| UL |
| RL |
| U−UL | ||
|
| 6V |
| RL |
| U−6V |
| 10Ω |
当开关S闭合，滑片P滑到变阻器最大阻值位置时：
| U′L |
| RL |
| U−U′L |
| R |
| 4V |
| RL |
| U−4V |
| 20Ω |
由①②解得：
U=12V，RL=10Ω
P额＝
| ||
| RL |
| (6V)2 |
| 10Ω |
（2）Q ＝I2Rt＝(
| U |
| RL+R |
| 12V |
| 10Ω+20Ω |
答：（1）电源电压是12V，灯L的额定功率为3.6W；
（2）当变阻器连入电路的阻值为最大值时，电流通过它3min所产生的热量为576J
| UL |
| RL |
| U−UL | ||
|
| 6V |
| RL |
| U−6V |
| 10Ω |
| U′L |
| RL |
| U−U′L |
| R |
| 4V |
| RL |
| U−4V |
| 20Ω |
| ||
| RL |
| (6V)2 |
| 10Ω |
| U |
| RL+R |
| 12V |
| 10Ω+20Ω |