问题 多选题

甲图中A、B是一对平行金属板,A板接地(电势UA=0),在两板间加上一周期为T的交变电压u(如乙图所示).现有一电子从A板上的小孔进入两板间的电场区内.设电子的初速度和重力的影响均可忽略,则(  )

A.若电子是在t=0时刻进入的,它将一直向B板运动

B.若电子是在t=T/8时刻进入的,它可能时而向B板运动,时而向A板运动,最后打在B板上

C.若电子是在t=3T/8时刻进入的,它可能时而向B板运动,时而向A板运动,最后打在B板上

D.若电子是在t=T/2时刻进入的,它可能时而向B板、时而向A板运动

答案

A、电子在t=0时刻进入时,在一个周期内,前半个周期受到的电场力向上,向上做加速运动,后半个周期受到的电场力向下,继续向上做减速运动,T时刻速度为零,接着周而复始,所以电子一直向B板运动,一定会到达B板.故A正确.

B、若电子是在t=

T
8
时刻进入时,在一个周期内:在
T
8
1
2
T,电子受到的电场力向上,向上做加速运动,在
1
2
T~T内,受到的电场力向下,继续向上做减速运动,
9T
8
T时刻速度为零,接着继续向B板运动,周而复始,所以电子时而向B板运动,时而向A板运动,最后打在B板上.故B正确.

C、若电子是在t=

3
8
T时刻进入时,与在t=
T
8
时刻进入时情况,在运动一个周期时间内,时而向B板运动,时而向A板运动,总的位移向左,最后穿过A板.故C错误;

D、若电子是在t=

T
2
时刻进入时,在一个周期内:在
T
2
~T,电子受到的电场力向左,向左做加速运动,在T~
3T
2
内,受到的电场力向右,继续向左做减速运动,
3T
2
时刻速度为零,接着周而复始,所以电子一直向A板运动,一定不会到达B板.故D错误.

故选AB

单项选择题 共用题干题
阅读理解

第三部分阅读理解(共20小题;每小题2分,满分40分)

阅读下列短文,从每题所给的四个选项(A、B、C和D)中,选出最佳选项,并在答题卡上将该项涂黑。

Looking back at my school yearbooks of the 1970s, 80s, and 90s, I find the style of uniform says a lot about the students and the character of the times. In the 1970s, students accessorized (配饰) their uniforms with platform shoes. During the 80s, hair was huge—literally. Students’ goofy hair filled up nearly every shot in the yearbook. In the 90s, accessories were all the rage (流行).

Uniform rules have always been strict. The restrictions (限制) are down to the inch: Gray or plaid skirts must be no shorter than two inches above the knee; a red, green, navy or white polo must be worn at all times; and only white, blue, black, and gray sneakers or flats are allowed. With all these rules, one can’t help but wonder how the typical rebellious (叛逆的) teenager can express her individuality.

Still, they manage. Senior Michelle Ferrier says she accessorizes by wearing bright colors like green and pink in bracelets, shoe laces, and headbands to stand out and be fashionable. For any stylish (时尚的) Carondelet student, accessories are key.

At Carondelet, one thing’s for sure: The older a student gets, the more she’s willing to bend the rules. Senior Natalia Piwek admits that she often pairs her “tights and colorful socks” with “illegal shoes”. If you happen to see a girl with colorful tights or boots running across the hallway, it’s most likely that she’s running from a dean.

In about two months, I will say goodbye to my uniform. I will miss the convenience of wearing it. However, I must say, wearing a uniform has taught me an important lesson that I will take with me when I go off to college: it doesn’t matter what you wear, but how you wear it. And how you carry yourself.

56.In the 1980s, _________.

A.uniform rules are sneer

B.students preferred goofy hair

C.accessories were popular with students

D.students wrote colorful shoes with uniforms

57.Which of the following combinations might the students think stylish?

a. grey bracelets     b. parks shoes laces C. green heathens

d. patterned tights   E. red polo

A.abe   B.bde   C.bcd   D.ace

58.According to the students, the restrictions of uniforms are_______.

A.atcler              B.understanding childish

C.childish           D.unselfishly

59.What’s the author’s attitude towards wearing school uniforms?

A.Objictive. B.Tolerant.   C.Agreenble.       D.Oppcaitive.

60.What is the main point of the text?

A.The history of student’s uniforms.

B.How to make school uniforms stylish.

C.Memories and lessons From school uniforms.

D.The debate oven restrictions on school uniforms.