在互相垂直的匀强磁场和匀强电场中固定放置一光滑的绝缘斜面，其_爱下问搜题

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问题问答题

在互相垂直的匀强磁场和匀强电场中固定放置一光滑的绝缘斜面，其倾角为θ．设斜面足够长，磁场的磁感应强度为B，方向垂直纸面向外，电场方向竖直向上，如图所示．一质量为m、带电量为q的小球放在斜面的最高点A，小球对斜面的压力恰好为零．在释放小球的同时，将电场方向迅速改为竖直向下，电场强度的大小不变，重力加速度为g．求：

（1）电场强度的大小；

（2）小球沿斜面下滑的速度v为多大时，小球对斜面的压力再次为零；

（3）小球从释放到离开斜面共经过多长时间？

答案

（1）小球放在最高点A时，小球对斜面的压力恰好为零，有 mg=Eq

解得 E=

mg

q

（2）电场反向后，小球做匀加速运动，所受洛伦兹力将增大，当小球对斜面的压力再次为零时，斜面对小球的支持力为零，有（mg+Eq）cosθ=Bqv

解得 v=

2mgcosθ

qB

（3）小球在平行于斜面方向上只受重力和电场力的分力作用，有

（mg+Eq）sinθ=ma

则得 a=2gsinθ

所以小球做匀加速运动，小球从释放到离开斜面一共历时 t=

v

a

=

mcotθ

Bq

答：

（1）电场强度的大小为

mg

q

；

（2）小球沿斜面下滑的速度v为

2mgcosθ

qB

时，小球对斜面的压力再次为零；

（3）小球从释放到离开斜面共经过的时间为

mcotθ

Bq

．

多项选择题

药品广告不能含有的内容是（）

A.免费试用

B.免费赠送

C.WHO推荐

D.无效退款

完形填空

In 1972, I returned to Miami Beach High School to speak to the drama class. Afterward I asked the drama teacher       any of my English teachers are still there. Irene Roberts, he tells me, is in the class        down the hall.
　　I was no one special in Miss Roberts’ class — just another student who did okay work. I don’t recall any one special bit of wisdom she passed on. Yet I cannot forget her        for language, for ideas and for her students. I        now, many years later, that she is the perfect example of a        teacher. I’d like to say something to her, I say, but I don’t want to        her from a class. Nonsense, he says, she’ll be        to see you.
　　The drama teacher       Miss Roberts into the hallway where stands this 32-year-old man she last saw at 18. “I’m Mark Medoff,” I tell her. “You were my 12th-grade English teacher in 1958.” She       her head to one side and looks at me, as if this angle might remember me in her       . And then, though armed with a message I want to        in many words, I can’t think up anything more memorable than this: “I want you to know,” I say, “you were        to me.”
　　And there in the hallway, this lovely woman, now nearing        age, this teacher who doesn’t remember me, begins to weep; and she encircles me in her arms.
　　        this moment, I begin to sense that everything I will ever know, everything I will ever pass to my students, is an inseparable part of a legacy(遗产) of our ancestors.
　　Irene Roberts holds me        in her arms and through her tears whispers        my cheek, “Thank you.” And then, with the briefest of looks into my forgotten face, she         back into her classroom,        to what she has done thousands of days through all the years of my       .
On reflection, maybe those were,       , just the right words to say to Irene Roberts. Maybe they are the very words I would like to speak to all those teachers through my life, the very words I would like spoken to me one day by some returning student: “I want you to know you were important to me.”

B．self-confident

C．self-conscious

C．Remembering

B．in addition