问题 选择题

如图所示,三根长直导线通电电流大小相同,通电方向为b导线和d导线垂直纸面向里,c导线向纸外,a点为bd的中点,ac垂直bd,且abadac。则a点磁感应强度的方向为(    )

A.垂直纸面指向纸外

B.沿纸面由a指向d

C.沿纸面由a指向b

D.沿纸面由a指向c

答案

答案:C

分析:该题考查了磁场的叠加问题.用右手定则首先确定三根通电直导线在a点产生的磁场的方向,利用矢量的叠加分析叠加后磁场大小变化和方向,从而判断各选项.

解答:解:

用右手螺旋定则判断通电直导线在a点上所产生的磁场方向,如图所示:

直导线b在a点产生磁场与直导线d在a点产生磁场方向相反,大小相等.则合磁场为零;而直导线c在a点产生磁场,方向从d指向b,即为

沿纸面由a指向b

故选C.

点评:磁感应强度既有大小,又有方向,是矢量.它的合成遵循矢量合成的平行四边形法则.

阅读理解

My newly-rented small apartment was far away from the centre of London and it was becoming essential for me to find a job, so finally I spent a whole morning getting to town and putting my name down to be considered by London Transport for a job on the underground. They were looking for guards, not drivers. This suited me. I couldn’t drive a car but thought that I could probably guard a train, and perhaps continue to write my poems between stations. The writers Keats and Chekhov had been doctors. T.S. Eliot had worked in a bank and Wallace Stevens for an insurance company. I’d be a subway guard. I could see myself being cheerful, useful, a good man in a crisis. Obviously I’d be overqualified but I was willing to forget about that in return for a steady income and travel privileges — those being particularly welcome to someone living a long way from the city centre.

The next day I sat down, with almost a hundred other candidates, for the intelligence test. I must have done all right because after about half an hour’s wait I was sent into another room for a psychological test. This time there were only about fifty candidates. The interviewer sat at a desk. Candidates were signaled forward to occupy the seat opposite him when the previous occupant had been dismissed, after a greater or shorter time. Obviously the long interviews were the more successful ones. Some of the interviews were as short as five minutes. Mine was the only one that lasted a minute and a half.

I can remember the questions now: “Why did you leave your last job?” “Why did you leave your job before that?” “And the one before that?” I can’t recall my answers, except that they were short at first and grew progressively shorter. His closing statement, I thought, revealed (揭示) a lack of sensitivity which helped to explain why as a psychologist, he had risen no higher than the underground railway. “You’ve failed the psychological test and we are unable to offer you a position.”

Failing to get that job was my low point. Or so I thought, believing that the work was easy. Actually, such jobs — being a postman is another one I still desire — demand exactly the sort of elementary yet responsible awareness that the habitual dreamer is least qualified to give. But I was still far short of full self-understanding. I was also short of cash.

小题1:The writer applied for the job chiefly because _________.

A.he wanted to work in the centre of London

B.he could no longer afford to live without one

C.he was not interested in any other available job

D.he had received some suitable training小题2:The writer thought he was overqualified for the job because _________.

A.he often traveled underground

B.he had written many poems

C.he could deal with difficult situations

D.he had worked in a company小题3:The length of his interview meant that _________.

A.he was not going to be offered the job

B.he had not done well in the intelligence test

C.he did not like the interviewer at all

D.he had little work experience to talk about小题4:What does the writer realize now that he did not realize then?

A.How unpleasant ordinary jobs can be.

B.How difficult it is to be a poet.

C.How unsuitable he was for the job.

D.How badly he did in the interview.小题5:What’s the writer’s opinion of the psychologist?

A.He was very aggressive.

B.He was unhappy with his job.

C.He was quite inefficient.

D.He was rather unsympathetic.

单项选择题