题干

三角形导线框abc放在匀强磁场中,磁感线方向与线圈平面垂直,磁感应强度B随时间变化的图象如图所示。t=0时磁感应强度方向垂直纸面向里,则在0~4 s时间内,线框的ab边所受安培力随时间变化的图象如图所示(力的方向规定向右为正)(  )

A:

B:

C:

D:

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答案(点此获取答案解析)

B

同类题1

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    I was at my parent's dinner table. Before me was a worn journal of thin and discolored pages. It was my grandfather's journal and now belonged to my father. My grandfather had passed away in the months leading up to my birth. I never got to visit the places he had frequently and people who had been a part of his life's journey.

    I was now about to enter his world, through the words he had left behind. Within minutes, I was attracted by the power of the written words. In the magical script before me, I was transported to another age when food was an everyday art, planned, prepared and enjoyed in the company of others, and a time when people had the heart to pause,their own lives to embrace (拥抱)each other's struggles. All this was conveyed to me in the beauty of the words that flowed together to connect with the writer's mind and understand the world they lived in.

    That kind of writing seems to be lost on us today. We have gotten used to writing in bite-sized pieces for a public looking for entertainment, and hungry for information. No wonder, there are nearly 200 million bloggers on the Internet and a new blog is created somewhere in the world every half a second. Instead of adding to our collective wisdom, most of these writings reflect the superficiality (肤浅)and impatience of our day and age.

    This not only robs us of the skill of writing impressive essays, it also prevents us from exploring what is truly important. Writing humbles us in a way that is vital for our character growth, by reminding us about the limits of the self and our appropriate place in the vast flow of life. Writing frees us by helping us explore the unknown so that we truly open up to the magic of the world around us. I saw all of this in the writing s of my grandfather. And I've seen it again and again in the writings of the greatest thinkers of humanity. Their writings reflect deep thought on issues of human importance.

同类题3

【加试题】某校举行班班有歌声比赛,参赛班级的成绩按照班级序号保存在“bbygs.accdb”数据库文件中,为了快速计算每个班级最终的分,设计了一个VB程序,该程序能够将8个评委的打分按照从高到低的顺序排序,然后去掉一个最高分和一个最低分,最后计算平均值作为参赛班级的成绩。程序界面如图:单击“提取”按钮Command1,在列表框list1中显示班级序号和该班级8个评委的打分成绩,单击“排序”按钮Command2后,在列表框list2中按降序排序。单击计算得分按钮Command3,在文本框text1中显示最终成绩。按此要求编写程序如下:

Dim class(1 To 30) As Integer '存放班级序号的数组定义为class

Dim score(1 To 8) As single '存放班级各评委打分的数组定义为score

Private Sub Formload()      '提取某班级的评委打分

   Dim conn As New ADODB.Connection

Dim rs As New ADODB.Recordset

   conn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + App.Path + "\bbygs.accdb "

   conn.Open

   Set rs.ActiveConnection = conn

   rs.Open "SELECT * FROM score " 

   n = 0

Do While Not rs.EOF   '到最后一条记录后退出循环

   n = n + 1

   class (n) = rs.Fields("班级序号").Value

   score (n) = rs.Fields("评委打分").Value

   rs.MoveFirst      '①指针移动到下一条记录

Loop

rs.Close

cn.Close

Set rs = Nothing

Set cn = Nothing

for i=1 to n

    List1.AddItem str(score(i))

Next i

End sub

Private Sub Command1Click()  

Dim i as integer

Dim j as integer

Dim t as single

For i = 1 to 8

  For j=1 to i

    If   ②  then  

t=score(j) 

score(j)=score(j+1)

score(j+1)=t

End if

  Next j

Next i

For i=1 to 8

   List2.AddItem str(score(i))

Next i

End sub

Private Sub Command2Click()   '计算班级平均得分

Dim i As Integer

Dim sum As single

For i=2 to 7

   sum=sum+score(i)

Next i

Text1.text=  ③ 

End sub