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如图所示,水平放置的平行金属导轨,相距l=0.50m,左端接一电阻R=0.20Ω,磁感应强度B=0.40T的匀强磁场方向垂直于导轨平面,导体棒ab垂直放在导轨上,并能无摩擦地沿导轨滑动,导轨和导体棒的电阻均可忽略不计,当ab以v=4.0m/s的速度水平向右匀速滑动时,求:

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         Who do you think was the most important woman of the past 100 years?

         Jane Addams(1860—1935)

         Addams helped the poor and worked for peace. She created shelters, education opportunities and services for people in need. In 1931, Addams became the first American woman to win the Nobel Peace Prize.

         Rachel Carson(1907—1964)

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         Angela Merkel(1954—)

         In 2005, Germans chose Angela Merkel as their first woman head of the country. She had been a scientist in the past. As Germany's leader, she has had an effect on the whole world.

         Sandra Day O'Connor(1930—)

         When Sandra Day O'Connor finished her class at Stanford Law School in 1952, she could not find work because she was a woman. However, she became the first woman to join the US Supreme Court(最高法院) in 1981 after years of hard work.

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          Marie Curie(1867—1934)

         Polish­born scientist Marie Curie discovered that some types of metal give off energy called radiation(辐射能). Her research led to new medical treatments and arms. She received the Nobel Prize in Physics in 1903 and in Chemistry in 1911.