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例谈欧姆定律的应用--应用继承和发展的关系培养学生的能力

作者:胡春宏  人气: 2279 次  时间:2010年03月13日  星级:


 欧姆定律用公式表示是:I=U/R。导出公式是(1)U=IR;(2)R=U/I。因此欧姆定律的应用可分为三类。下面通过一道测电阻的例题,谈谈如何引导学生在原有知识的基础上,发展思维,实现创新,从而培养学生应用知识解决实际问题的能力。
  一、R=U/I
  伏安法测电阻的原理是R=U/I。这表明一个导体的电阻R可通过测量同一时刻加在导体两端的电压U和通过它的电流I来计算。
  创设问题情境1:有一个未知电阻Rx,要测量它的电阻值,现有下列器材:电流表、电压表、开关、滑动变阻器,电源、导线若干。怎样测量Rx的电阻?
  〖学生练习〗画出你设计的电路图,并在右边的实物图上连线。
    
图1               图2
  二、U=IR
  创设问题情境2:若闭合开关后,发现电压表坏了,找来一个阻值已知的电阻器R0。怎样测出待测电阻Rx的值?
  分析:在上面设计的电路图上去掉电压表。电压表的作用是测量电阻器两端的电压Ux,现在有电流表和电阻器R0,根据变形公式可知U0=I0·R0,若把电流表和电阻器串联,可测出通过R0的电流I0,要使U0与Ux相等,要把它们并联。可见,一个电阻器与一个电流表串联其作用相当于一块电压表,如图中的虚线部分所示。

图3 图4
  实验的步骤:(1)断开开关,照图4那样连接电路,将变阻器的滑片P置于阻值最大处;(2)闭合开关,移动滑片P到适当处,读取电流表的示数I0,根据U0=I0·R0,算出电压;因为Rx与R0并联,所以Ux=U0;(3)断开开关,将Rx与R0换位,闭合开关,读取电流Ix,根据Rx=Ux/Ix算出电阻;(4)移动滑片P到不同位置,重做几次实验,将测算出的电阻值求平均值,即为Rx的阻值。
  三、I=U/R
  创设问题情境3:若闭合开关后,发现电流表坏了,找来一个阻值已知的电阻器R0。怎样测出待测电阻Rx的值?
  分析:在上面设计的电路图上去掉电流表。电流表的作用是测量电阻器中的电流Ix,现在有电压表和电阻器R0,根据变形公式可知I0=U0/R0,若把电压表和电阻器并联,可测出R0两端的电压U0,要使I0与Ix相等,要把它们串联。可见,一个电阻器与一个电压表并联其作用相当于一块电流表,如图中的虚线部分所示。
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