如图所示，平行金属导轨MN和PQ与水平面成θ角，导轨两端分别与两个阻值均为R的固定电阻相连，匀强磁场垂直穿过导轨平面。质量为m、电阻为R/2的导体棒以一定的初速度沿导轨向上滑动，在滑动过程中导体棒与金属导轨始终垂直并接触良好。已知t1时刻导体棒上滑的速度为v1，此时导轨两端电阻R消耗的电功率均为P；t2时刻导体棒上滑的速度为v2，忽略平行金属导轨MN和PQ的电阻且不计空气阻力。则
A．t1时刻导体棒受到的安培力的大小为
B．t1时刻导体棒克服安培力做功的功率为4P
C．t1~t2这段时间内导体棒克服安培力做的功为
D．t1～t2这段时间内导体棒克服安培力做的功为4p（t2-t1）
AB
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| It was a holiday weekend. The police officers were sitting in a hot room __1__ instructions from their captain. One of these officers __2_ Ed Williams. He and ten other officers were on special __3__because they knew that over the weekend two weeks __4__over 400 people had died from accidents __5__by drivers. Over 4,000 people had received serious injuries, all caused by drunken drivers. The __6__ would try their best to prevent __7__accidents before they happened. Joe Forest was __8__himself at a family party. It was getting late and Joe told his sister that he was __9__. Joe's sister asked him to stay and wait a few hours __10__he drove. "Don't worry. I'm going to be fine. I'm going to drive slowly. I only had a few __11__,"Joe said. Officer Williams was at his post __12_ the passing cars. A green car was coming, __13_ from left to right. Officer Williams stopped __14_ and told Joe to get out. He asked Joe to walk __15_ the white line. Joe took the test. Officer Williams told Joe that he must appear in court the next week and he __16_ drive his car home. Joe called his sister to come and drive him __17_ . That was Joe's first offense (违法). He appeared in court and received a ＄400 fine. He was not __18_ to drive for sixty days. Other drivers were __19_ , for Joe hadn't killed them. But what about the future, was Joe going to stop __20_ ? | |||
| ( )1. A. giving ( )2. A. knew ( )3. A. duty ( )4. A. passed ( )5. A. met ( )6. A. captains ( )7. A. these ( )8. A. enjoying ( )9. A. tired ( )10. A. before ( )11. A. glasses ( )12. A. watching ( )13. A. weaving ( )14. A. the man ( )15. A. beside ( )16. A. had to ( )17. A. out ( )18. A. allowed ( )19. A. glad ( )20. A. enjoying himself | B. receiving B. saw B. advice B. were enough B. caused B. drivers B. such B. helping B. drunken B. when B. minutes B. waiting B. looking B. the car B. in B. mustn't B. away B. promised B. unlucky B. breaking the law | C. accepting C. were C. visit C. later C. killed C. passengers C. above C. drinking C. leaving C. unless C. drinks C. serving C. broken C. driving C. onto C. must C. at once C. able C. lucky C. drinking and driving | D. learning D. was D. time D. earlier D. discovered D. officers D. more D. eating D. back D. while D. miles D. searching D. driven D. working D. along D. didn't have to D. home D. willing D. astonished D. drinking at his sister's |