如右图所示，MN右侧有一正三角形匀强磁场区域（边缘磁场忽略不计），上边界与MN垂直。现有一与磁场边界完全相同的三角形导体框，从MN左侧垂直于MN匀速向右运动。导体框穿过磁场过程中所受安培力F的大小随时间变化的图象以及感应电流i随时间变化的图象正确的是（取逆时针电流为正）
BC
题目分析：线框进入磁场过程，磁通量增大，根据楞次定律，感应电流沿逆时针方向为正，出磁场过程磁通量减少，感应电流为顺时针方向为负。线框匀速进入磁场设速度为v，时间为t，则切割磁感线的有效长度是磁场中线框首尾连接的线段即
，如下图，但有效长度与速度不垂直，分解后为
，所以产生的感应电流
与时间成正比，随时间均匀增大，安培力
与时间的平方成正比，所以选项A错。当线框与磁场区域完全重合时，感应电流最大，安培力最大，此后为出磁场过程，与进磁场过程对称，所以选项BC对D错。
| 阅读理解。 | ||
Paul also tipped Spain to beat Netherlands in the final. "We have had a lot of offers for Paul but he will definitely be staying with us and returning to his usual job-making children smile," Sea Life spokeswoman Tanja Munzig in Oberhausen said after presenting Paul with the World Cup replica. There's no rational (合理的) reasons why Paul always got it right. Bettors around the world made small fortunes based on Paul's guess, said Graham Sharpe, media relations director at William Hill in London. "I've seen a lot of things in my lifetime but this is the first time I've ever seen people making their picks based on what an octopus tells them. We had people coming in saying they didn't know how to place a bet but heard about this German octopus and wanted to bet with him. It's funny. But he kept getting it right." said Sharpe. Sharpe said that anyone who had placed a 10-pound accumulator bet (累计下注) on Paul's prediction from the start of the World Cup would have won 3,000 pound ($ 4,500) by the end of the tournament. Paul's home at Sea Life Aquarium (水族馆) in Oberhausen has been crowded with visitors and media from across Europe. Many networks broadcast his predicting live. Many networks broadcast his picks live. Hundreds were on hand to watch the World Cup replica lowered into his tank on Monday, July 12, 2010. | ||
| 1. Paul was rewarded a replica of the World Cup because he ____. | ||
| A. will celebrate his birthday soon B. predicted the results of matches accurately C. helped beat Netherlands in the final D. helped many fans earn much money | ||
| 2. If a fan had placed a 200-pound accumulator bet on Paul's prediction, he would have won ____ during the tournament. | ||
| A. $ 30,000 B. $ 45,000 C. $ 60,000 D. $ 90,000 | ||
| 3. The underlined word "tipped" in the 2nd paragraph probably means ____. | ||
| A. ordered B. announced C. predicted D. helped | ||
| 4. From what Sharpe said we can learn that ____. | ||
| A. it's amazing for Paul to make more exact prediction than people B. octopus like Paul may all have the ability to predict exactly C. it's common for people to believe in sea life for prediction D. we'd better not bet with animals when we have trouble making our picks |
