问题 计算题

质量m=2.0×10-4kg、电荷量q=1.0×10-6C的带正电微粒静止在空间范围足够大的匀强电场中,电场强度大小为E1。在t=0时刻,电场强度突然增加到E2=4.0×103N/C,场强方向保持不变。到t=0.20s时刻再把电场方向改为水平向右,场强大小保持不变。取g=10m/s2。求:

(1)原来电场强度E1的大小?

(2)t=0.20s时刻带电微粒的速度大小?

(3)带电微粒运动速度水平向右时刻的动能?

答案

解:(1)当场强为E1的时候,带正电微粒静止,所以mg=E1q

所以

(2)当场强为E2的时候,带正电微粒由静止开始向上做匀加速直线运动,设0.20s后的速度为v,由动量定理有 (E2q-mg)t=mv,解得:v=2m/s

(3)把电场E2改为水平向右后,带电微粒在竖直方向做匀减速运动,设带电微粒速度达到水平向右所用时间为t1,则0-v1=-gt1,解得:t1=0.20s

设带电微粒在水平方向电场中的加速度为a2,根据牛顿第二定律qE2=ma2,解得:a2=20m/s2

设此时带电微粒的水平速度为v2,v2=a2t1,解得:v2=4.0m/s

设带电微粒的动能为Ek,Ek==1.6×10-3J

阅读理解

阅读短文,根据要求完成下面各小题。

     In Britain you are allowed to drive a car when you are seventeen. You have to get a special two-year driving

license before you can start. When you are learning, someone with a full license always has to be in the car with

you because you aren't allowed to drive the car on the road alone. You don't have to go to a driving school …a

friend can teach you. 3. The person with you isn't allowed to take the money for the lesson unless he's got a

teacher's license.

     Before you are allowed to have a full license, you have to take a driving test. You can take a test in your

own car, but it has to be fit for the road. In the test you have to drive round for about half an hour and then

answer a few questions. If you don't pass the test, you are allowed to take it again a few weeks later if you

want to. In 1970 a woman passed her fortieth test after 212 driving lessons. When you've passed your test, you

don't have to take it again, and you are allowed to go on driving as long as you like. 4. Britain's oldest driver

was a man who drove in 1974 at the age of 100.

     Before 1904 everyone was allowed to drive, even children. Then from 1904 car drivers had to have a license.

But they didn't have to take a test until 1935. On the early days of car driving, before 1878, cars weren't allowed

to go faster than four miles an hour, and someone had to lead the car with a red flag.

(一)根据短文内容简要回答问题。

1.Is a fifteen-year-old boy allowed to drive alone on the road?

___________________________________________________________

2.What do you have to do during the driving test?

___________________________________________________________

(二)将短文中划线的两个句子译成汉语。

3. ___________________________________________________________

4. ___________________________________________________________

(三)请给短文拟一个适当的标题。

5.___________________________________________________________

单项选择题