如图，L是“12V 6W”的小灯泡，Rl是定值电阻，R2是最大阻值为24Ω的滑动变阻器，电源电压和灯泡的电阻不变。求：
（1）灯泡的阻值。
（2）闭合开关S、Sl，变阻器滑片在b端时，小灯泡恰能正常发光，这时电流表示数为0.9A，求Rl的阻值。
（3）闭合开关S，断开Sl时，小灯泡消耗的最小功率。
解：（1）由
得
灯丝的电阻值：
（2）当闭合开关S、S1，变阻器滑片在b端时，变阻器接入电阻为0Ω，R1与L并联且小灯泡恰能正常发光。
U＝U1＝U2＝12V
I1＝I－IL＝0.9A－0.5A＝0.4A
（3）当闭合开关S，断开S1时，L与R2串联。要使小灯泡消耗的功率最小，电路中的电阻应最大，即变阻器滑片移至a端。
RL＝R2＝24Ω
灯泡消耗的最小功率：
| 阅读理解。 | ||||
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