题干

如图所示,电阻为R、质量为m的矩形导线框ABCD自某一高度自由落下,通过一有界的匀强磁场。若线框恰好以恒定速度通过磁场,在不计空气阻力的情况下,线框中产生的焦耳热是多少?已知AB、CD边长均为l,AD、BC边长均为h,磁场区域宽度为h。

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答案(点此获取答案解析)

解:法一:以矩形导线框为研究对象。设线框匀速运动的速度为v,则线框穿过磁场的时间t=
2
h
v
由平衡条件可知,线框匀速穿过磁场时,所受重力与安培力大小相等。设线框中的电流为I,则有
BIl=mg
由闭合电路欧姆定律,有I=

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    My daughter had a bullying(欺凌) problem a couple of years ago. She kept receiving rude text messages from kids at school. Dealing with school bullying wasn't easy, but I finally ended it. When you realize things are serious, it is no time to be silent.

    If your kids are also receiving such text messages, take them directly to the headmaster and show him. Describe to him how it is affecting your kids. Tell him you will not tolerate it and that you want him to put an end to it now.

    School bullying is a dangerous and serious issue these days, and any good headmaster will take your charges seriously. He will probably address the issues with the parents. If they are crappy(糟糕的) parents, then they may do nothing to their child. You may have to repeat this process several times, but you need to keep a check on those text messages and don't allow your child to delete them.

    Now it is time to draw your child close and take care of him/her, because he/she needs it. My daughter did not like me being involved either, but you are the only person your kid can depend on to solve this problem, so do it and mean business. Always be polite, but straightforward. The headmaster and parents must know you mean business.

    If repeated meetings with the headmaster (and parents, if he allows it) do not work, then I would consider hiring a lawyer. That will send the message loud and clear. When you are talking about financial loss, most parents will stand up and take notice and act to keep their kids under control!

    I know a lawyer is a big expense and not something you do lightly, but when bad things like school bullying are happening to your children, you must act and act quickly.