问题 问答题

如图所示A为一稳压管,它的作用是保证DE两端的电压UDE恒定不变,当流过稳压管有电流在5mA和40mA之间时,UDE就稳定在10V上,R2为一可变电阻,它的最小值为500Ω,最大值为无穷大(即断路).设电源电压U=20V,则当R2变化时,R1的值应在______范围内,才能使流入稳压管的电流最大不超过40mA,最小不低于5mA?

答案

根据题意可知:U=20V,U2=10V

所以U1=U-U2=10V

当负载取最小值时稳压管中的电流达到最大,则电阻R1最小,根据欧姆定律,有:

R1=

U1
I
=
10V
40mA
=250Ω;

当负载取最大值时稳压管中的电流达到最小,则电阻R1最大,根据欧姆定律,有

R1=

U1
I′
=
10V
5mA
=2000Ω

故答案为:250Ω<R1<2000Ω.

阅读理解

阅读理解。

     Around the world more and more people are taking part in dangerous sports and activities. Of course,

there have always been people who have looked for adventure-those who have climbed the highest

mountains, explored unknown parts of the world or sailed in small boats across the greatest oceans.

Now, however, there are people who seek an immediate thrill from a risky activity which may only last a

few minutes or even seconds.

     I would consider bungee-jumping to be a good example of such an activity. You jump from a high

place (perhaps a bridge or a hot-air balloon)200 meters above the ground with an elastic(有弹性的)

rope tied to your ankles. You fall at up to 150 kilometers. It is estimated that 2 million people around the

world have now tried bungee-jumping. Other activities which most people would say are as risky as

bungee-jumping involve jumping from tall buildings and diving into the sea from the top of high cliffs.

     Why do people take part in such activities as these? Some psychologists suggest that it is because life

in modern societies has become safe and boring. Not very long ago people's lives were constantly under

threat. They had to go out and hunt for food, diseases could not easily be cured, and life was a

continuous battle for survival.

     Nowadays, according to many people, life offers little excitement. They live and work in comparatively safe environments;they buy food in shops;and there are doctors and hospitals to look after them if they become ill. The answer for some of these people is to seek danger in activities such as bungee-jumping.

1. A suitable title for the text is "        ".

A. Dangerous Sports:What and Why?

B. The Boredom of Modern Life

C. Bungee-jumping:Is It Really Dangerous?

D. The Need for Excitement

2. More and more people today are         .

A. trying activities such as bungee-jumping

B. climbing the highest mountains

C. coming close to death in sports

D. looking for adventures such as exploring unknown places

3. People take part in dangerous sports nowadays probably because           .

A. they have a lot of free time

B. they can go to hospital when they are injured

C. their lives are short of excitement

D. they no longer need to hunt for food

4. What does the word "thrill" in the first paragraph mean?

A. a strong feeling of excitement

B. a feeling of unhappiness

C. a feeling of feeling upset

D. a feeling of being frightened

填空题