题干

如下图所示,固定在匀强磁场中的水平导轨ab、cd的间距L1=0.5m,金属棒ad与导轨左端bc的距离L2=0.8m,整个闭合回路的电阻为R=0.2Ω,匀强磁场的方向竖直向下穿过整个回路。ad杆通过细绳跨过定滑轮接一个质量为m=0.04kg的物体,不计一切摩擦,现使磁感应强度从零开始以
Δ
B
Δ
t
=0.2T/s的变化率均匀地增大,求经过多长时间物体m刚好能离开地面?(g取10m/s2
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答案(点此获取答案解析)

解:物体刚要离开地面时,其受到的拉力F等于它的重力mg,而拉力F等于棒ad所受的安培力,即
mg=BIl1
其中B=
Δ
B
Δ
t
·t
感应电流由变化的磁场产生
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    Having a teenager at home can be a real headache. Many teenagers smoke, drink and even fight. They often do something dangerous.

    Earlier studies have shown that teenagers are more likely (可能) to make irrational decisions than people from any other age group, including children and adults. Is it that teenagers are too young to tell right from wrong? Not really. So what's the reason?

    According to Laurence, a teacher from a US university, the reason is that teenagers care a lot about how their peers (同龄人) view them—that is “peer influence”.

    As children enter their teenage years, they spend more time with their friends and classmates, and also they care more about what they think of them. This makes teenagers make decisions without thinking about the costs.

    In a test, a group of teenagers were asked to play a video driving game. When they played with their friends watching around them, they took more chances and drove more carelessly because that would increase their possibility of winning. But when they played alone, they drove more safely.

    Why do peers have such a big influence on teens' behavior?

As Laurence sees it, a teenager's brain is like a car with a good accelerator (油门) but a weak brake (刹车). The “accelerator” is fully developed by teenage years while the “brake” is still not fully developed. When teenagers are watched by their peers, they usually push hard on the accelerator. With their weak brake, it is likely that they are going to end up in an accident.

    But the good news is that a violent (暴力的)teenager doesn't necessarily become a violent adult. About two-thirds to three-quarters of violent youth grow out of it. “They get more self-controlled.”