英语周报2019到2022八年级上册答案

Possible version:Tom thought for a moment and was about to agree, but he changed his mind. " I can't, BenYou know Aunt Polly wants the fence to be perfect. It has to be done very carefully.I don'tthink you can do it well enough. "He said to Ben, who looked a little discouraged. "Oh, Tom, I'llbe careful. Please let me have a try I would like to give you my apple if you agree, "requestedBen In the end, Tom gave up the brush with unwillingness on his face, but joy in his heartWhile ben worked at the fence in the hot sun Tom sat under a tree, eating the apple, andlanning how to get more help. many of his other friends walked past him. Each one came topllaugh, but remained to whitewash the fence because they all got interested in it. One by one, theywhitewashed the fence, each giving Tom a reward, big or small. If he hadn't run out ofwhitewash, Tom would have owned everything belonging to his friends. He had a most wonderful andidle Saturday, sitting in the shade and enjoying the rewards that his friends gave him

14.【解析】(1)金属棒P1在区域ABCD的磁场中运动时产生的电动势为E=BLv(1分)代入数据解得E=0.6V(1分)回路中的感应电流为/≈E(1分)解得Ⅰ=1A(1分)由平衡条件得F1=2 mosin0+BIL(1分)解得F1=1.3N(1分)(2)当金属棒P1、P2均在磁场中运动时,拉力F的功率最大拉力为F2=2 mosin+4BIL(分)解得F2=2.2N(1分)拉力F的最大功率为P=F2v(1分)解得P=4.4W(1分)

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