masses. Like charges repel; like masses attract. The gravitational constant G is many orders of magnitude smaller than the Coulomb constant k . 3 A plastic rod is rubbed against a wool shirt, thereby acquiring a charge of –0.8 µ C. How many electrons are transferred from the wool shirt to the plastic rod? n e = q /(– e ) n e = (–0.8 × 10 6 )/(–1.6 × 10 –19 ) = 5 × 10 12 . 4 A charge equal to the charge of Avogadro's number of protons ( N A = 6.02 × 10 23 ) is called a faraday. Calculate the number of coulombs in a faraday. 1 faraday = N A e 1 faraday = 6.02 × 10 23 × 1.6 × 10 –19 C = 9.63 × 10 4 C 5* How many coulombs of positive charge are there in 1 kg of carbon? Twelve grams of carbon contain Avogadro's number of atoms, with each atom having six protons and six electrons. Q = 6 × n C × e ; n C = N A ( m C /12) Q = (6 × 6.02 × 10 23 × 10 3 × 1.6 × 10 –19 /12) C = 4.82 × 10 7 C 6 Can insulators be charged by induction? No 7 ∙∙ A metal rectangle B is connected to ground through a switch S that is initially closed (Figure 22-28). While the charge + Q is near B, switch S is opened. The charge + Q is then removed. Afterward, what is the charge state of the metal rectangle B ? ( a ) It is positively charged. ( b ) It is uncharged. ( c ) It is negatively charged. ( d ) It may be any of the above depending on the charge on B before the charge + Q was placed nearby. ( c ) 8 ∙∙ Explain, giving each step, how a positively charged insulating rod can be used to give a metal sphere ( a ) a negative charge, and ( b ) a positive charge. ( c ) Can the same rod be used to simultaneously give one sphere a positive charge and another sphere a negative charge without the rod having to be recharged? ( a ) Connect the metal sphere to ground; bring the insulating rod near the metal sphere and disconnect the sphere from ground; then remove the insulating rod. The sphere will be negatively charged. ( b ) Bring the insulating rod in contact with the metal sphere; some of the positive charge on the rod will be

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Chapter 22 The Electric Field I: Discrete Charge Distributions transferred to the metal sphere. ( c ) Yes. First charge one metal sphere negatively by induction as in ( a ). Then use that negatively charged sphere to charge the second metal sphere positively by induction. 9* ∙∙ Two uncharged conducting spheres with their conducting surfaces in contact are supported on a large wooden table by insulated stands. A positively charged rod is brought up close to the surface of one of the spheres on the side opposite its point of contact with the other sphere. ( a ) Describe the induced charges on the two conducting spheres, and sketch the charge distributions on them. (
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## This note was uploaded on 01/27/2009 for the course UNKNOWN d taught by Professor D during the Spring '09 term at 東京大学.

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ch22 - CHAPTER 22 The Electric Field I Discrete Charge...

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