[题目]某同学设计了一个路灯自动控制门电路.如图所示.天黑了.让路灯自动接通.天亮了.让路灯自动熄灭.图中RG是一个光敏电阻.当有光照射时.光敏电阻的阻值会显著地减小.R是可调电阻.起分压作用.J为路灯总开关--控制继电器(图中未画路灯电路).关于此电路.下列说法正确的是()A．天黑时.RG增大.A为高电位.Z为低电位.J使路灯亮B．天黑时.RG增大.A为低� 题目和参考答案——青夏教育精英家教网—

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【题目】某同学设计了一个路灯自动控制门电路，如图所示，天黑了，让路灯自动接通，天亮了，让路灯自动熄灭。图中RG是一个光敏电阻，当有光照射时，光敏电阻的阻值会显著地减小，R是可调电阻，起分压作用。J为路灯总开关——控制继电器（图中未画路灯电路）。关于此电路，下列说法正确的是（）

A．天黑时，RG增大，A为高电位，Z为低电位，J使路灯亮

B．天黑时，RG增大，A为低电位，Z为高电位，J使路灯亮

C．R调大时，傍晚路灯亮得早一些

D．R调大时，傍晚路灯亮得晚一些

【答案】BD

【解析】当光照变弱时光敏电阻RG的阻值变大，分担的电压大，则门电路获得的电势低，非门电路输入端电压变小，继电器得到的电压变高，路灯被点亮，所以B对，A错；在相同的光照情况下，若电阻R增大，使得R的分得电压变大，使非门电路的输入端电压变大，继电器得到的电压偏低，路灯不被点亮。只有天气更暗时，更晚一些，天色更暗时，光敏电阻更大一些，门电路获得的电势低，非门电路输入端电压变小，继电器得到的电压变高，路灯被点亮，故C错误，D正确。

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