问题 选择题

如图所示,ab是一个可以绕垂直于纸面的轴O转动的闭合矩形导体线圈,当滑动变阻器R的滑片P自右向左滑动的过程中,线圈ab将(  )

A.发生转动,但电源的极性不明,无法确定转动方向

B.顺时针转动

C.逆时针转动

D.静止不动

答案

当滑动变阻器的滑片P自右向左滑动时,引起线圈电流减小,通电螺旋管穿过线框的磁场强度减小,引起闭合矩形线框的磁通量减小,感应电流的磁场总是阻碍引起感应电流的磁通量的变化.

线框所在位置的磁场方向大致是水平的,要阻碍引起感应电流的磁通量的减小,所以线框逆时针转动.

故选:C.

单项选择题
阅读理解

Nearly all our food comes from the soil. Some of us eat meat, of course, but animals live on plants. If there were no plants, we would have no animals and no meat. So the soil is very necessary for life. The top of the ground is usually covered with grass or other plants. There may be dead leaves and dead plants on the grass. The waste matter (粪) from animals also falls on it.

All soil needs food. If we do not give it any, the plants will be weak. Farmers found that animals’ waste is the best food for the soil, but chemical fertilizers (化肥) are also very useful. The same crop (庄稼) should not be grown in the same place every year; it’s better to have a different crop. A change of crop and the use of a good fertilizer will keep the land good.

小题1:What would happen first without plants?

A.People would have more crops.

B.Animals would die.

C.There would be more food.

D.There would be less soil.小题2:What does the writer tell us in the first paragraph (段落)?

A.Leaves, plants, grass and waste live on each other.

B.There would be no animals without meat.

C.There would be no plants without animals.

D.People, animals, plants and soil live on each other.小题3:Which is the best food for soil in the field?

A.Chemical fertilizers

B.Plants and grass.

C.Animals’ waste.

D.Different crops.小题4:Why do we often grow a different crop in the same place every year?

A.Because we can keep the soil rich this way.

B.Because we don’t have enough land.

C.Because we want to have a different food.

D.Because we do this as a habit.小题5:What is the best title for the passage?

A.Soil and Waste

B.Plants and Animals

C.Food and Waste

D.Soil and Food