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【题目】如图所示,E=10 V,r=1 Ω,R1=R3=5 Ω,R2=4 Ω,C=100 μF。当S断开时,电容器中带电粒子恰好处于静止状态。求:

1S闭合后,带电粒子加速度的大小和方向;

2S闭合后流过R3的总电荷量。

【答案】1g,方向竖直向上 24×10-4 C

【解析】1开始带电粒子恰好处于静止状态,必有qE=mg且qE竖直向上。S闭合后,qE=mg的平衡关系被打破。

S断开时,带电粒子恰好处于静止状态,设电容器两极板间距离为d,有UCE=4 V, =mg

S闭合后,UC′=E=8 V

设带电粒子加速度为a,

-mg=ma,解得a=g,方向竖直向上。

2S闭合后,流过R3的总电荷量等于电容器上电荷的增加量,所以ΔQ=CUC′-UC=4×10-4 C

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