题干

如图所示的装置,闭合S,滑片P向左移动时,弹簧测力计示数增大,请在电磁铁周围作两条磁感线,并标出电源的正负极(用电路符号表示).

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答案(点此获取答案解析)

解:当滑片P向左移动时,滑动变阻器接入的电阻变小,电路中的电流变大,电磁铁磁性变强,若弹簧测力计示数增大,则由此可以判断,电磁铁的上端为S极;根据右手螺旋定则知,通电螺线管的上端是S极,下端是N极,在磁体外部,磁感线是N极发出,回到S极,电流由上端流入,下端流出,故电源的左端为正极,右端为负极,如下图所示:

同类题4

阅读下列短文,从短文后各题所给的A、B、C、D四个选项中,选出最佳选项。

    Every living thing on Earth is either a plant or an animal, right? Wrong! Many organisms(有机体)in our world are neither. Some of them float through the air, and some lie hidden in the ground. They do not need sunlight to survive, and if the weather becomes too cold, they can become inactive until conditions improve.

    What are these? They are called fungi(菌类), and you see them almost every day. Many kinds of fungi seem disgusting, such as the green black things that appear on the food which are left too long in the refrigerator. But other kinds are not so bad. For example, the yeast that is used to make bread and mushrooms (蘑菇) are both fungi.

    What makes fungi different from plants and animals? An animal can move around by itself. Fungi get around too, but they don't have feet, wings, or tails. They have to wait for wind or some other outside force to move them. Plants are different from fungi because they have chlorophyll(叶绿素). It helps them make food from the energy in sunlight. Fungi do not have chlorophyll.

    Some fungi are bad, while others are not. Some fungi cause diseases, such as athlete's foot, which makes feet uncomfortable. Other kinds of fungi can be used to make medicines, which have saved many lives.

    Fungi clean up more than wounds, though. They are the world's first recyclers. Without fungi, our world would be a mess. Since fungi cannot make their own food as plants do, they must get their food elsewhere. Many fungi get their food from dead plants. They break them down and turn them into soil. The fungi get a meal, and the world gets a housecleaning.