问题 计算题

如右图所示的电路中,两平行金属板A、B水平放置,两板间的距离d="40" cm.电源电动势E="24" V,内电阻r="1" Ω,电阻R="15" Ω。闭合开关S,待电路稳定后,将一带正电的小球从B板小孔以初速度v0="4" m/s竖直向上射入板间。若小球带电荷量为q=1×10-2 C,质量为m=2×10-2 kg,不考虑空气阻力。那么,滑动变阻器接入电路的阻值为多大时,小球恰能到达A板?此时电源的输出功率是多大?(取g="10" m/s2)

答案

8 Ω;23 W

题目分析:小球进入板间后,受重力和电场力,且到A板时速度为零,设两板间电压为UAB,由动能定理得:

所以滑动变阻器的电压为:

设通过滑动变阻器的电流为I,由欧姆定律得:

滑动变阻器接入电路的电阻为:

电源的输出功率为:

选择题
单项选择题

In 1965, the United States made important changes in its immigration laws, allowing many more immigrants to come and entirely eliminating the older laws’ bias (偏见, 偏向) in favor of white European immigrants. As a result, the United States is now confronted with a new challenge—taking in large numbers of new immigrants who are nonwhite and non-European. About 90 percent are from Asia, Latin America, and the Caribbean. In addition to the large numbers of legal immigrants, for the first time the United States has significant numbers of illegal immigrants. Many worry about what the impact will be on the American society. Can the American economy expand enough to offer these new immigrants the same opportunities that others have had What will be the effect on the traditional value system that has defined the United States for over 200 years

Many Americans see wonderful benefits for their country. Ben Wattenberg, a respected expert on American culture, believes that the "new immigration" will be of great help to the nation. According to Wattenberg, something very important is happening to the United States: It is becoming the first universal nation in history. Wattenberg believes that the United States will be the first nation where large numbers of people from every region on earth live in freedom under one government. This diversity, he says, will give the nation great influence and appeal to the rest of the world during the 21st century.

Perhaps the United States will be described not as a "melting pot" or a "salad bowl" but as a "mosaic"—a picture made up of many tiny pieces of different colors. If one looks closely at the nation, the individuals of different colors and ethnic groups are still distinct and recognizable, but together they create a picture that is uniquely American. "E Pluribus Unum" —the motto of the United States from its beginning—means one composed of many. "Out of many, one. \

"A universal nation" means ().

A. a nation whose citizens spread all over the world

B. a nation that unites all the countries in the world

C. a nation that influences the rest of the world

D. a nation where people from all over the world live under one government