问题 选择题

关于电流与电压的说法,正确的是(  )

A.电路中有电流时,就一定有电压,因为电压是产生电流的原因

B.电路中只要有电压,就一定会有电流

C.导体内有大量的自由电荷,只要构成通路,导体中就会有电流

D.以上说法都不正确

答案

A 电荷的定向移动形成电流,电路中有电流时,就一定有电压,因为电压是产生电流的原因.此说法正确;

B 要得到电流,必须满足两个条件:有电压,通路状态.电路状态不明确,有无电流无法判断.此说法错误;

C 要得到电流,必须满足两个条件:有电压,通路状态.虽然是通路,有无电压不确定,有无电流无法判断.此说法错误;

D 因为选项A正确,此说法错误.

故选A.

阅读理解

阅读理解。

     We have three players today. Each of them will tell us about himself. Let's welcome them.

     Earth(地球):I am the third planet (行星) from the sun. I have one moon (卫星). I have many

mountains. I give a home to lots of plants and animals. I have much water on me.

     Mars(火星):I am the fourth planet from the sun. I am a planet with high mountains. I have two

small moons. People call them Phobos and Deimos. I have a very high mountain. People call it

Olympus Mons.

     Mercury(水星):Well, I am the nearest planet to the sun. I am also the second smallest planet.

I don't have any moons. I am the fastest (最快的) of all the planets when I go around the sun. It

only takes me 88 earth days (One earth day is 24 hours). 

1. The underlined word "them" in the first paragraph refers to (指的是) _______.

A. the three planets        

B. moons

C. 88 earth days            

D. mountains

2. Earth is different from other planets in that_______.

A. it has a moon          

B. it has much water on it

C. it's the only planet of the sun

D. it's the only planet with mountains

3. According to the passage, where can we see the high mountain -Olympus Mons?

A. On the sun.          

B. On Mars.

C. On the earth.        

D. On Mercury.

4. How long does it take Mercury to go around the sun?

A. 24 hours.        

B. 88 hours.

C. 408 hours.        

D. 2112 hours.

5. What can we NOT learn from the passage?

A. Mercury has no moons.

B. Mercury is very near to the sun.

C. Mars goes around the earth.

D. Phobos and Deimos are moons of Mars.

单项选择题 A1/A2型题