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问题 计算题

（12分）如图所示，在y>0的区域内有沿y轴正方向的匀强电场，在y<0的区域内有垂直坐标平面向里的匀强磁场。一电子（质量为m、电量为e）从y轴上A点以沿x轴正方向的初速度v0开始运动。当电子第一次穿越x轴时，恰好到达C点；当电子第二次穿越x轴时，恰好到达坐标原点；当电子第三次穿越x轴时，恰好到达D点，D点未在图中标出。已知A、C点到坐标原点的距离分别为d、2d。不计电子的重力。求

（1）电场强度E的大小；

（2）磁感应强度B的大小；

（3）电子从A运动到D经历的时间t．

答案

(1)  (2)  (3)

题目分析：（1）电子在电场中做类平抛运动

设电子从A到C的时间为t1

　　1分

　  1分

      1分

求出   E =     1分

（2）设电子进入磁场时速度为v，v与x轴的夹角为θ，则

 　θ = 45°　1分　

　　　  　　

电子进入磁场后做匀速圆周运动，洛仑兹力提供向心力

       1分

由图可知　 　　 　 1分

得        　　  　1分

（3）由抛物线的对称关系，电子在电场中运动的时间为 3t1= 1分

电子在磁场中运动的时间    t2 =       2分

电子从A运动到D的时间   t=3t1+ t2 =     1分

填空题

Some consumer researchers distinguish_____(1) "rational" motives and "emotional" (or "non-rational") motives. They use the term "rationality"_____(2) the traditional economic sense that assumes_____(3) consumers behave rationally when they carefully consider all alternatives_____(4) choose those that give them the greatest utility (i.e., satisfaction). _____(5) a marketing context, the term "rationality" implies that the consumer selects goods based_____(6) totally objective criteria, such_____(7) size, weight, price, and so on. "Emotional" motives imply the selection of goods____(8) to personal or subjective criteria — the desire_____(9) individuality, pride, fear, affection or status.The assumption underlying this distinction is____(10) subjective or emotional criteria do not maximize utility or satisfaction. _____ (11), it is reasonable to assume that consumers always attempt to select alternatives that, ____(12) their view, serve to maximize satisfaction. Obviously, the assessment of satisfaction is a very personal process, based_____(13) the individual’’s own needs as_____(14) as on past behavioral, social, and learning experiences. What may appear_____(15) irrational to an outside observer may be perfectly rational ______(16) the context of the consumer’’s own psychological field. For example, a product purchased to enhance one’’s self-image (such as a fragrance) is a perfectly rational form of consumer behavior. ____(17) behavior did not appear rational to the person who undertakes it_____ (18) the time that it is undertaken, obviously he or she would not do it._____(19) the distinction between rational and emotional motives does not appear to be warranted.Some researchers go so far_____(20) to suggest that emphasis_____ (21) "needs" obscures the rational, or conscious, nature of most consumer motivation. They claim that con sumers act consciously____(22) maximize their gains and minimize their losses; that they act not____(23) subconscious drives but from rational preferences,____(24) what they perceive to be_____(25) their own best interests.
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单项选择题

腐植酸铬抗温可高达（）℃以上。

A、100

B、150

C、160

D、200
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