题干

利用电场和磁场来控制带电粒子的运动,是一项重要的物理应用技术.在等量正点电荷所激发的电场中,可以使带电粒子做匀速圆周运动.如图所示,AB为两个点电荷连线的中垂线,粒子在其上P点获得一垂直纸面向外的速度,恰好在中垂面上做匀速圆周运动.以下判断正确的是(   )

A:该粒子可能带正电

B:该粒子一定带负电

C:若点电荷M、N的带电量同时等量地缓慢增大,粒子仍在原轨道做匀速圆周运动

D:若点电荷M、N的带电量同时等量地缓慢增大,粒子将无法在原轨道做匀速圆周运动

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B,D

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    If it really is what's on the inside that counts, then a lot of thin people might be in trouble.

    Some doctors now think that the internal(内部的) fat surrounding important organs like the heart or liver could be as dangerous as the external fat which can be noticed more easily.

    “Being thin doesn't surely mean you are not fat,” said Dr Jimmy Bell at Imperial College. Since 1994, Bell and his team have scanned nearly 800 people with MRI machines to create “fat maps” showing where people store fat.

    According to the result, people who keep their weight through diet rather than exercise are likely to have major deposits of internal fat, even if they are slim.

    Even people with normal Body Mass Index scores can have surprising levels of fat deposits inside. Of the women, as many as 45 percent of those with normal BMI scores (20 to 25) actually had too high levels of internal fat. Among men, the percentage was nearly 60 percent.

    According to Bell, people who are fat on the inside are actually on the edge of being fat. They eat too many fatty and sugary foods, but they are not eating enough to be fat. Scientists believe we naturally store fat around the belly first, but at some point, the body may start storing it elsewhere.

    Doctors are unsure about the exact dangers of internal fat, but some think it has something to do with heart disease and diabetes(糖尿病). They want to prove that internal fat damages the body's communication systems.

    The good news is that internal fat can be easily burned off through exercise or even by improving your diet. “If you want to be healthy, there is no shortcut. Exercise has to be an important part of your lifestyle.” Bell said.