Whats the difference between progressive wave and stationary waves ?

[Nidzz93]

Wats the difference between progressive wave and stationary waves ?

 

[HallsofIvy]

Pretty much exactly what it says. Imagine a long bull whip, say, that you quickly "whip" up and then down exactly once. The ripple that moves down the whip is a "progressive" (I would say "traveling") wave.

Now take the loose end of that whip and attach it to a wall. If you "whip" it up and down, that ripple will move down the whip, hit the wall, and come back, inverted. If you do that again and again at exactly the right frequency (the "natural" frequency which depends upon the density of the whip material and tension) the waves reflecting off the wall will add to and subtract from the incoming waves so that you have nodes (0 motion) at some points, highest and lowest points half way between them. That is the nodes and hightest and lowest are "stationary".

 

[RedX]

If you do that again and again at exactly the right frequency (the "natural" frequency which depends upon the density of the whip material and tension) the waves reflecting off the wall will add to and subtract from the incoming waves so that you have nodes (0 motion) at some points, highest and lowest points half way between them. That is the nodes and hightest and lowest are "stationary".

What happens if you don't wave the whip at a natural frequency?

When you pluck a stringed instrument you inject it with a continuum of frequencies. But you only hear the discrete natural frequencies. So what happens to the other frequencies?

 

[olivermsun]

When you pluck a stringed instrument, you are making a "kink" in the string which contains the fundamental and its overtones (not a continuum!). The highest overtones dissipate much more quickly, and so the tone quickly settles to something more like the fundamental and a few overtones (which gives the instrument its characteristic "sound").

 

[RedX]

When you pluck a stringed instrument, you are making a "kink" in the string which contains the fundamental and its overtones (not a continuum!). The highest overtones dissipate much more quickly, and so the tone quickly settles to something more like the fundamental and a few overtones (which gives the instrument its characteristic "sound").

O yes, you're quite right. When you pluck a string, you lift the string until it forms a triangle with respect to the equilibrium situation (i.e., the string is a straight line), and then let it go. That triangle (i.e., initial position of wave at time=0) can be written as the sum of waves with wave numbers corresponding to the natural frequencies. I thought for a moment that the triangle could not be written as such a sum, but that's not true - every possible configuration of the string can be written as such a sum, including a sine wave of wave number not equal to the natural wave numbers!

Why do the higher overtones dissipate much quickly? Is it because of air resistance?

 

[Drakkith]

Why do the higher overtones dissipate much quickly? Is it because of air resistance?

To take a guess I would say that every time the string reaches near maximum amplitude, it strains the string, which means that higher frequencies reach peak amplitude more frequently and lose energy each time.

 

[ZealScience]

Basic difference between stationary and progressive wave is that progressive waves are transporting energy, but stationary waves trap energy on the antinodes (I mean in vacuum here, no damping). When the end of the node is removed, energy would be carried away.

Why do the higher overtones dissipate much quickly? Is it because of air resistance?

To this question, I think it is because of higher frequency has higher energy carried with it. Higher energy like kinetic energy, means it interacts more vigorously with, for instance, air molecules, then energy dissipate faster.

 

[RedX]

To this question, I think it is because of higher frequency has higher energy carried with it. Higher energy like kinetic energy, means it interacts more vigorously with, for instance, air molecules, then energy dissipate faster.

There is a similar thread on the front page that talks about the harmonics on a guitar string that are activated just by lightly putting your finger on the string and plucking on it. The explanation given was that by placing your finger lightly on the string, you are forcing certain shapes, particularly the higher harmonics. However, if this is true, then shouldn't you hear nothing at all, since the higher harmonics die out because they vibrate faster? Yet you can hear the high-pitched sound.

 

[RedX]

Actually, forget overtones. A stringed instrument usually has several strings whose fundamental are all different. Do you have to pluck the higher-pitched string to a greater amplitude in order to sustain the note the same length?

 

[olivermsun]

Something like that, yes. It's a fun and easy experiment that I recommend trying! ;)

 

[seonshrestha]

does crest and trough cancel each other in stationary waves?and why are these waves called stationary waves ?

 

[Drakkith]

does crest and trough cancel each other in stationary waves?and why are these waves called stationary waves ?

If there are two waves traveling in opposite directions and they are out of phase they will cancel each other out, yes.

See here for more: http://en.wikipedia.org/wiki/Standing_wave

 

[seonshrestha]

does this apply to electromagnetic waves too?

 

[A.E]

Electromagnetic waves are traveling waves.

 

[Drakkith]

does this apply to electromagnetic waves too?

Not quite in the same way. I can't explain it fully, perhaps someone else could.

 

[Delta Kilo]

Sure you can have electromagnetic standing waves, in a suitable resonant cavity. They are used in eg. magnetrons in microwave ovens.

 

[beantwin]

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.

 

[sophiecentaur]

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).

 

[sophiecentaur]

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]

Thank you. That answers my question.

 

[sophiecentaur]

No probs.

 

[RedX]

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]

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]

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]

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]

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

 

[seonshrestha]

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]

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]

However, it is quite possible that waving your arm up and down in a fluid could produce a propagating wave! ;)

 

[sophiecentaur]

But, at a distance, the only wave to propagate would be longitudinal.

 

[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!)