题干

如图所示,一个总电阻为R的导线弯成宽度和高度均为d的“半正弦波”形闭合线框,竖直虚线之间有宽带为d、磁感应强度为B的匀强磁场,方向垂直于线框所在的平面.线框以速度v向右匀速通过磁场;ab边始终与磁场边界垂直,从b点到达边界开始到a点离开磁场为止,在这个过程中(   )

A:线框中的感应电流先沿逆时针方向后沿顺时针方向

B:ab段直导线始终不受安培力的作用

C:平均感应电动势为 12 Bdv

D:线框中产生的焦耳热为 B2d2vR

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答案(点此获取答案解析)

A

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    Science has a lot of uses. It can uncover laws of nature, cure diseases, make bombs, and help bridges to stand up. Indeed science is so good at what it does that there's always a temptation(诱惑) to drag it into problems where it may not be helpful. David Brooks, author of The Social Animal: The Hidden Sources of Love, Character,and Achievement, appears to be the latest in a long line of writers who have failed to resist the temptation.

    Brooks gained fame for several books. His latest book The Social Animal, however, is more ambitious and serious than his earlier books. It is an attempt to deal with a set of weighty topics. The book focuses on big questions: What has science revealed about human nature? What are the sources of character? And why are some people happy and successful while others aren't?

    To answer these questions, Brooks surveys a wide range of disciplines(学科). Considering this, you might expect the book to be a dry recitation of facts. But Brooks has structured his book in an unorthodox(非常规的), and perhaps unfortunate, way. Instead of introducing scientific theories, he tells a story, within which he tries to make his points, perhaps in order to keep the reader's attention.So as Harold and Erica, the hero and heroine in his story, live through childhood, we hear about the science of child development and as they begin to date we hear about the theory of sexual attraction. Brooks carries this through to the death of one of his characters.

    On the whole, Brooks' story is acceptable if uninspired. As one would expect, his writing is mostly clear and, to be fair, some chapters stand out above the rest. I enjoyed, for instance, the chapter in which Harold discovers how to think on his own. While Harold and Erica are certainly not strong or memorable characters, the more serious problems with The Social Animal lie elsewhere. These problems partly involve Brooks' attempt to translate his tale into science.