# phase-which wave started first?

Discussion in 'Feedback and Suggestions' started by milly molly mandy, Feb 27, 2009.

1. ### milly molly mandy Thread Starter Member

Sep 4, 2008
25
0
hello,

I'm trying to understand all that stuff about waves being out-of-phase, leading by 180 degrees or lagging by 90 degrees etc.

Although I'm doing quite well really, largely due to a dogged persistence and refusal to give up, there is one thing at least that no matter how much I try, I cannot get the hang of.

That is, I don't know how to tell, when you're looking at a diagram of say 2 waveforms out of phase, WHICH WAVE STARTED FIRST.?

Is there actually a way of telling which wave started first?

By the way, I found that Worksheet provided by this Forum, on drawing a Sine wave by doing the angles of a circle, not only very informative, but also fun to do.

2. ### Papabravo Expert

Feb 24, 2006
10,338
1,850
The answer depends on your definition of "started first". There are a number of different possibilities, and that information is either lost or irrelevant by the time you look at the steadystate behavior.

If you are concerned about transient behavior then you can specify which one gets turned on first.

3. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292
Greetings!

This is the CLASSIC ambiguity issue. It's also related to the concept of GROUP velocity in that, without having some DISCONTINUITY, or other identifyable STARTING POINT, it's imposssible to tell which came first!

(For this same reason, it's impossible to determine range of a stationary object with a CW radar!)

However....all is not lost! IN a continuous AC circuit, for power calculaltions and such, a 90 % lead is the same thing as a 270 % lag.

From a practical standpoint, if you're using an oscilloscope for comparing two phases, you can always trigger the scope from the channel you want to use as your reference.

For many applications, it can be useful to have an interruptor, or "CHOPPER" synchronized with the crossover point of your reference wave.

IN any case...don't feel bad. If you could completely resolve the ambiguity issue for any system, you'd be writch and famous@!

Eric

4. ### Ratch New Member

Mar 20, 2007
1,068
3
milly molly mandy,

Irrelevant. Phase is a relative measurement determined by what is the reference wave. That determination does not have to be based on which started first or whether both started at the same time. Only the existence of the two waves is necessary to determine a phase value.

Ratch

5. ### milly molly mandy Thread Starter Member

Sep 4, 2008
25
0
phase-which wave started first?

Thank you everyone for your interesting answers and your clear and lively explanations, which I enjoyed reading. I think I understand that what you are saying is that what matters is not which one wave started first, but the difference between them, although it is possible to choose a "reference" wave (what a useful word, thank you for reminding me of it) . It sounds a bit like the chicken and the egg. Thank you for enlightening me. What you said about a lead of 90 degrees being in a sense the same as a lag of 270 is a useful thing to know. Alaska?! wow! North to Alaska uh!

Last edited: Feb 28, 2009