问题 多选题
如图为理想变压器原线圈所接交流电压的波形.原、副线圈匝数比n1:n2=10:1,串联在原线圈电路中电流表的示数为1A,下列说法正确的是(  )

A.变压器输出端所接电压表的示数为20V

B.变压器的输出功率为200W

C.变压器输出端的交流电的频率为50Hz

D.穿过变压器铁芯的磁通量变化率的最大值为20/n2Wb/s

答案

BCD

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任务型读写。

请认真阅读下列短文,并根据所读内容在文章后表格中的空格里填入最恰当的单词.

注意:每个空格只填1 个单词.

     Frederick M. Hess is the director of education policy studies at the American Enterprise Institute,

which is a nonprofit organization that conducts research on many public issues. He says that a long

summer vacation doesn't make sense in today's world.

     Can American students afford to take a summer vacation? In a summer vacation, millions of kids

spend valuable time sending messages, watching TV, playing video games and doing shopping in the

mall. They will also be putting their academic futures at risk.

     Summer vacation once made sense in the past when you didn't need an education to get a good

job. But now things have changed. For today's students, academic skills are important to students'

future success, but such skills are affected in the summertime. Many nations don't give kids an

Americanstyle summer vacation. They offer no more than seven consecutive( 连续的)weeks of vacation. Most American school districts offer up to thirteen weeks. To compete in the global marketplace,

Americans must be prepared to go up against international competitors.

     Summer vacation also causes challenges for today's families. In the 1960s, more than 60% of

families had a stayathome Mom. Now, twothirds of American children live in households where every

adult works. For these families, summer vacation can be more burden than break. Someone must watch

the kids.

     But the biggest problem may be how summer vacation hurts academic achievement. Researchers

have found that disadvantaged students lose significant ground in the summertime.

     A longer school year does not have to be an invitation to hard boring work. Rather, it should allow

timepressed teachers to conduct richer and more imaginative lessons. Schools would have more time to

devote to athletics, languages, music and the arts. Summer vacation can be a grand thing. But in the 21st

century, it may also be outdated.

                              Title: Long Summer 1.________