问题 多选题

在粗糙的斜面上固定一点电荷 Q,在 M 点无初速度的释放带有恒定电荷的小物块,小物块在 Q 的电场中沿斜面运动到 N 点静止.则从 M 到 N 的过程中(  )

A.小物块所受的电场力减小

B.小物块的电势能可能增加

C.小物块电势能变化量的大小一定小于克服摩擦力做的功

D.M 点的电势一定高于 N 点的电势

答案

物块在运动过程中所受摩擦力大小不变,假设物块受库仑引力,则物块将做加速度逐渐增大的加速运动,不会停止,因此物块受到的库仑力为斥力.

A、由于物块与电荷Q距离增大,根据F=

kq1q2
r2
可知,电场力减小,故A正确;

B、物块下滑中电场力做正功,电势能减小,故B错误;

C、由于物块初末速度都为零,根据功能关系可知,重力势能和电势能的减小之和等于克服摩擦力做的功,故C正确.

D、现在只能判断电荷Q和物块带电电性相反,不知Q带电的正负,无法判断M、N两点电势高低,故D错误.

故选AC.

问答题 案例分析题
阅读理解

阅读理解

     It was a Sunday and the heavy storm had lasted all night.The morning after the storm,though,was

beautiful:blue skies,warm air and a calm,inviting sea touching the shore gently.

     My father realised it was a good day for fishing and invited my sister and me to go with him.I was

only 14 and fishing had never been my thing,but I decided to go all the same.I'm_so_glad_I_did.

     On the road to the harbour we could see the terrible destruction on the coast,but the harbour itself

was in fairly good shape.After all,it was protected by the arms of a bay that had only one tiny channel

to the sea.As we got on board,we noticed two big humps(脊背) in the distance.

     On approaching them,we saw it was a mother whale with her baby.We couldn't believe it-there aren't

any whales along the coast here.The storm must have driven them across the ocean into the bay,in which

the still water was so badly polluted that nothing could survive.

     The little baby whale-actually as big as our boat-was obviously stuck and could not move.The mother

dived under the water and came up suddenly,making big whirlpools(漩涡) and waves."She's trying to

help her baby,but on the wrong side,"my father said.At this point,my father moved our boat in a semicircle to the other side and,heading the boat towards the baby whale,pushed it gently.With our several gentle

pushes the big hump turned over and disappeared under water.Then it swam up right beside its mum.

They struggled in their desperate attempts to escape but missed the exit and started heading in the wrong

direction.We hurried up to the whales and tried to lead them towards the bay channel.Slowly,they let us

lead them,sometimes rising from the water right beside us to breathe-and to give us a trusting look with

those huge eyes.Once they hit their first part of clean water flowing straight from the sea,the mum gave

us a wave with her tail and off they swam into the distance.

     In the excitement it had felt like only a few minutes,but we had been with those wonderful animals for

almost an hour and a half.That was the simple and lasting beauty of the day.Nearly four decades later,I

still look back fondly to that golden day at sea.

1. The author says "I'm so glad I did."(in Para. 2)because_____  .

A. he witnessed the whole process of fishing

B. he enjoyed the beauty of the calm sea

C. he experienced the rescue of the whales

D. he spent the weekend with his family

2. The harbour survived the storm owing to ________.

A. the shape of the harbour

B. the arms of the bay

C. the still water in the channel

D. the long coast line

3. The mother whale failed to help her baby because ________.

A. she had stayed in the polluted water for too long

B. the whirlpools she had made were not big enough

C. she had no other whales around to turn to for help

D. the waves pushed her baby in the wrong direction

4. What is the theme of the story?

A. Saving lives brings people a sense of happiness.

B. Fishing provides excitement for children.

C. It's necessary to live in harmony with animals.

D. It's vital to protect the environment.