初中物理第二册电话一节是联系实际的好教材。但是有些肯动脑的学生在学过“电话”这一节后往往会向老师提出这样的问题:电话听筒里的铁心能不能不用永磁铁,而改用软铁(像电磁铁的铁心一样)? 答:如果用软铁做铁心,在听筒线圈中不通过电流时,铁心对薄铁片没有吸力,薄铁片处于平衡位置;当线圈中通以图1所示 The second volume of the junior high school physics book is a good textbook for practical purposes. However, some brain-study students often ask the teacher questions after learning the “phone” section: Can the iron in the telephone handset not use permanent magnets instead of soft iron (like the core of an electromagnet)? : If iron core is made of soft iron and no current is passed through the coil of the handset, the iron core does not attract the thin iron sheet. The thin iron sheet is in an equilibrium position;