题干

如图所示,一根横截面积为S的均匀长直橡胶棒上均匀带有负电荷,每米电荷量为q,当此棒沿轴线方向做速率为v的匀速直线运动时,由于棒运动而形成的等效电流大小为(   )

A:vq

B:qv

C:qvS

D:qvS

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答案(点此获取答案解析)

A

同类题3

阅读下列短文,完成后面小题。
                                                                                         人类未来的食品来源——昆虫
        昆虫也能供人类当食品吃?是的。现在墨西哥国立自治大学生物学学院内,一位女教授胡列塔•拉莫斯•埃洛蚀伊正领导着她的动物专业研究小组,研究着将昆虫当食物的课题呢!
        其实,早在许多年以前,在那些因受地理条件限制,发现农业生产有困难的地区,如非洲和亚洲的一些国家,人们已经把昆虫作为食物了。有的还作为粮食来吃哩!即使在一些发达的国家里,如美国等,专门商店或饭馆、餐厅里也经常出售用昆虫做的罐头或菜肴,较多见的是蚂蚁、蛾蝶、蛾状毛虫,蜜蜂幼虫、蚕蛹、龙舌兰红囊、蜜蜂等。
        至于在墨西哥,人们食昆虫同样是家常便饭的事。据不完全统计,全国有9个州的居民爱食昆虫,所食的昆虫种类多达57种,主要的有蚂蚁、直翅目昆虫、甲虫、蝉,甚至苍蝇、蚊子、臭虫、黄蜂、白虱、蜻蜓、蝴蝶等。
        不过,女教授和她的助手们的研究成果表明,昆虫身上确实含有人类维持生命所必需的大量的各种营养,甚至比一般动物所含的价值还高。以蚂蚁一类昆虫来说,每100克含蛋白质为20.4克。而100克鸡肉和鱼肉所含蛋白质分别是20.2克和18.9克,100克蛋类所含蛋白质更少,仅6.4克。其他像维生素B1和维生素B2等,100克蚁类所含的量也要比100克鱼、肉、家禽等所含的量高得多。因此,女教授认为,人类在缺乏其他粮食作物的情况下,只有昆虫才可用作代用品。换句话说,人类如果有系统地加以利用,那么昆虫定将成为未来的可信赖的食品。

同类题5

阅读下面短文,从短文后各题所给的四个选项(A, B,C和D)中,选出可以填入空白处的最佳选项。

    I am an American living in France for twenty years. I used to complain about how the French were 1 for friendship. I had a(n)2 time making a living here.

    When I came I felt a need to meet people,3 I went to the cinema night for women's day. I was excited about the way to connect to French women through the 4 we would have after the film, perhaps to make friends and get more chance of 5.

    Outside the cinema was a woman with long gray hair who had a slightly 6 smell and was asking for money. I 7 my wallet to give her a Euro and then something made me stop and enter into 8 with her. I said, “Look, people are seeing a film that could 9 your life. I'd rather buy you a ticket than give you money.”

    So she came in,10 behind me because she said people 11 the theater didn't like her. She sat next to me in the cinema. Then I noticed some women were looking at me, with expressions of 12.

    I encouraged the women to come the following night to an event. I 13 her to an agency that helped women in her 14. Then, a woman came towards me and said, “Did you buy a ticket for the woman?” When I said yes, she said, “Please come to the community center to 15 other employees there.”

    This was the 16 of a deep friendship and colleague relationship. The woman I took to the cinema that night wasn't 17. Actually, she was a secretary and spoke English.

    For me, though, she was18 my angel. I made a great friend 19 reaching out to her, also got a great job, and 20 myself of many previous negative judgments about the French.