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Ch 5 Exercises

# Ch 5 Exercises - 182 The Silicon Web Physics for the...

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182 The Silicon Web: Physics for the Internet Age In 2 sec, the number of electrons is 2 × 6.25 × 10 19 = 12.5 × 10 19 . Q5.4 Work = energy = 5 C × 1.5 V = 7.5 J Q5.5 In the wire connecting the right plate to the minus end of the battery, the voltage is equal everywhere. There is no net force on the electrons. EXERCISES AND PROBLEMS Exercises E5.1 A metal ball has an excess charge of 1,000 electrons, and a second metal ball has a defi cit of 1,000 electrons. (a) If the two balls are connected by a long, thin piece of copper metal, what will happen to the charge on the negatively charged metal ball? (b) If the two balls are connected by a long, thin piece of glass, what will hap- pen to the charge on the negatively charged metal ball? Explain. E5.2 You have three insulated metal balls, A, B, and C, of equal size. You rub a Tefl on (plastic) rod with rabbit fur, and then touch the rod to ball A to transfer charge. To charge ball B, you rub a glass rod with silk then touch the rod to ball B. Assume that the quantity of charge transferred in each case equals − × 1 10 16 e (recall that e is the charge of 1 electron). Ball C is uncharged. Consider each case below separately, starting from the same initial conditions described above. (a) If you connect ball A to ball B through a neutral copper rod, what is the net charge on each ball afterwards? (b) If you connect ball A to ball B through a neutral glass rod, what is the net charge on each ball afterwards? (c) If you connect ball A to ball C through a neutral copper rod, what is the net charge on each ball afterwards? (Think of the charges on A repelling each other.) (d) If you connect ball B to ball C through a neutral copper rod, what is the net charge on each ball afterwards? (e) If you connect ball B to a copper wire that connects to a copper plumbing pipe that goes into the Earth, what is the net charge on ball B afterwards? E5.3 Explain why it is scientifi cally valid (and perhaps amusing) to claim that the Earth’s magnetic “north pole” is actually in Antarctica, not in northern Canada. E5.4 A small copper block has an excess charge of 2,000 electrons. A small gold block has an excess of 1,000 electrons. A small silver block has a defi cit of negative charge corresponding to 1,000 electrons. (a) Describe the forces between each pair of objects if separated by 1 mm. Which force is the strongest? (b) If the silver block is put into contact with the gold block, describe the force between the copper block and the combined silver/gold object. (c) If the silver block is put into contact with the copper block, describe the force between the combined silver/copper object and the gold object. TAF-K10173-08-1107-005.indd 182 TAF-K10173-08-1107-005.indd 182 4/25/09 12:51:18 AM 4/25/09 12:51:18 AM

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Electricity and Magnetism 183 E5.5 Referring to the three blocks in E5.4: (a) Sketch the electric fi eld arrows (as in Figure 5.7) surrounding each object when isolated. How would a small piece of dust having 100 excess electrons be affected by the fi eld around each object? Which force is strongest (for a fi xed distance between charged object and piece of dust)?
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Ch 5 Exercises - 182 The Silicon Web Physics for the...

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