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在中学物理教参中常见到这样的题目:“由公式R=V/I能否说明:导体的电阻与导体两端的电压成正比,与通过导体的电流强度成反比?”一般答案是:“不能。因为电阻是导体本身的一种性质,它的大小决定于导体的长度、横截面积和材料。”此答案本身没错。那么怎样理解导体的电阻大小“决定于导体的长度、根截面积和材料”呢?是不是在导体的长度、横截面积和材料确定之后,其电阻就固定不变了呢?请看一例:由某材料(电阻率为ρ)制成的一个长方体,其长、宽、高分别为c、b、c。当电流I分别沿长度a、宽度b和高度c方向通过该导体时,其电阻各为多少?
This problem is commonly seen in middle school physics teachings: “Is the formula R=V/I able to explain: The resistance of a conductor is proportional to the voltage across the conductor and inversely proportional to the current intensity through the conductor?” The general answer is :“No. Because the resistance is a property of the conductor itself, its size is determined by the length, cross-sectional area, and material of the conductor.” This answer is correct. So how do we understand the resistance of the conductor “ depends on the length of the conductor, root cross-sectional area and material ”? Is not the length of the conductor, cross-sectional area and material after the determination, the resistance is fixed in it? An example: a rectangular parallelepiped made of a material (resistance ρ) whose length, width, height are c, b, and c, respectively. When the current I passes through the conductor in the direction of length a, width b, and height c, respectively, what is the resistance of each?