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Whats the difference between progressive wave and stationary waves ?

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sophiecentaur
#19
Jul16-11, 05:32 PM
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Quote Quote by beantwin View Post
I've always sensed that our description of light as a wave is misleading. I can imagine it ACTUALLY being a progressive wave moving up and down because wouldn't that contradict the very idea of special relativity? Some light would would travel faster than others because not only is there the forward motion at light speed, but also the up and down (or side to side if you prefer) of the wave. Higher frequencies would travel faster to achieve the same speed as slower waves that need not oscillate as frequently.

So the question I ask is are we just calling it a traditional wave because it is simply convenient to do so or because light is, in fact, a wave jumbling back and forth while traveling at the ultimate speed limit across the universe?

Feedback would be helpful. I haven't had the chance to study this more as of yet, so I may, and probably am, overlooking a critical element that would resolve this dilemma.
Hi beantwin
An electromagnetic wave is transverse but that doesn't mean anything is "jumbling" up and down. It's called transverse because the varying electric and magnetic fields are at right angles (transverse) to the direction that the wave is travelling.
We call it a "traditional wave" because the same maths apply to em and all other, more tangible, waves.
beantwin
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Jul16-11, 05:35 PM
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Thank you. That answers my question.
sophiecentaur
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Jul16-11, 05:37 PM
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No probs.
RedX
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Jul16-11, 07:46 PM
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Quote Quote by sophiecentaur View Post
You get standing waves in signal lines and in some designs of RF filters. They are a real embarrassment in the lines between transmitters and antennae where then can introduce excessively high volts (High Voltage Standing Wave Ratio / VSWR).
I'm a little confused about this. Isn't the impedance of an antenna infinity, since an antenna is an open circuit? Therefore an antenna always has full reflection of the transmitter wave, and you get standing waves both in the antenna and in the line leading up to the antenna. Or is the impedance of an antenna say 73 ohms instead of infinity?

If you have a 73 ohm line, and attach it to a 73 ohm antenna, do standing waves form in the line? In the antenna?
sophiecentaur
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Jul17-11, 03:40 AM
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A wire dipole, fed at the centre, will present a load of 73ohms at just one frequency. (At which the dipole is an exact half wave long. All the energy is radiated into free space so this 'looks like' a pure resistance. Any practical antenna needs to radiate a range of frequencies (bandwidth) so it cannot be perfectly 'matched' to free space. A very short dipole will appear as a small Capacitance in series with a tiny resistance; that is almost an open circuit. A very long dipole can look like anything from a near open circuit to a near short circuit.
An antenna feeder will always have standing waves in it but these may be reduced by 'matching networks' at the antenna feed point.
RedX
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Jul17-11, 06:03 PM
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Quote Quote by sophiecentaur View Post
A wire dipole, fed at the centre, will present a load of 73ohms at just one frequency. (At which the dipole is an exact half wave long. All the energy is radiated into free space so this 'looks like' a pure resistance. Any practical antenna needs to radiate a range of frequencies (bandwidth) so it cannot be perfectly 'matched' to free space. A very short dipole will appear as a small Capacitance in series with a tiny resistance; that is almost an open circuit. A very long dipole can look like anything from a near open circuit to a near short circuit.
An antenna feeder will always have standing waves in it but these may be reduced by 'matching networks' at the antenna feed point.
If you have an antenna that is three quarter-wavelength long, then what would it behave like? I would like to say short circuit, but the fact that energy is radiating into space suggests that it can't be a short circuit, that the resistance has to be at least the radiation resistance.

Also, could you model a short dipole as a transmission line terminated in a circuit consisting of a resistor in series with a capacitor, with the resistance of resistor equal to radiation resistance and not line resistance?
sophiecentaur
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Jul18-11, 02:16 AM
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You can look up the exact impedance by I would say the input would be like about 30ohm resistance in series with several tens of ohms of capacitative reactance. The radiation resistance goes down and the capacitance goes down as the dipole gets shorter.
sophiecentaur
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Jul18-11, 03:02 AM
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Electricity is such a well joined up topic, in the way that radiated energy 'appears' as a resistance.
seonshrestha
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Jul19-11, 10:32 AM
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thanz. and i have got one more question.
why does transverse wave only travels in solid medium? and longitudinal wave in solid, liquid and gas? my physics teacher said something about shear strain and its relation to these waves but i did not get him quite right. please can anyone help ?
sophiecentaur
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Jul19-11, 10:38 AM
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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. In a solid, however (and on a string) you can actually transmit a force due to a shear force.
olivermsun
#29
Jul21-11, 09:22 AM
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However, it is quite possible that waving your arm up and down in a fluid could produce a propagating wave! ;)
sophiecentaur
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Jul21-11, 10:40 AM
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But, at a distance, the only wave to propagate would be longitudinal.
olivermsun
#31
Jul21-11, 11:21 AM
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Would a gravity wave be longitudinal? (I was careful to say up and down!)
sophiecentaur
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Jul21-11, 11:23 AM
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I guess it would have to be - as there isn't a negative gravitational field.
olivermsun
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Jul21-11, 11:25 AM
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Quote Quote by sophiecentaur View Post
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
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Jul21-11, 11:28 AM
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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
#35
Jul21-11, 11:33 AM
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Quote Quote by sophiecentaur View Post
Surface waves are a combination of both longitudinal and transverse.
I'm aware of that. I was responding your earlier statement:
Quote Quote by sophiecentaur
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
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Jul21-11, 11:48 AM
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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.


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