如图所示，电源电动势为E，内阻r=R，定值电阻R₁和R₂的阻值均为R．平行板电容器接在R₂两端，两极板长度和距离均为d，足够大屏幕与电容器右端距离为d，OO₁为电容器中心轴线．一个不计重力、质量为m、电荷量为q的带正电粒子从O点以一定的初速度沿OO₁方向射入电场，离开电场时的位置与电容器下极板的距离为 $\frac{d}{4}$ ．

    "Early to bed, early to rise makes people healthy, wealthy(富有的)and wise(聪明的). "

    This is an old English saying. Children should have ten hours' sleep every night, or they can't do their work very well. They will not be wise, either. The body needs exercise. Walkin9, running, swim-ruing and playing ball games are all exercise. Exercise keeps the body strong and makes the mind bright.

算一算，填一填．

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