30. 图甲是汽车的一种自动测量油箱内油量的简易装置,
R是阻值均匀的变阻器,它的滑片连在杠杆的一端,当油箱内油面发生变化时,浮标通过杠杆使滑片
P在电阻
R上滑动,引起油量表的指针偏转。油量表实际上是一个有“
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E0%3C%2Fmn%3E%3Cmi%3E%EF%BD%9E%3C%2Fmi%3E%3Cmn%3E0.6%3C%2Fmn%3E%3Cmi%3EA%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
”和“
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmn%3E0%3C%2Fmn%3E%3Cmi%3E%EF%BD%9E%3C%2Fmi%3E%3Cmn%3E3%3C%2Fmn%3E%3Cmi%3EA%3C%2Fmi%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
”两个量程的电流表,电源电压为12
V且保持不变,
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmsub%3E%3Cmi%3ER%3C%2Fmi%3E%3Cmn%3E0%3C%2Fmn%3E%3C%2Fmsub%3E%3Cmo%3E%3D%3C%2Fmo%3E%3Cmn%3E10%3C%2Fmn%3E%3Cmi%3E%CE%A9%3C%2Fmi%3E%3Cmo%3E.%3C%2Fmo%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
当长方体油箱中有55升汽油时,油量表的指针满偏,此时
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmrow%3E%3Cmsub%3E%3Cmi%3ER%3C%2Fmi%3E%3Cmn%3E0%3C%2Fmn%3E%3C%2Fmsub%3E%3C%2Fmrow%3E%3C%2Fmath%3E)
两端的电压为6
V;当油箱中的汽油还有5升时,滑片
P刚好在
R的中点,油量表的示数如图乙所示,此时报警电路
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmo+stretchy%3D%22false%22%3E%28%3C%2Fmo%3E%3C%2Fmath%3E)
图中未画出
![](http://math.21cnjy.com/mml2svg?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmo+stretchy%3D%22false%22%3E%29%3C%2Fmo%3E%3C%2Fmath%3E)
开始自动报警,提醒司机尽快加油。油箱中汽油的体积
V和变阻器接入电路的电阻
R的关系如图丙所示。问:
![](http://tikupic.21cnjy.com/8b/33/8b33728a5021d3a519604f51e5c66885.png)