问题 单项选择题

下列全控型器件中开关速度最快的是()。

A.GTO

B.GTR

C.MOSFET

D.IGBT

答案

参考答案:C

单项选择题

微波炉为什么能加热食物呢?让我们从水说起。水分子是由一个氧原子、两个氢原子构成的,氧原子对电子的吸引力很强,所以水分子中的电子比较集中在氧原子那一头,相应的氢原子那头就少一些。整体来看,水分子就一头带着正电,另一头带着负电。在化学上,这样的分子就被叫做“极性分子”。

在通常的水里,水分子是杂乱无章地排列的,正电负电冲哪个方向的都有。当水处在电场中的时候,正电的那头就会转向电场的负极,而带负电那头会转向电场的正极,这就所谓的“异性相吸,同性相斥”。

如果是一个静止的电场,水分子们排好队也就安静下来了;如果电场在不停地转,那么水分子就会跟着转,试图和电场保持一个方向的队型;如果电场转得很快,那么水分子们也就转得很快——摩擦生热,水的温度就升高了。

电磁波就相当于这样一种旋转的电场。用在微波炉上的电磁波每秒钟要转20几亿圈,水分子们以这样的速度跟着转,自然也就“浑身发热”,温度在短时间内就急剧升高了。一旦微波停止,旋转电场消失,水分子们也就重新恢复了杂乱无章的状态。在这个过程中,水分子本身并没有被微波改变。

不仅是水,其他极性分子也都可以被微波加热。通常的食物中都含有水和其他极性分子,所以在微波作用下可以被迅速加热。而非极性的分子,比如空气以及某些容器,就不会被加热。我们平常热完食物后觉得容器也热了,往往是被高温的食物给“烫”热的。

关于水分子,下列哪一项不正确?()

A.是由两个氢原子和一个氧原子构成 

B.属于极性分子 

C.其中的氧原子对电的吸引力强 

D.电场中的水分子杂乱无章

阅读理解

阅读理解

     A volcanic eruption in Iceland has sent ash across northern Europe. Airlines have stopped or changed

the flights across the Atlantic Ocean, leaving hundreds of passengers stuck in airports.

     Grimsvom is one of the largest and most active volcanoes in Europe. What makes Grimsvom different

is that it lies under a huge glacier(冰川) of ice up to 12 meters thick. The hot volcano heats up the ice

above it, which then forms a layer(层)of water between the glacier and the volcano.This layer of water

puts pressure on the volcano, keeping it stable. As the water flows out from under the glacier, the pressure lifts. The lava(岩浆) from the volcano then comes up to the surface. This is exactly what happened today.

     Now, airlines have to make changes to their flights so as not to fly through the clouds of volcanic ash.

According to KLM,one of Europe's biggest airlines, airplanes cannot go under the cloud or over it. Going

through the cloud can result in ash getting stuck in the airplane's engines, causing damage to the plane.

     The eruption has also caused problems for animals in Iceland. The volcano left ash and sharp glasslike

rocks all over the countryside. Farmers are keeping their animals inside to stop them from eating

ashcovered grass or the sharp object.

  

1.What makes Grimsvom different from other volcanoes?  

A.It is below ice.

B.It lies under the sea.

C.It is the largest volcano.

D.Its lava affects the airlines.  

2.What keeps Grimsvom still?  

A.The slow flow of water.

B.The low water temperature.  

C.The thick glacier.

D.The water pressure.  

3.Which of the following is the result of the volcanic eruption?

A.People stop traveling in Europe.

B.Airlines suffer from the loss of planes.  

C.It becomes dangerous for animals to eat outside.

D.Farmers have lost many of their animals.  

4.This text is most probably taken from________.

A.a research paper  

B.a newspaper report

C.a class presentation  

D.a geography textbook