（1）2∶1 （2）4∶1（3）2∶1  

（1）线圈以速度v匀速进入磁场，当CD边在磁场中时，线框中感应电动势其中L为CD边的长度．                                            

此时线框中的感应电流为，其中R为线框的总电阻．   

同理，线圈以速度2v匀速进入磁场时，线框中的感应电流最大值为

所以第二次与第一次线圈中最大电流之比为2∶1                     （4分）

（2）线圈以速度v匀速进入磁场，当CD边在磁场中时，CD边受安培力最大，最大值为 ．由于线圈做匀速运动，所以此时外力也最大，且外力大小等于安培力大小，此时外力的功率为．                 

同理，线圈以速度2v匀速进入磁场时，外力的最大功率为．

所以第二次与第一次外力做功的最大功率之比为4∶1                   （4分）

（3）线圈以速度v匀速进入磁场，线框中的感应电流为

设AD边长为l，则线框经过时间完全进入磁场，此后线框中不再有感应电流．所以第一次线框中产生的热量为         

同理，线圈以速度2v匀速进入磁场时，线框中产生的热量为                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

所以第二次与第一次线圈中产生热量之比为2∶1                       （4分）