题干

在做“用电流表、电压表测电阻”的实验中,连接电路时电键必须处于____状态(选填“断开”或“闭合”).现有待测电阻Rx、滑动变阻器、电压表、电流表、电键、若干导线和多节干电池,小李按照图(a)所示的电路正确连接电路,在闭合电键前,他应将滑动变阻器的滑片P移到____(选填“A”或“B”)端,闭合电键后测得Rx两端的电压是4伏,电流表示数如图(c)所示是____安,计算此时电阻Rx的阻值是____欧.实验时要测量多组数据,以便将计算结果取平均值来减小误差,下列做法可行的是____

A.按照图(a)所示的电路,移动滑动变阻器的滑片P的位置;

B.按照图(b)所示的电路,多更换几个相同的待测电阻Rx;

C.按照图(b)所示的电路,改变串联干电池的节数.

上一题 下一题 0.0难度 选择题 更新时间:2020-01-15 10:05:57

答案(点此获取答案解析)

断开,B,0.2A,20Ω,A、C

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    I still remember an old lady, a customer on the paper route in my hometown when I was 12 years old. She taught me a lesson in1that I shall never forget.

On a(n)2afternoon, a friend and I were throwing stones onto the 3of the old lady's house. The stone that I found was too smooth,4it slipped from my hand as I let it go and5straight not for the covering on top of the house but for a small window on the house's back porch. At the sound of broken glass, we knew we were6.We turned and7.

    I was too 8 about getting caught that first night to be concerned about the old lady with the broken window in the freezing weather. However, a few days later, when I was sure that I hadn't been9, I started to feel guilty about her10.She still greeted me with a smile each day11I gave her the paper , but I was no longer able to act comfortably in her 12.

    I13my mind that I would save my paper delivering money , and in three weeks I had the seven dollars that I calculated would pay for her window . I put the money in an envelope with a note14that I was sorry for breaking her window and15that the seven dollars would cover the cost of repairing it.

    I waited until it was dark , moved16to the old lady ' s house and put the letter I didn ' t sign through the letter slot in her door . I felt 17 and could have the freedom of , once again , looking straight into the old lady's kind eyes .

    The next day, I handed the old lady her paper and was able to 18 the warm smile that I was receiving from her. She thanked me for the paper and gave me a bag of cookies she had made herself. I thanked her and proceeded(开始) to eat the cookies as I continued my19.

After several cookies . I felt an envelope and pulled it out of the bag. When I opened the envelope , I was 20. Inside were the seven dollars and a short note that said , " I'm proud of you.”