问题 问答题

如图所示,理想变压器原副线圈的匝数比是10:1,其中定值电阻R上标有“100Ω,1.0A”,电容器C上标有“1μF,100V”,为使电阻和电容器均能安全工作,原线圈两端的输入电压的最大值不能超过______V,若原线圈两端输入电压u=500

2
sin100πt(V),则电流表的示数______0.5A(填“大于”、“等于”或“小于”)

答案

定值电阻R两端的电压为有效值,最大为U=IR=100V,电容器的耐压值是最大值,所以电容器两段的电压最大为U=100V

则副线圈的电压最大为100V

所以原线圈两端的输入电压的最大值为100×10=1000V

若原线圈两端输入电压u=500

2
sin100πt(V),则原线圈电压为500V,

所以电阻两端的电压为U=50V,通过电阻的电流为I=

50
100
=0.5A,电容器也有电流通过,

所以电流表的示数大于0.5A

故答案为:1000;大于

阅读理解

Tom and his best friend Rob were out of school for the summer. They had been swimming, gone to the movies, played video games, and even built a castle out of some old cardboard boxes. But today they were so bored. “At least we would have something to do.” Tom thought for a while and suggested, “Let’s make some popcorn and lemonade and sell it outside.”

The boys went to the kitchen and asked Tom’s mom if she would help them make some lemonade while they popped the popcorn. Tom’s mom got the lemons from the refrigerator and also a small amount of sugar. She mixed up the lemonade while the boys started popping the popcorn.

Tom and Rob got a small table from the garage and set it up near the sidewalk. They made a red sign telling everyone that popcorn and lemonade was for sale. The boys sat for about ten minutes before they got their first sale. Mrs. Jenkins, one of their neighbors, was walking her dog. “It’s so hot today.” “I sure can use a drink,” she said. Soon the boys were busy handing out popcorn to Mr. Baker. He was walking down the street sending the mail. “I need a break,” he said. “Walking all over the neighborhood gave me an appetite.”

The boys had only three popcorn bags left and enough lemonade for three more cups. “Wow, I didn’t think the popcorn and lemonade would sell that fast. We made six dollars,” said Tom. “If we get bored again before school starts, we can try selling more lemonade with cookies(饼干) next time,” laughed Rob.

小题1:What did Tom suggest?

A.Swimming with their friends.

B.Buying some sugar and cookies.

C.Making some popcorn and lemonade.

D.Going to the movies and playing games.小题2:Where did Tom and Rob set up the table?

A.Near the sidewalk.

B.In the kitchen.

C.At the supermarket.

D.Outside the school.小题3:What did Tom and Rob learn from this experience?

A.People might prefer to buy things on sale.

B.They could do something new to have fun.

C.Mr. Baker liked sending mails very much.

D.Mrs. Jenkins needed help to walk the dog.

单项选择题