问题 选择题

如图所示,理想变压器的原、副线圈匝数比为1:5,原线圈两端的交变电压为 氖泡在两端电压达到100V时开始发光,下列说法中正确的有( )

A.开关接通后,氖泡的发光频率为100Hz

B.开关接通后,电压表的示数为100 V

C.开关断开后,电压表的示数变大

D.开关断开后,变压器的输出功率不变

答案

AB

题目分析:在解题时要注意变压器的三个决定关系,副线圈电压由原线圈决定,则不论负载如何变化,电压表的读数不变,始终为有效值100 V,B正确;每个交流电周期内氖管发光两次,每秒发光100次,则氖管发光频率为100 Hz,开关断开后,负载电阻增大、电流减小,则变压器的输出功率减小,故选AB

点评:本题难度较小,解决此类问题主要是掌握三个决定关系:输入电压决定输出电压、输出电流决定输入电流、输出功率决定输出功率

阅读理解

阅读理解。

    In many British schools, the pupils usually wear their school uniforms to school on weekdays. However, recently the students at LVS Ascot Junior School in England wore something quite different. What they wore was what people wear when they go to bed - pyjamas (睡衣). They did this not only for fun, but for a local charity called Christopher’s Smile.

    Christopher’s Smile was set up in 2008 by Karen & Kevin Capel whose only son Christopher died of cancer at a young age. They hope their charity will help pay for more research into children’s cancers. Since then, the charity has raised a lot of money and gotten more and more volunteers. Every year, thousands of people take part in different activities such as charity walk or run to show their support for Christopher’s Smile.

    The students of Grade 6 at LVS Ascot Junior School also wanted to do something for the sick children. They organized the event “Pyjama Day” to support Christopher’s Smile. “We wanted to raise money for our charity in a way that the whole school could join in it, so each pupil paid £1 to wear his or her pyjamas to school on Pyjama Day,” said the young organizers. Both the students and their teachers took part in the event with great interest. Together they not only raised some money, but also had a good time.

1. Who set up Christopher’s Smile? 

A. LVS Ascot Junior School’s teachers.          

B. The local government.

C. LVS Ascot Junior School’s students.          

D. Christopher’s parents.

2. Christopher’s Smile was set up to        .  

A. tell people some knowledge about children’s cancers

B. have fun for the students in LVS Ascot Junior School

C. help the local government do more things for students

D. raise money for more research into children’s cancers

3. What is not mentioned in the passage?  

A. Charity walk.        

B. Charity run.      

C. Charity concert.    

D. Pyjamas Day.

4. Which of the following is TRUE?

A. It has been four years since Christopher’s Smile was set up.

B. Few pupils wear school uniforms to school on weekdays in Britain.

C. In LVS Ascot Junior School, only students join in the event “Pyjama Day”.

D. All students in England need to wear pyjamas to school on Pyjama Day.

5. The best title for the passage may be       

A. The Christopher’s Smile  

B. Fun and charity  

C. The Pyjama Day  

D. Children’s cancers

单项选择题