Group Velocity / Phase Velocity Animation - Case 4: Zero Phase Velocity | Part 4

[@beinginspiredbyjennifersan3338]

Thank you for these videos you have really helped my understanding of this content. .. One thing that would have been better would have been if there was some sound or voice over explaining what was going on. It took me a minute but overall it was easier to interpret than my physics text and for that I thank you.

[@meyavuz]

+'Being Inspired' by Jennifer Sanderson Thanks. Great to hear that they are helpful. Yes, having sound would be nice but this is what I can do for now. :)

[@davidgmos]

Thank you very much.

[@meyavuz]

Thanks

[@rupeshkashyap4083]

great video really ..thanks ...i wonder what would i say about group velocity in this case positive or negative ??.....as the wavepacket seems travelling in both directions

[@meyavuz]

well actually the group is the one moving with the magenta dot which is positive. Just follow the magenta dot.

[@tamyk64]

great video !

[@meyavuz]

Thanks

[@SushilKumar-so6tl]

its nice ,thanks

[@meyavuz]

Thanks

[@ДмитрийАникин-р5ц]

Thanks for the video! Here's the thing I don't understand: what if red and blue waves would have speed of light? As I see from the video group velocity'd be more than speed of light, but it's impossible.

[@meyavuz]

Дмитрий Аникин Group velocity can exceed speed of light (search for superluminal phase velocity in google) as it has been shown in various papers even experiments. What cannot exceed is the speed of individual wave components (so called energy chunks) that makes up the whole signal. Individual signals cannot exceed speed of light but where they superimpose can seem to have speed larger than speed of light. Think of it as relative speed between two plane wave components traveling opposite to each other.

[@pacogarcia4628]

I wonder if this is the case for standing waves in a rope...

[@meyavuz]

Paco García Interesting question. In the rope case the end point is fixed, hence it kinds of creates this negative direction wave so you maybe right, this case is very similar to rope standing waves

[@myterracegarden2329]

Thnq v mch

[@meyavuz]

thanks

[@qingfengwang2404]

very nice!

[@meyavuz]

Thanks

[@Souravkumar-iw7gy]

can u please tell how to simulate I have an assingment due

[@meyavuz]

Just two sinusoids with different frequencies (but close to each other) then add them up in different time instances

[@vozdejubilo7114]

Amigo, qué expresión utilizas para graficar la envolvente?

[@MehmetEmreYavuz]

I used something similar to: www.mathworks.com/matlabcentral/fileexchange/3142-envelope1-1

[@meyavuz]

Gracias

[@joxa6119]

Is this the case when Vphase and Vgroup is equal?

[@meyavuz]

No here Vphase is zero but Vgroup is larger than zero.

[@shuvroroy6280]

is it a standing wave???

[@meyavuz]

+Shuvro Roy It looks like it but the wavelengths of left and right moving are different from each other. So it is not actually although you can think of these components as the incident and the reflected waves from an interface and their combination creates the standing wave. But the wavelengths are not same, these are combination of two wave components at different wavelengths

[@shreyjain3965]

+meyavuz For standing wave both group velocity and phase velocity need to be zero.

[@jeremiahmullikin]

Shrey Jain this is definitely not a standing wave.

[@toma78320]

Which software do you use pls ?

[@meyavuz]

these are all done in Matlab.

[@toma78320]

+meyavuz thanks! is there any website where i can find the code or a similar one ?

[@meyavuz]

+toma78320 I don't know any website as I wrote my own. You can write yours easily by just looking at the legends I am using in the video. Just do the same thing and it should work.

[@janalbrecht5099]

is it possible to get it in german?

[@meyavuz]

Hi. What exactly you want to get it in German? The video doesn't have much text. Can you clarify? Thanks

[@janalbrecht5099]

hey i know there are only a few words, i just want to understand some comments because i dont get it why that is not a standing wave. I think now i know : )

[@janalbrecht5099]

i asking for it bacause there exist no kind of explanation like this in german where you can see the difference between the velocities like this great video

[@meyavuz]

Mein Deutcsh ist nicht sehr gut:) It is not a standing wave simply because the wavelengths and wavenumbers of these two components are not same and they are independent of each other. In the standing wave, the negative traveling wave is the "reflected" from the positive direction traveling wave. hence the wavenumbers and wavelengths are same, also one is created from the other one. I had another animation on standing waves here: kzbin.info/www/bejne/qWawc6GlZbWNoKc

[@jeremiahmullikin]

Why does this have a positive group velocity? It should be a zero group velocity too. It looks like it happened because the red and blue dots didn't meet at 0 at the same exact time.

[@jeremiahmullikin]

Guess you'd call this a partial standing wave.

[@meyavuz]

Just look at the envelope and you will see the envelope moving to the left which represents the group velocity. The dots here don't do anything except facilitate the viewer to see the wave movement. They don't need to meet at 0, consider the whole wave package not points.

[@meyavuz]

The wavelengths of left and right moving waves are different, so not a standing wave, you were right in your previous comment. I corrected accordingly. Thanks

[@jeremiahmullikin]

meyavuz thank you sir. I'm wondering if you can help me (and other people) with modeling a problem. We need to see a simulation (similar to what you did in this video) but with a group velocity from 0.9 c to 0.1c (so a tapered waveguide), with two identical waves in superposition. So the left side 0.9 c, and the right side 0.1c. I envision everything being exactly the same except the group velocity. From what I understand, phase velocity is irrelevant in a waveguide. Also, we can modestly reimburse you for your time via PayPal.

[@meyavuz]

I didn't understand completely. So within the waveguide, the group velocity will change at different locations? I am not sure how it is exactly achieved but with just two wavecomponents traveling to left and right, it might not be possible. Is the input wideband signal and are we talking about 2D signals? Do you have a reference equations etc? I need to see what is being implemented. If you can send me photo of the equations/book page etc, that might help. Send to meyavuz (at) gmail.com. Then I can tell you whether I can help or not. Thanks.