Waves phase difference

  • Thread starter Crystal037
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  • #1
Crystal037
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Homework Statement:
Here we have subtracted the phase 4pi from S2 than S1. Why
Relevant Equations:
y=a cos (wt)
IMG-20191205-WA0004.jpg

See fig(a), S1Q=7lambda
S2Q=9lambda
I think since source S2 is lagging behind. So, we should add the phase 4pi instead of subtracting it from the cosine function. Wouldn't subtracting the phase further delay the wave more.
 
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  • #2
Tom.G
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I vote for a very poorly worded explanation! Using the Fig 10.9(a) I agree with your statement.

Unfortunately, if you look at their describing equation (S2- S1=2λ, near the top of the page) the equations they give are consistent.

edit: Fortunately in this case it doesn't matter because the sign of the phase difference doesn't matter here. That's the usual case when working with optics. If working with electrical signals (for instance comparing two signals) it may make a difference. /edit:

Just another case of poor copy editing on the part of the book publisher.

Cheers,
Tom

p.s. Good catch Crystal!
 
  • #3
Crystal037
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So am I right that y2=a cos(wt+4pi)
 
  • #4
Tom.G
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So am I right that y2=a cos(wt+4pi)
Yes. See also the edit/insertion to my first response.

Cheers,
Tom
 
  • #5
Crystal037
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Thanks Tom. G but how will it make a difference in electrical signal.
 
  • #6
Tom.G
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One example would be in data transmission using Phase Shift Keying (PSK). This method uses a fixed frequency transmitting data but varies the phase dependent on the data value at that instant.

https://www.google.com/search?hl=en...QIHf5lCWYQ4dUDCAY&uact=5#imgrc=zA9FSK-cMjgV4M

Notice that the phase is shifted by 180° as the data changes between '0' and '1'.

(I hope the image shows, it doesn't appear in my Preview. It's the big image on the right of the link with a Black background.)

image found with:
https://www.google.com/search?&q=phase+shift+keying+waveform

Cheers,
Tom
 

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