问题 选择题

如图所示,A、B是一条电场线上的两点,若在A点释放一初速度为零的电子,电子仅受电场力作用,并沿电场线从A运动到B,其速度随时间变化的规律如图4所示.设A、B两点的电场强度分别为EA、EB,电势分别为φA、φB,则(  )

A.EA=EB

B.EA<EB

C.φAB

D.φA>φB

答案

由图可知:电子做初速度为零的匀加速直线运动.匀加速运动说明所受电场力恒定,因此为匀强电场,即EA=EB,电子由静止从A运动到B,所以电场力由A指向B,因此电场线方向从B指向A,沿电场线电势降低,即有φA<φB,故BCD错误,A正确.

故选A.

单项选择题
阅读理解

To us it seems so natural to put up an umbrella to keep the water off when it rains, but actually the umbrella was not invented as protection against the rain, Its first use was as a shade against the sun!

Nobody knows who first invented it, but the umbrella was used in very ancient times. Probably the first to use it were the Chinese in the 11th century BC.

We know that the umbrella was used in ancient Egypt and Babylon as a sunshade. And there was a strange thing connected with its use: it became a symbol of honor. In the Far East in ancient times, the umbrella was allowed to be used only by those in high office.

In Europe, the Greeks were the first to use the umbrella as a sunshade. And the umbrella was in commonly used in ancient Greece. But it is believed that the first persons in Europe to use the umbrella as protection against the rain were the ancient Romans.

During the Middle Ages, the use of the umbrella practically disappeared. Then it appeared again in Italy in the late sixteenth century. And again it was considered as a symbol of power. By 1680, the umbrella appeared in France and later in England.

By the eighteenth century, the umbrella was used against rain throughout most of Europe.

Umbrellas have not changed much in style during all this time, though they have become much lighter in weight. It wasn’t until the twentieth century that women’s umbrellas began to be made , in a whole variety of colors.

小题1:According to this passage, the umbrella was probably first invented in ancient_______.

A.China

B.Egypt

C.Greece

D.Rome小题2:Which of the following statements is not true about the umbrella?

A.No one exactly knows who the inventor of the umbrella was

B.The umbrella was first invented to be used as protection against the sun.

C.The umbrella changed much in style in the eighteenth century

D.In Europe, the Greeks were the first to use the umbrella as a sunshade.小题3:A strange feature of the umbrella’s use is that it was used as__________.

A.protection against rain

B.a symbol of honor and power

C.a shade against the sun

D.women’s decoration小题4:In Europe, the umbrella was first used against the rain_______________.

A.during the Middle Ages

B.in Rome

C.by the 18th

D.in Greece小题5:This passage talks mainly about_______________.

A.when and how the umbrella was invented

B.why the umbrella was so popular in Europe

C.the development of the umbrella

D.The history and use of the umbrella