问题 填空题

一个质量为m、直径为d、电阻为R的金属圆环,在范围很大的磁场中沿竖直方向下落,磁场的分布情况如图所示,已知磁感应强度竖直方向的分量By的大小只随高度变化,其随高度y变化关系为By=B0(1+ky)(此处k为比例常数,且k>0),其中沿圆环轴线的磁场方向始终竖直向上,在下落过程中金属圆环所在的平面始终保持水平,速度越来越大,最终稳定为某一数值,称为收尾速度.俯视观察,圆环中的感应电流方向为______(顺时针,逆时针);圆环收尾速度的大小为______.

答案

向下运动的过程中穿过的磁通量在增大,根据楞次定律,得感应电流的方向俯视观察沿顺时针方向.

圆环下落高度为y时的磁通量为:

Φ=BS=Bπ

d2
4
=B0(1+ky)π
d2
4
.①

设收尾速度为vm,以此速度在△t时间内磁通量的变化量

△Φ=△BS=B0

d2
4
vm△t   ②

根据法拉第电磁感应定律有:E=

△Φ
△t
=B0
d2
4
vm  ③

圆环中感应电流的功率PE=

E2
R
 ④

重力做功的功率PG=mgvm  ⑤

根据能量守恒定律有:PE=PG

由以上各式解得:vm=

16mgR
π2d4
B20
k2

故答案为:顺时针,

16mgR
π2d4
B20
k2

阅读理解

阅读理解。

     In 1989, an earthquake almost flattened (铲平) America, killing over 30,000 people in less than four minutes.

In the earthquake, a father left his wife safely at home and rushed to the school where his son was supposed

to be, only to discover (发现) that the building was as flat as a pancake.

     He was surprised at what he saw, but he suddenly remembered the promise (承诺) he had made to his son:

"No matter what happens, I'll always be there for you!" And tears began to fill his eyes. He began to think of the

place where his son's classroom might be. He rushed there and started digging through the ruins (废墟). As he

was digging, other parents tried to pull him off, saying, "It's too late! They are all dead! You can't help! Go home!

There is nothing you can do!" But he only replied, "Are you going to help me now?" And then he continued to dig

for his son, stone by stone. The policemen also tried to pull him off and asked him to go home. But he only replied,

"Are you going to help me now?" But everyone was sure it was over. No one helped. He went on alone to see if

his son was still alive, 8 hours… 12 hours… 36 hours… then, in 38th hour, he pulled back a big stone and heard

his son's voice. He screamed, "Armand!" He then heard, "Dad, it's me. I told the other kids not to worry. I told

them if you were alive, you'd save me and when you saved me, they'd be saved! You have promised

no matter what happens, you will always be there for me! You did it, Dad!"

     "Come out, boy!"

     "No, Dad! Let the other kids out first, because I know you'll get me! No matter

what happens, I know you will always be there for me!"

1. How long did the terrible earthquake last? [ ]

A. 8 hours

B. About four minutes

C. 36 hours

D. 38 hours

2. Why did others try to stop the father? [ ]

A. Because they thought what he did was useless.

B. Because they thought the father was foolish.

C. Because the policemen would save the children.

D. Because they didn't want to save the children.

3. What did the boy do when he heard his father's voice? [ ]

A. He believed his father.

B. He asked his father to save others first.

C. He cried loudly and couldn't say anything.

D. He got out without his father's help.

4. What's the main idea of the story? [ ]

A. East or west, home is the best.

B. Don't worry about anything.

C. Father's promise.

D. God will save you.

选择题