【題目】某同學(xué)設(shè)計了一個路燈自動控制門電路，如圖所示，天黑了，讓路燈自動接通，天亮了，讓路燈自動熄滅。圖中RG是一個光敏電阻，當(dāng)有光照射時，光敏電阻的阻值會顯著地減小，R是可調(diào)電阻，起分壓作用。J為路燈總開關(guān)——控制繼電器（圖中未畫路燈電路）。關(guān)于此電路，下列說法正確的是（）

A．天黑時，RG增大，A為高電位，Z為低電位，J使路燈亮

B．天黑時，RG增大，A為低電位，Z為高電位，J使路燈亮

C．R調(diào)大時，傍晚路燈亮得早一些

D．R調(diào)大時，傍晚路燈亮得晚一些

【答案】BD

【解析】當(dāng)光照變?nèi)鯐r光敏電阻RG的阻值變大，分擔(dān)的電壓大，則門電路獲得的電勢低，非門電路輸入端電壓變小，繼電器得到的電壓變高，路燈被點(diǎn)亮，所以B對，A錯；在相同的光照情況下，若電阻R增大，使得R的分得電壓變大，使非門電路的輸入端電壓變大，繼電器得到的電壓偏低，路燈不被點(diǎn)亮。只有天氣更暗時，更晚一些，天色更暗時，光敏電阻更大一些，門電路獲得的電勢低，非門電路輸入端電壓變小，繼電器得到的電壓變高，路燈被點(diǎn)亮，故C錯誤，D正確。

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