问题 问答题

一台直流电动机,测得它的线圈电阻为2Ω,接在输出电压为200V的直流电源两极间,正常工作时测得通过电动机线圈的电流为10A.

(1)求它正常工作时的效率(即转化为机械能的功率与消耗电功率的比值).

(2)通过计算分析,说明为什么当电动机转子被卡住后通电很容易烧毁电动机?

答案

(1)90%        (2)热功率P’ =20000W.线圈生热过多,会烧毁电动机.

(1)正常工作时消耗的电功率为P=UI,生热功率P=I2R,因此效率,代入数据解得η=90%

(2)如果线圈停止转动,则通过线圈的电流I’=U/R,这时消耗的电能全部转化为内能而生热,其生热功率为P’=UI’=U2/R,代入数据解得P’=(2002/2)W=20000W.线圈生热过多,会烧毁电动机.

阅读理解

There is a story of a country where the rate of inflation(通货膨胀) is so high that clever people pay for a taxi ride before, instead of after the trip. The story may not be true, but inflation was almost that serious in Germany from July 1920 until December 1923. Prices went up so fast that by the end of 1923 they were 50 billion percent higher-arise of almost 2,500% a month.

There was so much paper money, and it had so little value, that people carried bags full of money around to pay for things. One woman tells the story of standing outside a shop with a basket full of 500,000 marks(马克) notes. She wanted to buy just one piece of meat, and she hoped she had enough money. But when she was looking, a thief robbed her. He didn't take her money, he threw it away and took the basket instead.

At first workers demanded to be paid every day, but as the situation became worse, they had to be paid twice a day. But they had to run out and spend the money at once, or it would lose its value. People bought anything that was for sale, but food was almost impossible to find. Farm workers refused to take money. They wanted to be paid in potatoes instead.

New policies ended the inflation in 1923, when the government introduced a new money, but about half of the German people lost everything in those three and a half years.

小题1:According to the text, in Germany the prices in 1920 were ______.

A.the lowest in history

B.the highest in history

C.higher than those in 1923

D.lower than those in 1923小题2:The thief stole the basket instead of the money in it because he thought ______.

A.the basket was what he needed most

B.the money was of no value

C.the basket was more valuable than the money

D.he couldn't buy a piece of meat with the money小题3:The farm workers demanded to be paid in potatoes because they believed ______.

A.the potato was valuable

B.the money might lose its value

C.the potato was too expensive

D.the money could not buy potatoes

问答题 简答题