问题 选择题

如图,一理想变压器原线圈接入一交流电源,副线圈电路中R1、R2、R3和R4均为固定电阻.开关S是闭合的,

为理想电压表,读数分别为U1和U2

 和 

为理想电流表,读数分别为I1、I2和I3.U1数值不变,现断开S,下列推断中正确的是(  )

A.U2变小、I3变大

B.U2不变、I3变小

C.I1变小、I2变小

D.I1变大、I2变大

答案

理想变压器的电压与匝数程正比,由于理想变压器原线圈接到电压不变,则副线圈电压不变,所以V2的示数U2也不变,

当s断开之后,并联电路的电阻变大,副线圈的电阻也就变大,由于副线圈电压不变,所以副线圈的总电流减小,即I2变小,

由于电流与匝数成反比,当副线圈的电流减小时,原线圈的电流也就要减小,所以I1变小,

由于副线圈的总电流减小,R1的电压减小,并联电路的电压就会增大,所以R3的电流I3就会增大,

所以C正确,ABD错误.

故选C.

阅读理解

In a surprising result, the No. 69 Middle School girls’ football team yesterday beat their school’s boys’ team. The boys’ team often helped the girls with their training (训练). They had never lost to the girls before, but this time the girls beat them 4 — 3.

After the game, Wu Nai, head of the boys’ team, was very unhappy. "We all thought this would be an easy game," he said. "We never thought a team of girls could beat us. This is the saddest day of my life. "

But Mr Hu, the boys’ PE teacher, said he thought the girls should win. "The boys were too confident (自信)," he said. "I told them before the match that they needed to play well. They all thought that girls’ football was a joke. Now they know better. They didn’t play care­fully, and they lost."

The match had started well for the boys. After 30 minutes, they were winning 2 - 0. Their best player, Lu Ming, scored in the thirtieth minute. Earlier, the mid-field player, Ma Zhengquan, had scored the first goal in only the second minute of the match.

After the first half hour, the boys seemed to become too confident. At first, the girls had felt a bit nervous (紧张), but then they became more and more confident. Just before half time, Li Xiaolin made the score 2 - 1.

In the second half, the boys were the first to score. It was from Lu Ming. After that the boys became lazy, but the girls kept on working hard. Hao Meiling scored in the 68th minute, to make the score 3 — 2. Then Li Xiaolin scored twice in the last six minutes to make the last score 4-3. It was a surprising finish.

The girls’ PE teacher, Miss Wang, was very pleased with their work. "They were great!" She said. "I told them they could win. I told them that the most important thing was teamwork. The boys’ team had some good players, but my girls were a better team!"

小题1:   Who scored the first goal in the match?

A.Lu Ming.

B.Ma Zhengquan.

C.Li Xiaolin.

D.Hao Meiling.小题2:  How many goals did Li Xiaolin score?

A.One.

B.Two.

C.Three.

D. Four.小题3:   Why could the girls’ team beat the boys’ in the football match?

A. Because the boys’ team helped the girls with their training.

B. Because the boys had never lost to the girls before.

C. Because the boys’ team won the first half.

D. Because the girls played together better and worked harder.小题4:   Which of the following is right?

A. The girls made only one goal in the first half.

B. The girls even made 3 more goals than the boys in the second half.

C. Many people thought the girls could beat the boys before the match.

D. The girls’ team had some better players than the boys’.小题5:   Choose the right order according to the passage.

a.   The girls’ PE teacher told them that they could win.

b.   Wu Nai, head of the boys’ team was very unhappy.

c.   The match started well for the boys.

d.   The girls kicked the second goal in the match.

A. a-b-c-d

B. b-c-d-a

C. d-c-a-b

D. a-c-d-b

多项选择题