根据振动图象或交流电图象要写出数学表达式,学生最感困难的是确定初相。利用参考圆求初相,固然是一种有效方法,但现行高中教材已删掉了参考圆,并且连表达振动曲线也只采用正弦,不再用余弦了,本文就不用参考圆确定初相的教法介绍如下。不用参考圆确定初相的简单法则: 简谐振动或交流电的正弦曲线若有一个正弦图跨在坐标系原点两边,则初相可用该正弦图跨在原点左边那部分所对应的角取正值表示;若曲线以完整的正弦图起于坐标系原点,则初相为零。如图1中,i_1的初相为φ_0,i_2的初相为零,i_3的初相为(π+φ_0)。应用上述“法则”时要注意两步,(1)看看曲线是否 According to the vibrational image or AC image, a mathematical expression should be written. The most difficult task for students is to determine the initial phase. Using the reference circle to find the initial phase is of course an effective method. However, the current high school textbook has deleted the reference circle, and even the expression of the vibration curve only uses the sine, and no longer uses the cosine. This article will not use the reference circle to determine the primary phase. The teaching method is described below. A simple rule for determining the initial phase without reference to a circle: Harmonic vibration or alternating current sinusoids If a sinusoidal pattern spans both sides of the origin of the coordinate system, the positive phase can use this sinusoidal pattern to take a positive value across the corresponding part of the left side of the origin. If the curve starts from the origin of the coordinate system with a complete sinogram, the initial phase is zero. In Fig. 1, the initial phase of i_1 is φ_0, the initial phase of i_2 is zero, and the initial phase of i_3 is (π+φ_0). There are two steps to be taken when applying the above “rules”, (1) to see if the curve is