问题 计算题

如图,一质量不计,可上下自由移动的活塞将理想气体封闭在圆筒内,筒的侧壁为绝缘体,下底M及活塞D均为导体并按图连接,活塞面积S=2cm2。闭合电键前,DM间距l1=5μm,闭合电键后,活塞D与下底M分别带有等量异种电荷,并各自产生匀强电场(D与M间的电场为各自产生的电场的叠加)。在电场力作用下活塞D发生移动。稳定后,DM间距l2=4μm,此时安培表读数为0.75A,伏特表读数为2V。

(1)求出活塞受到的电场力大小F(大气压强,活塞移动前后气体温度不变);

(2)求出活塞D所带电量q;

(3)一段时间后,一个电阻发生故障,安培表读数变为0.8A,伏特表读数变为3.2V,请判断是哪个电阻发生了故障?是短路还是断路?筒内气体压强变大还是变小?

(4)已知R3=4Ω,能否求出R1、电源电动势E和内阻r的阻值?如果能,求出结果,如果不能,说明理由。

答案

解:(1),解得

活塞受到的气体压强差为:

活塞受力平衡,电场力大小:

(2)D与M间的电场强度:

因为E是D与M所带的等量异种电荷产生的合场强,所以M所产生的场强:EM=E/2

,得

(3)R2断路;筒内气体压强变大

(4)R2断路后,伏特表的读数就是R1两端的电压,因此有:

可得:

因为R4未知,所以不能求出r的值

阅读理解

阅读理解。

     "Mum, what does it mean when someone tells you that they have a skeleton (骨骼) in the closet (衣橱)?"

Jessica asked. "A skeleton in the closet?" her mother paused thoughtfully. "Well, it's something that you would

rather not have anyone else know about. For example, if in the past, someone in Dad's family had been arrested

for stealing a horse, it would be a skeleton in his family's closet. He really wouldn't want any neighbor to know

about it."

     "Why pick on my family?" Jessica's father said with anger. "Your family history isn't so good, you know.

Wasn't your great-great-grandfather a prisoner who was transported to Australia for his crimes?" "Yes, but

people these days say that you are not a real Australian unless your ancestors arrived as prisoners." "Gosh, sorry

I asked. I think I understand now," Jessica cut iin before things grew worse.

     After dinner, the house was very quiet. Jessica's parents were still quite angry with each other. Her mother

was ironing clothes and every now and then she glared at her husband, who hid behind his newspaper pretending

to read. When she finished, she gathered the freshly pressed clothes in her arms and walked to Jessica's closet.

Just as she opened the door and reached in to hang a skirt, a bony arm stuck out from the dark depths and a

bundle of white bones fell to the floor. Jessica's mother sank in a faint (晕倒), waking only when Jessica put a

cold, wet cloth on her forehead. She looked up to see the worried faces of her husband and daughter.

     "What happened? Where am I?" she asked. "You just destroyed the school's skeleton, Mum," explained

Jessica. "I brought it home to help me with my health project. I meant to tell you, but it seemed that as soon as

I mentioned skeletons and closets, it caused a problem between you and Dad." Jessica looked in amazement as

her parents began to laugh madly. "They're both crazy," she thought.

1. According to Jessica's mother, "a skeleton in the close" means _____.

A. a family honor

B. a family secret

C. a family story

D. a family treasure

2. What can we learn about some Australians' ancestors form Paragraph 2?

A. They were brought to Australia as prisoners.

B. They were the earliest people living in Australia.

C. They were involved in some crimes in Australia.

D. They were not regarded as criminals in their days.

3. Why did Jessica bring a skeleton home?

 A. She was curious about it.

B. She planned to keep it for fun.

C. She needed it for her school task.

D. She intended to scare her parents.

4. Jessica's parents laughed madly at the end of the story probably because _____.

A. they were crazy

B. they were over excited

C. they realized their misunderstanding

D. they both thought they had won the quarrel

单项选择题