phy F6 phase difference問題

好明顯相位差是π/2﹐問題是 θ_Y=θ_X+π/2抑或θ_Y=θ_X-π/2
當X是波峰時Y在平衝位置。令Y=sinθ_Y,則Y=0=>θ_Y=0,π但X=1=>θ_X=π/2=>θ_Y=θ_X-π/2=>ANSWER D


2009-08-06 17:57:14 補充：
上面解說錯了。應該令X=sinθ_X,
若θ_Y=θ_X+π/2=>Y=-cosθ_X
若θ_Y=θ_X-π/2=>Y= cosθ_X

因為由圖可以看出﹐X的波形下降時﹐Y的波形上升﹐所以θ_Y=θ_X-π/2

The trace of the CRO sweeps from left to right. Hence, on the x-axis, which represents the time axis, the left hand side indicates an earlier time and the right hand side indicates a later time. In other words, the waves are moving from right to left on the screen.
Thus, it is not difficult to see that at time t=0 (i.e. the origin), the crest of Y has already passed that point, but the crest of X is just passing the same point. Therefore, Y leads X.
The phase difference between X and Y can be found by comparing the difference in positions of the crests (or troughs) of X and Y on the CRO screen. It is easy to see that a period equals to 5 units on the screen. the difference between crests of X and Y are separated by 1.25 units, hence a phase difference of (1.25/5) x 2.pi = pi/2.