20.
(2023高二下·宁波期末)
电动汽车通过能量回收装置增加电池续航.在行驶过程中,踩下驱动踏板时电池给电动机供电,松开驱动踏板或踩下刹车时发电机工作回收能量.某兴趣小组为研究其原理,设计了如图所示的模型:两个半径不同的同轴圆柱体间存在由内至外的辐向磁场,磁场方向沿半径方向,有一根质量为
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3Em%3C%2Fmi%3E%3C%2Fmath%3E)
、长度为
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3EL%3C%2Fmi%3E%3C%2Fmath%3E)
、电阻为
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3ER%3C%2Fmi%3E%3C%2Fmath%3E)
的金属棒
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3EM%3C%2Fmi%3E%3Cmi%3EN%3C%2Fmi%3E%3C%2Fmath%3E)
通过导电轻杆与中心轴相连,可绕轴无摩擦转动,金属棒所在之处的磁感应强度大小均为
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3EB%3C%2Fmi%3E%3C%2Fmath%3E)
,整个装置竖直方向放置.中心轴右侧接一单刀双掷开关:踩下驱动踏板,开关接通1,电池给金属棒供电,金属棒相当于电动机,所用电池的电动势为
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3EE%3C%2Fmi%3E%3C%2Fmath%3E)
,内阻为
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3Er%3C%2Fmi%3E%3C%2Fmath%3E)
;松开驱动踏板或踩下刹车,开关自动切换接通2,金属棒相当于发电机,给电容器充电,所接电容器电容为
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3EC%3C%2Fmi%3E%3C%2Fmath%3E)
.初始时电容器不带电、金属棒
![](//math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi%3EM%3C%2Fmi%3E%3Cmi%3EN%3C%2Fmi%3E%3C%2Fmath%3E)
静止,电路其余部分的电阻不计.
![](//tikupic.21cnjy.com/2023/06/25/00/2c/002cdd86a3a364b29e4dfcf575d10180_296x206.png)