# 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 +

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