问题 选择题

欧姆在探索通过导体的电流和电压、电阻关系时由于无电源和电流表,他就利用金属在冷水和热水中产生电动势代替电源,用小磁针的偏转检测电流,具体做法是:在地磁场作用下处于水平静止的小磁针上方,平行于小磁针水平放置一直导线,当该导线中通有电流时,小磁针会发生偏转;当通过该导线电流为I时,小磁针偏转了30°,问当他发现小磁针偏转了60°,通过该直导线的电流为(已知直导线在某点产生的磁场与通过直导线的电流成正比)(  )

A.2I

B.

2
I

C.3I

D.

3I

答案

当通过电流为I,根据题意可知:地磁场、电流形成磁场、合磁场之间的关系为:

当夹角为30°时,有:B1=kI=Btan30°   ①

当夹角为30°时,有B2=kI1=Btan60°    ②

由①②解得:I1=3I,故ACD错误,B正确.

故选C.

多项选择题 案例分析题
完形填空

The sound of the rain hitting the windows wakes me up and it sounds just exactly as it did 15 years ago, which reminded me of my girlhood.

It was my second day of ____36___ and it was raining heavily. I couldn’t __37_____ the habit I had at home, so I fell asleep right at my ___38____. I woke up to find myself not in the classroom but carried on Dad’s back. We were ____39__ home. It was the last time I felt __40____ to him.

After that, he never carried me or hugged me as other fathers did to their children. I was not bothered by this ___41____. I was proud of it, for I felt _____42____ and “grown up”. However, there were times when I felt depressed by not having sought __43___ from him.

One day, I was writing Chinese characters as homework. I hated doing this more than anything. Dad came and stood behind me for a moment, __44____ me scratching(乱涂)the paper. Then he said, “ This kind of writing needs ___45_____. Your writing is not strong enough. Press harder.”

I followed his ____46_____ for the next two weeks, but __47____ I could see little improvement, I followed my own method---the __48___ way. His advice didn’t improve my writing but our __49____. As I grew older, I found out that he had finished his ___50___ in Grade 6, which was the highest grade in town then, so he was considered to be a ___51___. People used to ask him to write something to put on the walls, but he never felt very useful.

He often said to me, “As you can see, this family is going to count on you. We are __52__ to improve you anything for your future. You have to get _53____ to change the life for yourself.”

As a result of his __54___, I have __55___ it through high school and through college.

36. A. graduation   B. employment      C. college       D. school

37. A. form    B. resist   C. follow D. break

38. A. dormitory    B. door   C. desk    D. bed

39. A. at  B. already       C. beyond       D. halfway

40. A. close    B. near    C. happy D. relaxed

41. A. gap      B. thought      C. action D. accident

42. A. lonely   B. depressed   C. independent       D. comfortable

43. A. comfort       B. help    C. support      D. explanation

44. A. helping B. directing    C. watching    D. blaming

45. A. skill     B. patience     C. time    D. practice

46. A. introduction B. manner      C. spirit   D. advice

47. A. even if  B. while  C. as       D. though

48. A. difficult       B. slow   C. efficient     D. fast

49. A. relationship  B. friendship   C. thought      D. life

50. A. schooling     B. life     C. hope   D. expectation

51. A. writer   B. scholar       C. scientist      D. specialist

52. A. tired     B. glad    C. useless       D. ready

53. A. an educationB. a rich boyfriend C. a well-paid job   D. a good writing

54. A. help     B. expectations       C. examination       D. watch

55. A. succeeded    B. got     C. made  D. found