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。

     Arnel Laparan grew up in Rosario. After his father died in 1970, five-year-old Amel had to work in a

small shop to help his family. The boy insisted on going to school and his mother finally sent him to a nearby

public school. "I was happy to study but my life became harder because I had to wake up earlier to do all my

jobs before I went to school," says Arnel. Worn down by all the work, he was ready to give up by the time

he reached the fifth grade. Then his teacher's words changed his life and made him study harder.

     Arnel finished high school at the top of his class, received a scholarship (奖学金) from the Philippine

Normal University in Manila and got a degree in education in 1985. He took a job teaching math at a Manila

private school and did volunteer teaching at the school's adult class in the evening. "The adults were from poor

families and had to work when they were very young," he explains. "It was touching to see how they tried to

get a high school education at their age."

     As time passed, he realized that helping needy students was his true calling. In 2004 he left his well-paid job

and retumed to Rosario. "My goal was to educate street children for free." He began holding classes under a

large tree. Slowly, more and more street children became interested in his teachirig. "I taught them the basics

of reading, writing and math," says Arnel. He worked harder, walking to nearby villages, where he would give

classes in parks or on playgrounds. To make ends meet, he began teaching children from rich families.

     In June this year Arnel borrowed a bicycle with a large sidecar from a neighbour. The sidecar holds a small

wooden desk, textbooks and school supplies, and a black- board set to the front bears Arnel's words:"Education

can be found anywhere. It is a road to progress."

     The bicycle allows Arnel to visit faraway areas. He sets up an outdoor classroom and invites teenagers to

his class three days a week. Many of the children have been out of school for years. Amel also offers adult

education to help out-of-school young people learn high school lessons. He does not receive any money but

accepts school supplies and food for the students. He says, "I want to bring education to as many children and

adults as possible. Everyone should have a chance."

1. Who decided Arnel should go to school at first?

A. His mother.

B. He himself.

C. His teacher.

D. The govemment.

2. Why did Amel retum to his hometown to teach the street children?

A. Because there were no schools in his hometown.

B. Because he didn't want to teach rich children only.

C. Because he considered it's his duty to help the poor.

D. Because the country's education should be improved.

3. How does Arnel make his living?

A. By serving rich families.

B. By accepting school supplies.

C. By receiving his family's help.

D. By offering lessons for adults.

4. Which of the following can be the best title of the passage?

A.Teacher on Wheels

B. Classes for the Poor

C. Road to Progress

D. Students in the Street

判断题