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1993年高考物理试卷中,多选题第19题,是个内涵很丰富的题目,虽然定性分析也可以得出评分说明中的结果,但从较高层次来理解和讨论该题仍有必要。 [题目] 图1中A、B是一对中间开有小孔的平行金属板,两小孔的连线与金属板互相垂直,两极板的距离为l。两极板间加上低频交流电压,A极电势为零,B板电势u=U_(0cosωl)。现在一电子在l=0时穿过A板上的小孔射入电场。设初速度和重力的影响均可忽略不计。则电子在两极板问可能 (A)以AB间某一点为平衡位置来回振动。 (B)时而向B板运动,时而向A板运动,但最后穿出B板。 (C)一直向B板运动,最后穿出B板,如果ω小于
In the 1993 college entrance examination physics paper, question 19 of the multiple-choice question was a very rich topic. Although the qualitative analysis can also give the results in the score description, it is still necessary to understand and discuss the topic from a higher level. [Topic] In Fig. 1, A and B are a pair of parallel metal plates with small holes in the middle. The connecting lines of the two small holes are perpendicular to the metal plates. The distance between the two plates is l. Between the two plates plus low-frequency AC voltage, A pole potential is zero, B plate potential u = U_ (0cosωl). Now an electron enters the electric field through the hole in the A plate at l=0. Let the effects of initial velocity and gravity be negligible. Then the electrons may be asked to oscillate back and forth at the two pole plates (A) back and forth at a certain point in AB. (B) moves to B board from time to time, sometimes moves to A board, but finally goes out B board. (C) has been moving to the B board, and finally out of the B board, if ω is less than