问题 选择题

如图所示,在一倾角为θ的光滑斜面上,放置一段通有电流强度为I、质量为m的导体棒a (电流方向垂直纸面向里),整个装置处于匀强磁场中,当磁场方向竖直向上、磁感应强度大小为B时,棒恰好处于静静止状态.保持棒中电流不变,让磁场方向在纸面内逆时针缓慢转过90°,该过程中导体棒始终处于平衡.则下列说法正确的是(  )

A.磁感应强度B先减小后增大

B.磁感应强度B先增大后减小

C.磁感应强度B一直减小

D.磁感应强度B一直增大

答案

要保持导体棒在斜面上静止,导体棒受力必须平衡,如图,作出磁感应强度B在三个不方向时力的合成图,由图看出,导体棒所受的安培力F先减小后增大,而F=BIL,I、L均不变,则知,磁感应强度B先减小后增大.

故选A

阅读理解

根据短文回答问题。

     Mr Brown looks worried these days. Something is wrong with John’s

head. He took the little boy to a hospital. The doctor looked over him and

said that the boy needed an operationor he would die. John’s mother is

often ill and she can’t work. John’s two sisters are at school. Sometimes

his family has little food to eat and Mr Brown has no money to pay for the

operation. 

      “Why not go and borrow some money from Mr Green?” said Mrs

Brown. “You helped him before.”

     Mr Green has much money. But he never helps anybody. So he has few

friends in the village and nobody likes him. Mr Brown was afraid that his

friends would know about it. So he went to the man’s house at night.

       “I want to ask you for some help, but can you keep a secret for me?”

     “Of course!”

     “My son needs an operation, Mr Green.” Said Mr Brown. “Or he will

die. Will you please lend me some money?”

     “Don’t worry, Mr Brown,” said the rich man. “I will take it as if  I

didn’t hear you!”

1.What’s Mr Brown afraid of ?

He is afraid that John________ ________.

2.How many people are there in John’s family?

There ________ ________.

3.Why can’t Mr Brown pay for the operation?

Because he is ________ _______.

4.Why did Mr Brown go to Mr Green’s house in the evening?

He was afraid that his friends would see ________ ________ ________ Mr

Green’s house.

5.What did Mr Green’s words mean?

Mr Green didn’t want to _______ _______money to Mr Brown.

单项选择题