问题 问答题

(1)当线圈的ab边刚进入磁场时,它可能做什么运动,并分析各种运动下h的条件.

(2)设ab边刚进入磁场和刚穿出磁场时都作减速运动,且加速度大小相等。求线框经过磁场的过程中产生的焦耳热。

(3)设线圈刚好以匀速运动进入匀强磁场,此时线圈中的电流为I0,且线圈的边长L=h磁场的宽度H=2h。请在坐标系中定性画出线圈进入磁场到离开磁场的过程中,线圈中的电流i随下落高度x变化的图象。(不需要计算过程,按图象评分,设电流沿abcda如方向为正方向,x以磁场上边界为起点。)

答案

(1)线圈可能做匀速运动、加速运动、减速运动

,线圈匀速进入磁场;    ,线圈减速进入磁场;,线圈加速进入磁场;

(2)

(3)

(1)线圈可能做匀速运动、加速运动、减速运动。(2分)

(3分)

,线圈匀速进入磁场;(1分)

,线圈减速进入磁场;(1分)

,线圈加速进入磁场;(1分)

(2)(8分)

设ab边刚进入磁场时的速度为V1,cd边刚进入磁场时的速度为V2

则ab边刚穿出磁场的速度为V1、cd边刚穿出磁场时的速度为V2

从cd边刚进入磁场到ab边刚穿出磁场的过程有:

 (3分)

从ab边刚进入磁场到cd边刚离开磁场的过程有:

    (3分)

解得:

  (2分)

(3)共6分

说明:在2h~3h的过程有两种情况均对,只要画出一种情况即可。

阅读理解

People know the dangers of fires. It’s good for a family to learn how to prepare for a fire. There are some suggestions:

Put a smoke alarm in the house. Smoke from a fire causes the alarm to go off. The alarm makes a loud sound. The sound tells everyone to leave the house at once.

Make escape plans. Everyone should know all the ways out of the house. If there is a fire, everyone should know all the ways out of the house. If there is a fire, everyone follows the plan to get out. Part of the plan is to check all the windows to make sure they can be opened easily.

Buy fire extinguishers(灭火器) in the house. Everyone in the family should know how to use them. Practice for a fire. We do fire practice because we teach children about fire safety. Everyone in the family should know the following fire rules:

*Don’t open a hot door. The fire can grow more quickly if you open the door.

*Stay close to the floor. Smoke can be more dangerous than fire. The best air is near the floor because smoke rises.

*What should you do if your hair or clothes start to burn? First, stop. Don’t run! The fire burns faster because of more air. Drop1 Fall to the floor. Then roll! Turning over and over will make the fire go out. Put a blanket around you to keep air away from the fire that may still be on you.

There are many possible causes for fires. A wise family is ready all the time. If there is a fire, don’t forget to call 119 for help.

小题1:What does it mean when a smoke alarm rings at home?

A.You have to get up.

B.Water is running to the floor.

C.Something is burning.

D.Someone breaks your window.小题2:The writer advises people to do the following to prepare for a fire EXCEPT that ________.

A.they practice for a fire

B.they make escape plans

C.they buy fire extinguishers

D.they use electrical cookers小题3:When a fire happens,        if you open the hot door.

A.the fire will grow more quickly

B.the electricity will be cut off

C.the door will soon be on fire

D.the house will fall down小题4:What are the right steps you should when your hair or clothes catch fire?

A.Stop,run,roll.

B.Stop,drop,roll .

C.Run,stop,roll.

D.Roll,drop,stop.小题5:What is the best title for this passage?

A.The Dangers of a fire

B.The causes of a Fire

C.Learn to Use a Fire Extinguisher

D.Be Ready for a Fire

单项选择题