图甲是温度自动报警器。控制电路中,电源电压U
控=5V,热敏电阻R
2的阻值与温度的关系如图乙所示;工作电路中,灯泡L标有“9V0.3A”的字样,R
4为电子嗡鸣器,它的电流达到某一固定值时就会发声报警,其阻值R
4=10Ω。在R
2温度为20℃的情况下,小明依次进行如下操作:闭合开关S
1和S
2 , 灯泡L恰好正常发光,此时R
0的电功率为P
0;将R
1的滑片调到最左端时,继电器的衔铁刚好被吸下,使动触点与下方静触点接触;调节R
3的滑片,当R
3与R
4的电压之比U
3:U
4=4:1时,电子嗡鸣器恰好能发声,此时R
0的电功率为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmsub%3E%3Cmtext%3E%3Cmsup%3EP++++%3Cmn%3E%27%3C%2Fmn%3E%3C%2Fmsup%3E%3C%2Fmtext%3E%3Cmn%3E0%3C%2Fmn%3E%3C%2Fmsub%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
,
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmsub%3E%3Cmtext%3E%3Cmsup%3EP++++%3Cmn%3E%27%3C%2Fmn%3E%3C%2Fmsup%3E%3C%2Fmtext%3E%3Cmn%3E0%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmsub%3E%3Cmrow%3E%3Cmtext%3E%EF%BC%9AP%3C%2Fmtext%3E%3C%2Fmrow%3E%3Cmn%3E0%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E%3D%3C%2Fmo%3E%3Cmn%3E4%3C%2Fmn%3E%3Cmo%3E%EF%BC%9A%3C%2Fmo%3E%3Cmn%3E9%3C%2Fmn%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
。已知电源电压、灯丝电阻都不变,线圈电阻忽略不计。求:
![](http://tikupic.21cnjy.com/b4/48/b448f7c89a9d6e059997dfabe21f070a.png)