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在图所示的电路中，电源电压保持不变，R1为定值电阻。闭合电键S，当滑动变阻器的滑片P向右移动时，电压表V2的示数将      ；电压表V1示数与电流表A示数的比值将      。（均选填“变大”、“不变”或“变小”）

答案

变大； 不变

：（1）由电路图知：电阻R1与R2串联，电压表V1测电阻R1的电压，电压表V2测电阻R2的电压，电流表A测电路电流；

（2）当滑动变阻器的滑片P向右移动时，滑动变阻器接入电路的电阻变大，电路总电阻R变大；

（3）电源电压U不变，电路总电阻R变大，则电路电流I=变小，电阻R1阻值不变，则它两端的电压U1=IR1变小，电阻R2两端的电压U2=U-U1变大，则电压表V2的示数将变大．

（4）由于电阻R1阻值不变，由R1=知，电压表V1示数与电流表A示数的比值将不变．

故答案为：变大；不变．

单项选择题

尿路感染的血行感染绝大多发生于（）。

A.性生活活跃期妇女

B.老年妇女

C.原先已有严重尿路梗阻者或机体免疫力极差者

D.未婚少女

E.尿路器械检查后
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单项选择题

Children start out as natural scientists, eager to look into the world around them. Helping them enjoy science can be easy ;there’s no need for a lot of scientific terms or expensive lab equipment. You only have to share your children’s curiosity. Firstly,listen to their questions. I once visited a classroom of seven-year-olds to talk about science as a job. The children asked me" textbook questions" about schooling, salary and whether I liked my job. When I finished answering, we sat facing one another in silence. Finally I said, "Now that we’ve finished with your lists, do you have questions of your own about science"
After a long pause ,a boy raised his hand," Have you ever seen a grasshopper(蚱蜢) eat When I try eating leaves like that, I get a stomachache. Why"
This began a set of questions that lasted nearly two hours.
Secondly, give them time to think. Studies over the past 30 years have shown that, after asking a question, adults typically wait only one second or less tor an answer, no time for a child to think. When adults increase their" wait time" to three seconds or more, children give more logical, complete and creative answers.
Thirdly, watch your language. Once you have a child involved in a science discussion, don’t jump in with" That’s right" or" Very good". These words work well when it comes to encouraging good behavior. But in talking about science, quick praise can signal that discussion is over. Instead, keep things going by saying" That’s interesting" or" I’d never thought of it that way- before", or coming up with more questions or ideas.
Never push a child to" Think". It doesn’t make sense, children are always thinking, without your telling them to. What’s more, this can turn a conversation into a performance. The child will try to find the answer you want, in as few words as possible, so that he will be a smaller target for your disagreement.
Lastly, show;don’t tell. Real-life impressions of nature are far more impressive than any lesson children can learn from a book or a television program. Let children look at their fingertips through a magnifying glass, and they’ll understand why you want them to wash before dinner. Rather than saying that water evaporates, set a pot of water to boil and let them watch the water level drop.

In which of the following paragraph (s) does the author tell us what to say to encourage children in a science discussion

A. The second and third.

B. The fourth and fifth.
C. The fifth and sixth.


D. The seventh.
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