解得                                              ④ 2分

所以：                                       ③

 解得                                                   ②1分

                                                    ① 1分

                                 ⑦  2分

                                        ⑥  4分

    由①、⑥两式得

                                            ⑤  2分                              

（2）在离开磁场过程中金属框产生的热为Q，由能量守恒得

                                                       ④  2分

由①、②、③、④式解得                                                            

                                                          ③  2分