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791_Physics ProblemsTechnical Physics

791_Physics ProblemsTechnical Physics - Chapter 28 131...

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Chapter 28 131 Q28.16 A wire or cable in a transmission line is thick and made of material with very low resistivity. Only when its length is very large does its resistance become significant. To transmit power over a long distance it is most efficient to use low current at high voltage, minimizing the I R 2 power loss in the transmission line. Alternating current, as opposed to the direct current we study first, can be stepped up in voltage and then down again, with high-efficiency transformers at both ends of the power line. Q28.17 Car headlights are in parallel. If they were in series, both would go out when the filament of one failed. An important safety factor would be lost. Q28.18 Kirchhoff’s junction rule expresses conservation of electric charge. If the total current into a point were different from the total current out, then charge would be continuously created or annihilated at that point. Kirchhoff’s loop rule expresses conservation of energy. For a single-loop circle with two
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