问题 问答题

小明同学认为:电风扇工作时,电能转化为叶片转动的机械能和线圈发热、摩擦等产生的内能.其中转化为机械能所做的功是______,转化为内能所做的功是______.(选填“有用功”或“额外功”)为了粗略探测电风扇正常工作的电能转化效率,小明用一个铭牌上标有额定电压为U,但其它字迹不清的小型直流电风扇来进行实验,实验设计如下:

(1)按如图所示连接好电路(图中为电风扇),闭合开关,叶片转动,移动变阻器滑片至某一位置时,叶片刚好不转动,读出此时电压表和电流表的示数分别为U0、I0,则线圈导体的电阻为______;

(2)向______ (选填“左”或“右”)移动变阻器滑片,使电压表示数为额定电压U,读出此时电流表的示数为I;

(3)若忽略摩擦,只考虑线圈发热损耗,请你写出电风扇正常工作时电能转化效率η的表达式 (用测出的已知量表示).

答案

电动机转化为机械能所做的功是有用功,转化为内能所做的功是额外功.

(1)当叶片刚好不转动时,根据欧姆定律得,线圈导体的电阻R=

U0
I0

(2)电动机不转动,说明电动机的电压低于额定电压,可向左移动滑动变阻器滑片,使变阻器分担的电压减小,使电动机的电压增大,达到额定电压.

(3)电风扇正常工作时,输入功率为P=UI,输出功率为P=UI-I2R

则效率为η=

P
P
=
UI-I2R
UI
=
UI0-IU0
UI0

故答案为:

有用功,额外功   

(1)

U0
I0
; (2)左;(3)
UI0-IU0
UI0

不定项选择题
阅读理解

Four people in England back in 1953, stared at Photo 51,It wasn’t much—a picture showing a black X. But three of these people won the Nobel Prize for figuring out what the photo really showed –the shape of DNA The discovery brought fame and fortune to scientists James Watson, Francis Crick, and Maurice Wilkins. The fourth, the one who actually made the picture, was left out.

Her name was Rosalind Franklin.”She should have been up there,” says historian Mary Bowden.” If her photos hadn’t been there, the others couldn’t have come up with the structure.” One reason Franklin was missing was that she had died of cancer four years before the Nobel decision. But now scholars doubt that Franklin was not only robbed of her life by disease but robbed of credit by her competitors

At Cambridge University in the 1950s, Watson and Click tried to make models by cutting up shapes of DNA’s parts and then putting them together. In the meantime, at King’s College in London, Franklin and Wilkins shone X-rays at the molecule(分子). The rays produced patterns reflection the shape.

But Wilkins and Franklin’s relationship was a lot rockier than the celebrated teamwork of Watson and Crick, Wilkins thought Franklin was hired to be his assistant .But the college actually employed her to take over the DNA project.

What she did was produce X-ray pictures that told Watson and Crick that one of their early models was inside out. And she was not shy about saying so. That angered Watson, who attacked her in return, “Mere inspection suggested that she would not easily bend. Clearly she had to to go or be put in her place.”

As Franklin’s competitors, Wilkins, Watson  and Crick had much to gain by cutting her out of the little group of researchers, says historian Pnina Abir-Am. In 1962 at the Nobel Prize awarding ceremony, Wilkins thanked 13 colleagues by name before he mentioned Franklin, Watson wrote his book laughing at her. Crick wrote in 1974 that  “Franklin was only two steps away  from the solution.”

No, Franklin was the solution. “She contributed more than any other player to solving the structure of  DNA . She must be considered a co-discoverer,” Abir-Am says. This was backed up by Aaron Klug, who worked with Franklin and later won a Nobel Prize himself. Once described as the  “Dark Lady of DNA”, Franklin is finally coming into the light.

小题1:What is the text mainly about?

A. The disagreements among DNA researchers.

B. The unfair treatment of Franklin.

C. The process of discovering DNA.

D. The race between two teams of scientists.

小题2:Watson was angry with Franklin because she     .

A.took the lead in the competition

B.kept her results from him

C.proved some of his findings wrong

D.shared her data with other scientists小题3:Why is Franklin described as  “Dark Lady of DNA”?

A. She developed pictures in dark labs.

B. She discovered the  black X-the shape of DNA.

C. Her name was forgotten after her death.

D. Her contribution was unknown to the public.

小题4:What is the writer’s attitude toward Wilkins, Watson and Crick?

A.Disapproving.

B.Respectful.

C. Admiring.

D.Doubtful.