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问题 问答题

某同学用电源、电压表、电流表、标有“20Ω  2A”字样的滑动变阻器、电键及导线若干做“测定小灯泡功率”实验．小灯上标有“3.8伏”字样，他正确连接电路，实验步骤正确，闭合电键S，观察到电流表示数为0.3安，电压表示数为0伏，灯不亮，他移动滑动变阻器的滑片，灯始终不亮．该同学断开电键，换用一个相同规格的灯接入电路，重新进行实验，当闭合电键，移动滑动变阻器的滑片，发现滑片在中点位置时，小灯正常发光．请分析前面小灯不能正常发光的原因______，电源电压为______ 伏，小灯正常发光时，变阻器连入的电阻为______欧，小灯的额定功率为______瓦．

答案

（1）正确连接电路时，滑动变阻器接入电路的电阻R=20Ω；

电流表有示数说明电路是通路，电压表无示数说明电压表测导线的电压，即小灯泡短路；

电源的电压为U=IR=0.3A×20Ω=6V．

（2）灯泡正常发光时UL=3.8V，R滑=10Ω，

滑动变阻器两端的电压U滑=U-UL=6V-3.8V=2.2V，

电路中的电流I′=

U滑
R滑
=
2.2V
10Ω
=0.22A，

小灯泡的额定功率P=ULI′=3.8V×0.22A=0.836W．

故答案为：小灯短路；6；10；0.836．

单项选择题

Of all the areas of learning the most important is the development of attitudes. Emotional reactions as well as logical thought processes affect the behavior of most people.
"The burnt child fears the fire" is one instance. Another is the rise of despots like Hitler. Both of these examples also point up the fact that attitudes stem from experience. In the one case the experience was direct and impressive, in the other case it was indirect and cumulative. The Nazis were formed largely by the speeches they heard and the books they read.
The classroom teachers in the elementary school are in a strategic position to influence attitudes. This is true partly because children acquire attitudes from those adults whose opinion they respect.
Another reason is that pupils often delve somewhat deeply into a subject in school that has only been touched upon at home or has possibly never occurred to them before. To a child who had previously acquired little knowledge of Mexico, his teacher’s method of handling such a unit would greatly affect his attitude towards Mexicans.
The media through which the teacher can develop wholesome attitudes are innumerable. Social attitudes (with special reference to races and nationalities) science matters of health and safety, the very atmosphere of the classroom--these are a few of the fertile fields for the induction of proper emotional reactions.
However, when children come to school with undesirable attitudes, it is unwise for the teacher to attempt to change their feelings by cheating or scolding them. She can achieve the proper effect by helping them obtain constructive experiences. Finally, a teacher must constantly evaluate her own attitudes, because her influence can be harmful if she has personal prejudices. This is especially true in respect to controversial issues and questions on which children should be encouraged to reach their own decisions as a result of objective analysis of all the facts.

In the second paragraph, the author gives two examples to show ______.

A. the development of attitudes is most important
B. people’s behavior can be shaped by direct or indirect experience
C. conscious instruction is important in shaping people’s behavior
D. unpleasant experience leaves a greater impression on people
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单项选择题 A1/A2型题

男性，36岁，尿蛋白（++），尿红细胞10～20个/HP，红细胞管型0～1个／HP，血压175／100mmHg，诊断为急性肾小球肾炎，经休息、限制水盐、利尿后血压仍高，应给予（）

A.抗菌药物

B.低蛋白饮食

C.降压药

D.洋地黄类药物

E.硝普钠
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