如图甲所示,质量为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3Em%3C%2Fmi%3E%3C%2Fmath%3E)
边长为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EL%3C%2Fmi%3E%3C%2Fmath%3E)
、电阻为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3ER%3C%2Fmi%3E%3C%2Fmath%3E)
的正三角形金属框
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EA%3C%2Fmi%3E%3Cmi+mathvariant%3D%22normal%22%3EC%3C%2Fmi%3E%3Cmi+mathvariant%3D%22normal%22%3ED%3C%2Fmi%3E%3C%2Fmath%3E)
由粗细均匀的金属棒组成,绝缘细线一端连接
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EA%3C%2Fmi%3E%3Cmi+mathvariant%3D%22normal%22%3EC%3C%2Fmi%3E%3C%2Fmath%3E)
的中点
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EG%3C%2Fmi%3E%3C%2Fmath%3E)
将金属框吊在天花板上的
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EO%3C%2Fmi%3E%3C%2Fmath%3E)
点,金属框处于静止状态,金属框部分处于垂直金属框平面向外的匀强磁场中,磁场的磁感应强度大小为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EB%3C%2Fmi%3E%3C%2Fmath%3E)
, 金属框
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EA%3C%2Fmi%3E%3Cmi+mathvariant%3D%22normal%22%3ED%3C%2Fmi%3E%3C%2Fmath%3E)
、
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EC%3C%2Fmi%3E%3Cmi+mathvariant%3D%22normal%22%3ED%3C%2Fmi%3E%3C%2Fmath%3E)
边的中点
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EE%3C%2Fmi%3E%3C%2Fmath%3E)
、
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3EF%3C%2Fmi%3E%3C%2Fmath%3E)
在磁场的水平边界上,重力加速度为
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3Eg%3C%2Fmi%3E%3C%2Fmath%3E)
。现让磁感应强度按如图乙所示的规律变化,图甲中磁场方向为正方向,感应电流以顺时针为正方向。
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmi+mathvariant%3D%22normal%22%3Et%3C%2Fmi%3E%3Cmo%3E%3D%3C%2Fmo%3E%3Cmn%3E0%3C%2Fmn%3E%3C%2Fmath%3E)
时刻悬线的拉力恰好为零,细线能承受的最大拉力为金属框重力的
![](https://math.21cnjy.com/MathMLToImage?mml=%3Cmath+xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F1998%2FMath%2FMathML%22%3E%3Cmn%3E2%3C%2Fmn%3E%3C%2Fmath%3E)
倍,则细线未断之前,金属框中的电流、流过金属棒的电荷量大小和细线上的拉力随时间变化的关系图像正确的是( )
![](http://tikupic.21cnjy.com/2024/03/29/08/37/08375cbf556d84300c204b41551d1a3f.png)