问题 选择题

如图所示为一未知电路,现测得两个端点A、B之间的电阻为R,若在A、B之间加上电压U,测得通过电路的电流为I,则该未知电路的电功率一定是为(  )

A.UI

B.

U2
R

C.I2R

D.UI-I2R

答案

A、无论是黑箱中是一个纯电阻元件,还是非纯电阻元件,都能用P=UI求电功率.故A正确.

B、当黑箱中是一个纯电阻元件时,电功率可以用P=

U2
R
求,若是非纯电阻元件,欧姆定律不成立,没有这个公式.故B错误.

C、当黑箱中是一个纯电阻元件时,电功率可以用P=I2R求,若是非纯电阻元件,则电功率P>I2R.故C正确.

D、P=UI-I2R可以求其他的功率,不能用来求电功率.故D错误.

故选:A

阅读理解

Mother’s Day is a holiday for mothers. It is celebrated(庆祝)in the United States, England, India and some other countries. In a short time, it becomes widely celebrated. Mother’s day falls on the second Sunday in May. On that day, many people send gifts(礼物)of love to their mothers.

Those whose mothers are still living often wear a pink or red rose or carnation(康乃馨), while those whose mothers are dead wear a white one.

The idea of a day for mothers was first given by Miss Anna Jarvis of Philadelphia. As a result of her hard work, the celebration of the first American Mother’s Day was held in Philadelphia(费城) on May 10, 1908. Soon the holiday became popular all over the country and around the world.

In China, people do the same on the day for mothers. And, in some cities, people sometimes ask a song to be broadcast(广播) for his or her mother only. This might cost a little money for some of them, but, as it is said, “Love is invaluable.”

On May 8, 1999, just the day before the Mother’s Day that year, a Chinese mother was killed in criminal bombing(罪恶轰炸). Her name is Shao Yanhuan, a heroic mother. All the Chinese will remember her forever.

小题1: Mother’s Day         .

A.is a holiday for mothers

B.is celebrated in the United Stated only

C.falls on May 10 every year

D.was given by Miss Anna Jarvis小题2:What was the date of the Mother’s Day in 1999?

A.May 7.

B.May 8.

C.May 9.

D.May 10小题3:On Mother’s Day, people       .

A.send some flowers to their mothers.

B.wear a pink or red rose or carnation

C.wear a white flower

D.send gifts of love to their mothers小题4: the underlined word “invaluable” means “   ”.

A.美好的

B.无价的

C.永恒的

D.无私的小题5:Maybe you’ve known the criminal bombing, which of the following ideas do you agree to?

A.Shao Yunhuan is a heroic mother.

B.A Chinese mother is the same as an American mother.

C.The bombing cannot get away with the crime (难逃罪责).

D.All of the above

填空题