Whats the difference between progressive wave and stationary waves ?

[olivermsun]

Would a gravity wave be longitudinal? (I was careful to say up and down!)

 

[sophiecentaur]

I guess it would have to be - as there isn't a negative gravitational field.

 

[olivermsun]

I guess it would have to be - as there isn't a negative gravitational field.

I was alluding to gravity waves -- waves with gravity as the restoring force, e.g., surface water waves -- not gravitational waves.

 

[sophiecentaur]

Surface waves are a combination of both longitudinal and transverse. The particles on a water wave actually go in circles (see cans and gulls floating on waves at the seaside). Look at a plethora of google hits on water waves for some pretty animations.

 

[olivermsun]

Surface waves are a combination of both longitudinal and transverse.

I'm aware of that. I was responding your earlier statement:

if you wave your arm from side to side in a fluid, the fluid in front will not be moved. If you push your hand forward, the fluid in front can be made to move. A side to side motion will not propagate because there is no way a force can be transmitted. It's only through a difference in pressure (i.e. a longitudinal displacement) that a wave can be made to propagate.

To point out that up and down disturbances commonly are observed to propagate within a fluid -- hence internal gravity waves.

(Actually, side to side disturbances are also observed to propagate, although not typically at the scale of one's arm!)

 

[sophiecentaur]

Bulk Waves: I refer you to Seismic waves. P waves (longitudinal) propagate through the solid mantle and the liquid outer core. S waves (transverse) only propagate through the mantle. Liquid 'slips' in shear mode so how can a transverse wave propagate if there is no transverse force between adjacent regions to allow a wave to propagate? I guess that where there is significant viscosity, a wave could propagate a short distance - until the energy dissipates due to the friction forces but not over a long distance. I cannot envisage a mechanism for transverse waves to propagate in a gas, when all you have is impulses transmitted from molecule to molecule via (pretty much elastic) collisions.

Surface Waves: On the surface of a solid and liquid propagate by transverse and longitudinal displacement. The transverse component is there because the average volume will remain the same during longitudinal displacement.

 

[olivermsun]

Surface Waves: On the surface of a solid and liquid propagate by transverse and longitudinal displacement. The transverse component is there because the average volume will remain the same during longitudinal displacement.

The restoring force for surface gravity waves acts transversely, so one might more usefully argue that the longitudinal component is there solely to satisfy volume conservation.

Meanwhile, all intuition goes by the wayside when you leave the free surface and consider internal gravity waves.

Again, my point is that waves other than longitudinal pressure waves can exist and propagate in a(n inviscid) fluid, because there are restoring forces other than those which oppose material stresses.

 

[sophiecentaur]

Again, my point is that waves other than longitudinal pressure waves can exist and propagate in a(n inviscid) fluid, because there are restoring forces other than those which oppose material stresses.

These are Surface waves - requiring an interface with another medium and not Bulk waves. The 'no-transverse wave' bit only applies to bulk, 'internal' waves. Clearly, there is a restoring force for the transverse component of a surface wave, even when this is not gravity but internal tension - as with Surface Acoustic Waves on a substrate such as quartz. There are also surface Seismic waves but they have a very short range compared with the bulk waves.

 

[olivermsun]

These are Surface waves - requiring an interface with another medium and not Bulk waves. The 'no-transverse wave' bit only applies to bulk, 'internal' waves.

Internal gravity waves in a continuously stratified fluid do not work across any clear interface. You may argue that these are another form of surface waves, but the physics really look quite different.

But where is this all going? When I said that waving your arm up and down could excite propagating waves, I did it to point out some interesting waves which are neither of the types you described, not to have a semantic argument.

 

[sophiecentaur]

Internal gravity waves in a continuously stratified fluid do not work across any clear interface. You may argue that these are another form of surface waves, but the physics really look quite different.

But where is this all going? When I said that waving your arm up and down could excite propagating waves, I did it to point out some interesting waves which are neither of the types you described, not to have a semantic argument.

I see how a stratified medium could support your 'gravity' waves but that's not far different from a surface wave, is it? If you had made it clear that this was what you were alluding to, originally, then things could have been a lot easier.
Of course waving my arms up and down in an ideal fluid could well launch a wave that would comprise longitudinal (compression ) displacements along with some vortices. But I do not see where you would get any shear forces which would be necessary for a transverse wave to propagate. This isn't semantics, it's basic fluid mechanics, surely.

 

[olivermsun]

I see how a stratified medium could support your 'gravity' waves but that's not far different from a surface wave, is it?

I suppose that would be a matter for the viewer to decide: http://www.phys.ocean.dal.ca/programs/doubdiff/demos/IW1-Lowfrequency.html
(movie link at the bottom).

Interestingly, the excitation in this particularly example is caused by moving the disk side-to-side.

 

[sophiecentaur]

A good article. Just like waves 'on' water just a much lower restoring force. A lot harder to see though and an interesting measurement technique. All very low frequency, I should think.

 

[RedX]

Electricity is such a well joined up topic, in the way that radiated energy 'appears' as a resistance.

Yeah, power radiated happens to be proportional to amplitude of current squared, just like impedance.

Quick question. The load at the end of a transmission line is transformed to a new impedance, and it is this impedance that the transmitter sees. Is the impedance that the transmitter sees from antenna radiation equal to the radiation resistance, or is the radiation resistance transformed to a different impedance at the transmitter? In other words, is the 73 Ohms at the load, or at the transmitter?

 

[sophiecentaur]

The 73ohms is what you see, looking into the feed point. Looking towards the transmitter, you see the output impedance of the transmitter, transformed by the length of 75 or 50 ohm feeder,