题干

如图所示,一导线弯成半径为a的半圆形闭合回路。虚线MN右侧有磁感应强度为B的匀强磁场。方向垂直于回路所在的平面。回路以速度v向右匀速进入磁场,直径CD始络与MN垂直。从D点到达边界开始到C点进入磁场为止,下列结论错误的是( )

A:感应电流方向不变

B:CD段直线始终不受安培力

C:感应电动势最大值E=Bav

D:感应电动势平均值

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答案(点此获取答案解析)

B

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       In Los Angeles, drivers spend sixty­one hours every year stuck in traffic. These drivers know too well how bad the traffic can be. “There're too many cars, and you can't move around a lot.”

       Professor Cyrus Shahabi also knows about traffic jams. He lives more than 65 kilometers from his office at the University of Southern     California in Los Angeles. He is always late even with the help of a navigation (导航) system.

       He decided to develop a program called ClearPath for that. He says his program uses historical data to predict traffic conditions even before the driver leaves the house.

   “What's unique is that we use a lot of data that's currently become available including traffic data, weather data, and we analyze that so that we can predict what's going to happen in front of you when you leave home.”

      Professor Shahabi says his system does more than just respond to current traffic conditions. With ClearPath, he says, a driver can decide what time he wants to leave, and ClearPath will give the fastest route. It looks at the entire road network, including surface streets as well as highways, before the driver hits the road. Professor Shahahi hopes to have ClearPath available nationwide and overseas once they can collect traffic data from other cities.

   “I always thought that Los Angeles had the worst traffic, but now I know that Shanghai, Beijing, Seoul, Tokyo, believe it or not, Singapore, Hong Kong certainly are examples that can immediately use this.”

       Professor Shahabi hopes to share this new technology with companies that already have navigation systems, such as Google and Apple.