题干

如图所示,一块长度为a、宽度为b、厚度为d的金属导体,当加有与侧面垂直的匀强磁场B,且通以图示方向的电流I时,用电压表测得导体上、下表面MN间电压为U.已知自由电子的电量为e.下列说法中正确的是(   )

A:M板比N板电势高

B:导体单位体积内自由电子数越多,电压表的示数越大

C:导体中自由电子定向移动的速度为 v=UBd

D:导体单位体积内的自由电子数为 BIeUb

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C,D

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    A 15-year-old student who invented a flashlight(手电筒) getting power from the holder's body heat is going home today from California with a big prize and a chance to do further research.

    Ann Makosinski was the only Canadian among the four winners at Google's international science competition. Thousands of young scientists from around the world took part in the competition.

    Winning the science and technology competition was "a surprise". Ann said, "I think it will have a great influence on my future."

    Ann thanked her family for encouraging her interest in science and said that her first toy was a box of transistors(晶体管).

    Ann's prize includes $ 25,000 and a "once in a-lifetime expericnce" from Google for her Hollow Flashlight, which has no moving parts or batteries.

    The idea for the invention came from seeing unwanted batteries and her friend's experience. When Ann visited a friend in the Philippines, she saw the friend couldn't study when it became dark because there was no electricity or light. She saw the need for a flashlight that has no batteries - Hollow Flashlight.

    In her project, Ann wrote "I made two flashlights that do not use any batteries or harmful materials. They do not create any noise and will always work. The flashlight needs at least a 5℃ temperature difference between the holder's body and the environment around to produce light."

    A video of Ann explaining how she creatcd the flashlight has been watched more than 1.4 million times on the Internet.

    Though Ann was successful, she has not made a decision about her career path. Ann hopes that she can find a way to join her love of film and science together.

    The four winners were chosen from 15 final competitors from eight countries. The competition attracted thousands of students in 120 countries.