题目内容
如图所示为示波器的原理图,电子枪中炽热的金属丝可以发射电子,初速度很小,可视为零。电子枪的加速电压为U0,紧挨着是偏转电极YY’、XX’,设偏转电极的极板长均为L1,板间距离均为d,偏转电极XX’的右端到荧光屏的距离为L2,电子电量为e,质量为m(不计偏转电极YY’和XX’之间的间距)。在YY’、XX’偏转电极上不加电压时,电子恰能打在荧光屏上坐标的原点。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241156322975328.gif)
求:(1)若只在YY’偏转电极上加电压U YY’=U1(U1>0),则电子到达荧光屏上的速度多大?(2)在第(1)问中,若再在XX’偏转电极上加上U XX’ =U2(U2>0),试在荧光屏上标出亮点的大致位置,并求出该点在荧光屏上坐标系中的坐标值。(13分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241156322975328.gif)
求:(1)若只在YY’偏转电极上加电压U YY’=U1(U1>0),则电子到达荧光屏上的速度多大?(2)在第(1)问中,若再在XX’偏转电极上加上U XX’ =U2(U2>0),试在荧光屏上标出亮点的大致位置,并求出该点在荧光屏上坐标系中的坐标值。(13分)
(1)
(2)
,![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632375640.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632344893.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632360701.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632375640.gif)
(1)加速:
偏转:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632422636.gif)
v=
-
电子在yy’中偏转的位移为y1
y1=
离开yy’后的运动时间为t2侧向的位移为y2 y2=v1×t2 t2=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632469446.gif)
解得到y=y1+y2=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632375640.gif)
同理可以得到
x=x1+x2=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632360701.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632391535.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632407311.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632422636.gif)
v=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632344893.gif)
电子在yy’中偏转的位移为y1
y1=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632453544.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632469446.gif)
解得到y=y1+y2=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632375640.gif)
同理可以得到
x=x1+x2=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115632360701.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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