问题 选择题

.两条通电的直导线互相垂直,如图15-2-13所示,但两导线相隔一小段距离,其中导线AB是固定的,另一条导线CD能自由转动.它们通以图示方向的直流电时,CD导线将(  )

图15-2-13

A.逆时针方向转动,同时靠近导线AB

B.顺时针方向转动,同时靠近导线AB

C.逆时针方向转动,同时离开导线AB

D.顺时针方向转动,同时离开导线AB

答案

答案:A

由安培定则可知直线电流产生的磁场是以直线电流为圆心的同心圆,AB中电流所产生的磁场在CD导线所在平面内的磁感线如图甲所示(俯视图).在图示的MN两点的磁感应强度对导线CD中的电流的力的作用如图乙所示,由左手定则可知,CD通电导线在M点受到的磁场力垂直于纸面向里,在N点受到磁场力垂直于纸面向外.由于磁场的对称性,MN两点的磁场力大小相等、方向相反,使得CD通电导线在图甲上做逆时针旋转,即CD电流将会转到与AB同向的位置.在图乙中,CD中的电流所受到磁场力由左手定则可知,CD将向AB靠近. 

翻译题
In Britain, people have different attitudes(态度) to the police. Most people generally appreciate(欣赏) them and the job they do—although there are some people who do not believe that the police _    have the power that they do.
  What does a policeman do in fact? It is not _    job to describe. After all, a policeman has a number of jobs in one. A policeman often has to control traffic, either on foot in the centre of a town, or in a police car on the roads. Indeed, in Britain, he might be in the Traffic Police and spend all, or a lot of, his time driving up and down main roads and motorways. A traffic policeman has to help keep the traffic moving, stop     _ motorists and help when there is an accident.
  A policeman has to help keep the peace, too. If there is a fight or some other disturbance(动乱), we _   _ the police to come and keep order. And they often have to _    the situation at great risk to their own     _.
  We expect the police to solve crimes, of course, so an ordinary(普通) policeman, even if he is not a detective, will often have to help _   _ and arrest criminals.
  And _   __ do we call when there is an emergency—an air crash, a _   , a road accident, or a robbery? We call the police. So a policeman has to be _    to face any unpleasant emergency that may happen in the modern world.
  The police do a necessary job, and they do it very well and I support them, but I do not envy policemen. I do not think that I could ever do the job of a policeman.
小题1:
A.shouldB.would C.could D.must
小题2:
A.a funny B.a pleasant C.an interesting D.an easy
小题3:
A.resting B.tired C.speeding D.drinking
小题4:
A.wait for B.believeC.think of D.expect
小题5:
A.turn to B.stop C.deal with D.treat
小题6:
A.safety B.families C.future D.friends
小题7:
A.remove B.question C.look for D.sentence
小题8:
A.how B.where C.what D.who
小题9:
A.power failure B.fire C.thunder stormD.thief
小题10:
A.provided B.promised C.prepared D.presented
单项选择题