如图1所示，一个匝数n=100的圆形线圈，面积S₁=0.5m²，电阻r=1Ω．在线圈中存在面积S₂=0.4m²、垂直线圈平面（指向纸外）的匀强磁场区域，磁感应强度B随时间t变化的关系如图2所示．将其两端a、b与一个R=2Ω的电阻相连接．试求：

【查阅资料】过氧化钠(Na₂O₂)是一种淡黄色固体，能与 CO₂、H₂O发生反应：2Na₂O₂＋2CO₂＝2Na₂CO₃＋O₂，2Na₂O₂＋2H₂O＝4NaOH＋O₂↑。

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    Reading can be a social activity. Think of the people who belong to book groups. They choose books to read and then meet to discuss them. Now the website BookCrossing.com turns the page on the traditional idea of a book group.

    Members go on the site and register the books they own and would like to share. BookCrossing provides an identification number to stick inside the book. Then the person leaves it in a public place, hoping that the book will have an adventure, traveling far and wide with each new reader who finds it.

    Bruce Pederson, the managing director of BookCrossing, says, “The two things that change your life are the people you meet and books you read. BookCrossing combines both.”

    Members leave books on park benches and buses, in train stations and coffee shops. Whoever finds their book will go to the site and record where they found it.

    People who find a book can also leave a journal entry describing what they thought of it. E-mails are then sent to the BookCrossers to keep them updated about where their books have been found. Bruce Peterson says the idea is for people not to be selfish by keeping a book to gather dust on a shelf at home.

    BookCrossing is part of a trend among people who want to get back to the “real” and not the virtual(虚拟). The site now has more than one million members in more than one hundred thirty-five countries.