阅读理解。
The rich and the poor
The world is divided (分为)into two main parts . The difference is that
one part is rich and the other is poor . In the poor part , a lot of people never get
enough to eat . In the rich part , a lot of people eat too much . In one part , child
starve (饿死)and in the other , a lot of people get fatter and fatter and have to
go on diets (吃特别饮食), or do special exercises in order to lose weigt.
The poorer countries have special problems . Sometimes the land is too
poor to grow anything on . The land can be improved (改善), but a lot of
things must be done first . The people must be educated an water must be
found.
But rich countries have problems , too . There are not always pleasant
places to live in . Sometimes the air is too dirty to breathe , and the rivers are
too dirty to swim in or to take water from . The roads are too crowded to drive
along . Large numbers of people do not have decent (象样的)housed to live
in . Some things will have to be done about these problems . The air and the
river will have to be cleaned , and more houses will have to be built.
Answer the questions :
1. What’s the difference between the two parts ?
___________________________
2. Do the poorer countries have many problems ? What for example ?
___________________________
3. Can anything be done about it ?
___________________________
4. What must be done, for example ?
___________________________
5. Can all this be done easily ?
___________________________
，电压表和电流表均为理想电表。若发电机向原线圈输入图乙所示的正弦交流电，图中Rt为NTC型热敏电阻（阻值随温度升高而变小），R1为定值电阻。下列说法中正确的是
，正弦交流电电压有效值
，即原线圈电压即电压表示数为
，选项A对。原副线圈电压与匝数成正比，，但原副线圈电流与匝数成反比，即变压器原、副线圈中的电流之比为
，选项B错。
时，电动势为0，即磁通量变化量为0，磁通量最大，此时线圈与磁场垂直，选项C错。热敏电阻温度升高时，电阻变小，副线圈总电阻变小，副线圈总电流即电流表示数变大，根据原副线圈电流比与匝数成反比，可判断原线圈电流变大，但电压是原线圈决定副线圈，所以电压表示数不变，选项D对。