题干

下列关于电与磁的说法正确的是(   )

A:磁场中某点的磁场方向是由放在该点的小磁针决定的

B:改变通电螺线管中电流的方向,可以改变通电螺线管的N、S极

C:发电机是利用电磁感应原理制成的,工作时将电能转化为机械能

D:只改变直流电动机的电流方向,直流电动机内线圈的转向不会发生改变

上一题 下一题 0.0难度 选择题 更新时间:2018-02-09 08:05:59

答案(点此获取答案解析)

B

同类题5

 Are you a happy person most of the time? Or do you easily get angry sometimes? Everyone has a different temperament (脾气). It is usually believed that both genes (基因) and environment may affect people's temperaments in different ways. But now scientists have found that the season of birth also plays a role.

 Scientists from Budapest, Hungary, studied 400 university students. In the study, the students needed to finish a questionnaire (问卷). The purpose of it was to find out which of four kinds of temperaments they most had. From the questionnaire, scientists found that the students always answered something like "My mood often changes for no reason" and "I love to deal with new projects, even if it is risky". These answers were then matched with their birthdays.

  They discovered that people born in summer easily change between sad and happy moods compared to people born in winter. Those who always feel positive are mostly born in spring and summer.

The study also found that those born in autumn less probably had a mood of depression which may easily drive them to cry, while those born in winter were not easier to be angry.

 Scientists said that this was probably because the seasons had an influence on certain chemicals in the human body. And the chemicals are important to control people's moods even in adult life.

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"It seems that when you are born may increase or decrease your chance of developing certain mood disorders (紊乱)," lead scientist Xenia Gonda told The Telegraph.