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一束初速不计的电子流在经U的加速电压加速后，立即在距两极板等距处垂直进入平行板间的匀强电场，如图所示，若A、B板间距离d，板长l，一个电子的带电量为e，质量为m．则

（1）电子离开加速电场时的速度V0是多少？

（2）要使电子能从B板边缘飞出，这两个极板应加的电压U′是多少？

答案

（1）在加速电场中：由动能定理得 eU=

1
2
m
v20

得v0=

2eU
m

（2）在偏转电场中，电子做类平抛运动，则有：

平行于极板方向：l=v0t

垂直于极板方向：y=

1
2
at2，a=
eE
m
=
eU′
md

联立以上各式得 y=

U′l2
4Ud

要使电子能从B板边缘飞出，则有y=

d
2
，联立解得，U′=
2Ud2
l2

答：

（1）电子离开加速电场时的速度V0是

2eU
m
．

（2）要使电子能从B板边缘飞出，这两个极板应加的电压U′是

2Ud2
l2
．

听力题

A school newspaper The Teens asked over one thousand teenagers how they spent their spare time. Here they report the r     小题1:of the survey. It’s not at all surprising to learn that most teens said they wanted more free time. Most have less than an hour a day for after-school activities.

P      小题2:activity was popular among both girls and boys, averaging between three to six hours a week. Favourite sports among girls were tennis, basketball and swimming. Boys said they liked football, basketball and skating.

Speaking of entertainment, music and TV were popular. About half of those surveyed said that music was their favourite activity. This i       小题3:both listening to and playing music. TV was also very popular. Three quarters said they preferred watching TV to reading a book or magazine.

Unexpectedly, as many as three out of ten teenagers mentioned that collecting things was something they liked to do in their spare time. They had v      小题4:collections from cartoon books and baseball cards to stickers, toys and coins.

V  小题5:friends was also popular. Eight out of ten teenagers said they met friends once a week. Nine out of ten said they contacted friends through the Internet. A   小题6:popular way of contacting friends was the telephone. Everyone we surveyed said they spoke to friends every day. Most said they spoke about twenty minutes a day. Surprisingly, teenagers s    小题7:went dancing, especially girls. Only one out of ten said they went to the dance regularly.
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单项选择题

* * 后脓肿切开引流时，切口应是（）

A.放射状切口

B.垂直切口

C.平行切口

D. * * 下缘弧形切口

E. * * “十”字形切口
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