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问题 计算题

（10分）载流长直导线周围磁场的磁感应强度大小为B＝kI/r，式中常量k>0，I为电流强度，r为距导线的距离。在水平长直导线MN正下方，矩形线圈abcd通以逆时针方向的恒定电流，被两根等长的轻质绝缘细线静止地悬挂，如图所示。开始时MN内不通电流，此时两细线内的张力均为T0。当MN通以强度为I1的电流时，两细线内的张力均减小为T1；当MN内的电流强度变为I2时，两细线的张力均大于T0。

(1)分别指出强度为I1、I2的电流的方向；

(2)求MN分别通以强度为I1和I2电流时，线框受到的安培力F1与F2大小之比；

(3)当MN内的电流强度为I3时两细线恰好断裂，在此瞬间线圈的加速度大小为a，求I3。

答案

（1）方向向左，方向向右（2）（3）（方向向右）或者（方向向左）

题目分析：（1）当MN通以强度为的电流时，两细线内的张力均减小为，知此时线框所受安培力合力方向竖直向上，则ab边所受的安培力的向上，cd边所受安培力方向向下，知磁场方向垂直纸面向里，则方向向左．当MN内电流强度变为时，两细线内的张力均大于．知此时线框所受安培力合力方向竖直向下，则ab边所受的安培力的向下，cd边所受安培力方向向上，知磁场方向垂直纸面向外，则方向向右．

（2）当MN中通以电流I时，线圈所受安培力大小为，，所以，

（3），

若加速度向下：根据第（2）问结论：

（方向向右）

若加速度向上：根据第（2）问结论：，

（方向向左）

选择题

当南半球是春季时，漳州是什么季节[ ]

A．冬季

B．春季

C．夏季

D．秋季

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单项选择题

Questions 1～5


Writing articles about films for The Front Page was my first proper job. Before then I had done bits of reviewing—novels for other newspapers, films for a magazine and anything I was asked to do for the radio. That was how I met Tom Seaton, the first arts editor of The Front Page, who had also written for radio and television. He hired me, but Tom was not primarily a journalist, or he would certainly have been more careful in choosing his staff.
At first, his idea was that a team of critics should take care of the art forms that didn’t require specialized knowledge: books, TV, theatre, film and radio. There would be a weekly lunch at which we would make our choices from the artistic material that Tom had decided we should cover, though there would also be guests to make the atmosphere sociable.
It all felt a bit of a dream at that time: a new newspaper, and I was one of the team. It seemed so unlikely that a paper could he introduced into a crowded market. It seemed just as likely that a millionaire wanted to help me personally, and was pretending to employ me. Such was my lack of self-confidence. In fact, the first time I saw someone reading the newspaper on the London Underground, then turning to a page on which one of my reviews appeared, I didn’t know where to look.
Tom’s original scheme for a team of critics for the arts never took off. It was a good idea, but we didn’t get together as planned and so everything was done by phone. It turned out, too, that the general public out there preferred to associate a reviewer with a single subject area, and so I chose film. Without Tom’s initial push, though, we would hardly have come up with the present arrangement, by which I write an extended weekly piece, usually on one film.
The space I am given allows me to broaden my argument—or forces me, in an uninteresting week, to make something out of nothing. But what is my role in the public arena I assume that people choose what films to go to on the basis of the stars, the publicity or the director. There is also such a thing as loyalty to "type" or its opposite. It can only rarely happen that someone who hates westerns buys a ticket for one after reading a review, or a love story addict avoids a romantic film because of what the papers say.
So if a film review isn’t really a consumer guide, what is it I certainly don’t feel I have a responsibility to be "right" about a movie. Nor do I think there should be a certain number of "great" and "bad" films each year. All I have to do is put forward an argument. I’m not a judge, and nor would I want to be.

A weekly lunch would be arranged in order to ______

A. help the writers get to know each other
B. provide an informal information session
C. distribute the work that had to be done
D. entertain important visitors from the arts circle
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