题干

某物理实验小组的同学利用实验室提供的器材测量一待测电阻的阻值.可选用的器材(代号)与规格如下:

电流表A1(量程250mA,内阻r1为5Ω);

标准电流表A2(量程300mA,内阻r2约为5Ω);

待测电阻R1(阻值约为100Ω);

滑动变阻器R2(最大阻值10Ω);

电源E(电动势约为10V,内阻r约为1Ω);

单刀单掷开关S,导线若干.

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       Each new school year brings fresh reminders of what educators call the summer learning gap. Some call it the summer learning setback. Simply speaking, it means the longer kids are out of school, the more they forget. The only thing they might gain is weight.

       Most American schools follow a traditional nine-month calendar. Students get winter and spring breaks and about ten weeks of summer vacation. Some schools follow a year-round calendar. They hold classes for about e ight weeks at a time, with a few weeks off in between. The National Association for Year-Round Education says there were fewer than three thousand such schools at last count. They were spread among forty-six of the fifty states.

       But many experts point out that the number of class days in a year-round school is generally the same as in a traditional school. Lead researcher Paul von Hippel said, “Year-round schools don't really solve the problem of the summer learning setback. They simply spread it out across the year.”

Across the country, research shows that students from poor families fall farther behind over the summer than other students. Experts say this can be prevented. They note that many schools and local governments offer programs that can help.

       But calling them “summer school” could be a problem. The director of the summer learning center at Johns Hopkins, Ron Fairchild, said research with groups of different parents in Chicago and Baltimore found that almost all strongly disliked the term “summer school”. In American culture, the idea of summer vacation is connected to beliefs about freedom and the joys of childhood. The parents welcomed other terms like “summer camp”, “enrichment”, “extra time” and “hands-on learning”.