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问题 计算题

如图所示，一个质量为m=0.016 kg、长为L=0.5 m、宽为d=0.1 m、电阻R=0.1 Ω的粗细均匀的矩形线框，从h1=5 m的高度由静止自由下落，然后进入匀强磁场，磁场的磁感应强度B=0.4 T，磁场方向与线框平面垂直，g=10m/s2。求：

（1）刚进入时线框的电流大小和方向；

（2）请通过计算判断，线框刚进入磁场时做什么运动？线框从刚进入磁场到完全进入磁场，产生多少热量？

（3）如果线框的下边ab通过磁场区域的时间t=0.15 s，求磁场区域的高度h2；

（4）请定性画出线框穿越磁场过程中电流随时间的变化关系（取顺时针为正）。

答案

解：（1）线框进入磁场时的速度为v1==10 m/s

产生的感应电动势为E=Bdv1=0.4V

感应电流I=，方向：顺时针

（2）所受的安培力为F=BId=0.16N，G=mg=0.16N，故线框做匀速运动

Q=mgL=0.08J

（3）线圈全部进入磁场用的时间t1==0.05 s，所以由题意得：

v1(t－t1)+g(t－t1)2=h2－L

代入数据解得h2=1.55 m

（4）

单项选择题 A1/A2型题

女性，30岁。农民。2周前田间劳动时右小腿皮肤划破。1周前起畏寒、高热，3天来咳嗽、气急，伴右侧胸痛就诊。影像学检查右侧肺炎伴液（脓）气胸。推测其最可能的病原体是（）

A.化脓性链球菌

B.金黄色葡萄球菌

C.厌氧菌

D.铜绿假单胞菌

E.军团杆菌
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单项选择题

Comparisons were drawn between the development of television in the 20th century and the diffusion of printing in the 15th and 16th centuries. Yet much had happened between. As was discussed before, it was not until the 19th century that the newspaper became the dominant pre- electronic (61) , following in the wake of the pamphlet and the book and in the (62) of the periodical. It was during the same time that the communications revolution (63) UP, beginning with transport, the railway, and leading on through the telegraph, the telephone, radio, and motion pictures into the 20th-century world of the motor car and the air plane. Not everyone sees that process in (64) . It is important to do so.

It is generally recognized, (65) , that the introduction of the computer in the early 20th century, (66) by the invention of the integrated circuit during the 1960s, radically changed the process, although its impact on the media was not immediately (67) . As time went by, computers became smaller and more powerful, and they became "personal" too, as well as (68) , with display becoming sharper and storage (69) increasing. They were thought of, like people, (70) generations, with the distance between generations much (71) .

It was within the computer age that the term "information society" began to be widely used to describe the (72) within which we now live. The communications revolution has (73) both work and leisure and how we think and feel both about place and time, but there have been (74) view about its economic, political, social and cultural implications. "Benefits" have been weighed (75) "harmful" outcomes. And generalizations have proved difficult.

（64）是（）

A.concept

B.dimension

C.effect

D.perspective
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