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带电质点在两金属板间的电场中运动时能否折返,从功和能的角度分析比较方便:先假设质点能够到达某个点,然后利用功与能的关系推导出质点过该点时动能的表达式,最后根据动能的正、负所具有的物理涵义确定出质点能否折返的条件. 例1 如图1所示,A、B为平行金属板,两板间距为d,分别与电源两极相连,两板中央各有一个小孔M和N.今有一带电质点,自A板上方相距为d的P点由静止自由下落,
It is more convenient to analyze the charged particles when they move in the electric field between the two metal plates. It is more convenient to analyze them from the perspective of work and energy: first assume that the particles can reach a certain point, and then use the relationship between work and energy to derive the kinetic energy of the particles when passing through the point. The expressions finally determine the conditions for the return of the mass point based on the physical meanings of positive and negative kinetic energy. Example 1 As shown in Figure 1, A and B are parallel metal plates, and the distance between the two plates is d, respectively. Connected at the poles, there is a small hole M and N in the center of the two plates. There is a charged material point, and the point P from the top of the A plate is d by the free fall.