CHAPTER22The Electric Field I: Discrete Charge Distributions1* ·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/r2. Differences: There are positive and negative charges but only positive masses. Like charges repel; like masses attract. The gravitational constant Gis 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 μC. How many electrons are transferred from the wool shirt to the plastic rod? ne= q/(–e) ne= (–0.8×10-6)/(–1.6×10–19) = 5×1012. 4 ·A charge equal to the charge of Avogadro's number of protons (NA = 6.02×1023) is called a faraday. Calculate the number of coulombs in a faraday. 1 faraday = NAe1 faraday = 6.02×1023×1.6×10–19C = 9.63×104C 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×nC×e; nC= NA(mC/12) Q= (6×6.02×1023×103×1.6×10–19/12) C = 4.82×107C 6 ·Can insulators be charged by induction? No 7 ··A metal rectangle Bis connected to ground through a switch Sthat is initially closed (Figure 22-28). While the charge +Qis near B,switch Sis opened. The charge +Qis 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 Bbefore the charge +Qwas 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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