问题 问答题

一个匝数为200的矩形线圈abcd位于匀强磁场中,磁感强度大小为0.8T、初始位置如图所示(线圈平面和磁场方向平行),线圈以ab为轴匀速转动,角速度为5rad/s,已知ab=15cm,ad=10cm,线圈的总电阻是100Ω.求:

(1)线圈转动过程中的感应电动势的最大值;

(2)从图示位置开始计时,线圈转动过程中的感应电流的瞬时表达式;

(3)线圈从初始位置开始,转过90°角的过程中,通过导线截面的电量.

答案

(1)感应电动势的最大值为Em=NBSω=12V.

    (2)从图示位置开始计时,线圈转动过程中的感应电动势的瞬时表达式形式为e=Emcosωt,

由欧姆定律得到感应电流的瞬时表达式为i=

e
R
=
Emcosωt
R
=0.12cos5tA

     (3)线圈从初始位置开始,转过90°角的过程中,

感应电动势平均值为  

.
E
=N
△Φ
△t

感应电流平均值为    

.
I
=
.
E
R

通过导线截面的电量q=

.
I
•△t

    联立得到   q=N

△Φ
R
=
NBS
R
=2.4×10-2C

答:(1)线圈转动过程中的感应电动势的最大值为12V;    

    (2)从图示位置开始计时,线圈转动过程中的感应电流的瞬时表达式为0.12cos5tA;

    (3)线圈从初始位置开始,转过90°角的过程中,通过导线截面的电量是2.4×10-2C.

选择题
阅读理解

阅读理解

     Going to outer space is a little like going camping.You have to carry with you all the food and

equipment you need for your trip, so your food can't be too heavy or hard to prepare.Plus, there are

no refrigerators to keep food cold and fresh.

     Food with all the water dried out is lightweight and doesn't spoil.So, many space foods are dried

on Earth and stored in special packets.Some, such as dried pears, taste good right out of the packet.

But to eat the turkey in this outerspace Thanksgiving dinner, an astronaut must add water and heat the

packets.

     On Earth, gravity is the force that keeps your feet on the ground and your sandwich on your plate.

But there's no gravity in space.To keep food from floating away, astronauts on the Space Shuttle attach

the packets to a special tray.The tray can be fixed to a wall or to the astronaut's lap.To prepare their

food, astronauts use liquid forms of salt and pepper.The liquid sticks to the food better.Regular salt

and pepper would float away, maybe up an astronaut's nose.Achoo!

     Liquids float right out of cups and glasses, though.So astronauts drink everything from a small bag

with a straw(饮料吸管) that can be closed.Astronauts say that most space foods taste pretty good.

Some, such as apples and pudding are the same as the foods you eat on Earth.

      Astronauts eat tortillas(玉米粉圆饼) instead of bread because they make fewer crumbs(碎屑).

Floating crumbs could get stuck in equipment or an astronaut's eye.

     Your body uses food in space the same way it does on Earth.Your body must get rid of waste in

space too.

     During take off and landing, astronauts can't leave their seats to go to the bathroom.They wear

diapers(尿布) under their spacesuits instead.

     The Space Shuttle has a bathroom the size of a small closet.The toilet has bars that keep astronauts

from floating away.

      A strong flow of air is used instead of water to flush waste down the toilet.Back on Earth it's flushed

away-or sometimes studied by scientists.

1. Why do astronauts use liquid forms of salt and pepper to prepare food?

A. Liquid doesn't spoil.

B. Liquid is convenient to use in space.

C. Liquid is lightweight.

D. Astronauts are in great need of water.

2. What do astronauts think of space foods?

A. Most of them taste pleasant.

B. They are mostly fruits.

C. Most of them are cold.

D. They contain little meat.

3. Why don't astronauts eat bread?

A. Bread tastes bad.

B. Tiny pieces may float.

C. Bread weights heavy.

D. Bread is rather hard to prepare.

4. We can infer from the passage that astronauts ______.

A. drink from cups and glasses

B. always wear diapers in space

C. can move freely during landing

D. must pay attention to the crumbs in space