题目内容
如图所示,一段长方体形导电材料,左右两端面的边长都为a和b.内有带电量为q的某种自由运动电荷.导电材料置于方向垂直于其前表面向里的匀强磁场中,内部磁感应强度大小为B。当通以从左到右的稳恒电流I时,测得导电材料上、下表面之间的电压为U,且上表面的电势比下表面的电势低,由此可得该导电材料单位体积内自由运动电荷数及自由运动电荷的正负别为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250045059473059.jpg)
A.
,负
B.
,正
C.
,负
D.
,正
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250045059473059.jpg)
A.
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004505963382.png)
B.
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004505963382.png)
C.
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004505994387.png)
D.
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004505994387.png)
C
本题考查带电粒子在电场和磁场组成的复合场中的运动情况,要求学生的综合分析能力较强.导电材料上表面的电势比下表面的低,故导电材料内部的电场方向为竖直向上.由于电流方向为水平向右,假设运动电荷为正电荷,则由左手定则可知电荷受到的洛伦兹力竖直向上,而运动电荷受到的电场力也竖直向上,不能平衡,所以运动电荷不是正电荷,即该电荷一定是负电荷.取长为l的这段导电材料进行分析,电荷通过这段导电材料所用的时间为t,则自由运动电荷运动的速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004506025264.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004506041297.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004506056335.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004506088552.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004506103385.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004506119667.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004506212307.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825004506228602.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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