题干

小明在探究“影响电磁铁磁性强弱的因素”的实验中,通电螺线管的磁感线如图所示,请在图中标出电源的正负极.

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解:磁感线总是从磁体的N极出发、回到S极,所以螺线管的左端为S极、右端为N极;右手握住螺线管,大拇指指向螺线管的右端(N极),根据四指的弯向可以标出螺线管的电流方向,如下图所示:

同类题1

    There is a famous English saying "You are what you eat", and it means what you put in your mouth will become part of you. Many experts(专家) use it to tell people it's important to have a good eating habit. For example, too much sweet food makes a person overweight. To avoid being fat, kids are told to keep off sweet food. However, a team of American scientists have discovered something new about eating sweet food.
"When we say some people 'sweet', we mean they are kind and friendly to us. Perhaps this word can help us learn more about people's feelings and acts." said Brian Meier, a scientist from the study team. "Taste is something we experience every day. What we have done is to find whether we can know someone's character and behavior depending on their taste."
    In one of their experiments, college students were asked questions about their character- whether, for example, they were soft-hearted or not. Then they were asked to make a list of their favorite food. According to their answers, the scientists found that the students who liked sweet  food were nicer.
    In their another experiment, the scientists found that people who preferred sweet food such as cookies and chocolate were more willing to help other people in need, and those who ate non-sweet food such as chips weren't. For example, people with a sweet tooth were more likely to volunteer to clean up their city, visit sick children in the hospital and so on.
    The result of the experiments is that people's helpfulness or being nice has something to do with their taste for sweet food. So eating sweet food is a useful way to make people nice and helpful. Next time, try some ice-cream.