问题 选择题

如图所示,两平行虚线间的区域内存在着匀强磁场,矩形金属闭合线圈abcd的ab边与磁场边界平行,已知磁场区域的宽度大于bc的边长.线圈从静止开始在向右的水平恒力F的作用下运动,在ab边刚进入磁场区域的瞬间,线圈受力恰好平衡.下面的说法中正确的是[  ]

A.在线圈进入磁场区域的过程中,恒力F对线圈做功的功率保持不变

B.在线圈整体在磁场中运动的过程中,恒力F对线圈做功的功率保持不变

C.在线圈离开磁场区域的过程中,恒力F对线圈做功的功率逐渐减小

D.在线圈穿越磁场区域的整个过程中,恒力F对线圈做的功等于线圈中感应电流产生的焦耳热 

答案

AC

在线圈进入磁场的过程中,线圈作的是匀速运动,因此恒力F对线圈做功的功率不变.在线圈完全进入磁场后,没有了感应电流,线圈只受拉力F作用,作加速运动,等到ab边到达磁场的右边界时,速度已经变大了,相应地感应电流也变大,所受的妆培力必大于F它将做减速运动,因此恒力F对线圈做功的功率逐渐减小.

完形填空

阅读下面短文并回答问题,然后将答案写到答题卡相应的位置上(请注意每题词数要求)。

In the spring of 2013, Carly Zalenski's eyes were filled with tears as the dusty bus ran down a dirt road in southern Vietnam. The 14-year-old girl and her family had traveled by plane from Canton, Ohio, to Ho Chi Minh City and then by bus deep into the Mekong Delta. Now, as they approached the village, hundreds of cheering schoolchildren lined the entrance to the Hoa Lac School, a two-story concrete building that Carly had raised money for.

Carly started helping others when she was eight, handing out Thanksgiving baskets at church to families in need. It was a snowy day, and she saw that one girl was wearing a pair of broken shoes and others didn't have warm coats. The next November, she went door-to-door asking for used coats, hats, gloves, and scarves, and then handed them out with the baskets.

But Carly wanted to do more - she wanted to "change lives," she says. She remembered that her grandmother's Rotary club had, years earlier, raised money to build a school in Vietnam. That was it, she decided. She'd build a school, too.

She put together a PowerPoint presentation on the people and culture of Vietnam. At 12, hardly able to see over the platform, she gave her first fund-raising speech. She spoke with enthusiasm" The kids in rural Vietnam don't have good schools, " she told a room of 200 Rotarians. "That's not fair. I want to raise money and improve the school condition to make their lives better. "

In two years, Carly set off with her family across Ohio, visiting three or four Rotary clubs a week. "We traveled like crazy people to all these meetings," recalled her mother, Kris.

Finally, Carly raised $ 50,000, which was matched by the Vietnam Children's Fund. At the donation ceremony in Hoa Lac, the school principal was impressed with the ninth grader. "How wonderful," he said through a translator, "that a girl wanted to do something for kids of her age so far away. "

小题1:In which year was Carly born? (Within 2 words)

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小题2:Why did Carly once collect used clothes from door to door? (Within 12 words)

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小题3:What made Carly determine to build a school in Vietnam? (Within 10 words)

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小题4:How did Carly help the kids in rural Vietnam when she was 12? (Within 5 words)

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小题5:What does the school principal in Hoa Lac think of Carly? (Within 6 words)

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单项选择题 A1/A2型题