⑥

 (2) 由公式                                           ⑤  1分

可得

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