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【简答题】
案例： 下面是教师在“电功率”一课中。向学生讲解纯电阻的电功率的教学片段。 教师：同学们。我们已经学习过功率的概念，请大家说一下它的定义。 学生：功率就是单位时间所做的功或消耗的能量。 教师：对．今天我们推导阻值为R的电阻的功率。 教师：功率的公式为P＝W／t，通过焦耳定律，推出了纯电阻的电功率的表达式：P＝I2R，P＝U2／R .对这三个公式，教师分别讲解。指出哪个是定义式，哪个是计算式。 学生1：老师．功率怎么和电阻成正比。又和电阻成反比呢? 教师：上课要注意听讲，电流确定，功率就和电阻成正比；电压确定，功率就和电阻成反比。学生2：老师．那我们用的交流电，好像电流和电压都是变化的，怎么计算电功率啊? 教师：我们只考虑电流或电压不变的情况，不考虑其他情况。你们把这几个公式用好就可以了。 问题：（1）请对上述教学片段进行评析。（15分） （2）重新设计一个教学片段，使学生深入理解电功率的概念以及表达式。（15分）
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