Range of frequency of electromagnetic waves

[Pushoam]

Theoretically speaking, does the frequency of em wave range from 0 to infinity?

 

[Comeback City]

Theoretically speaking, does the frequency of em wave range from 0 to infinity?

Well it can't be zero. It can approach zero, though. It also can't be infinity (being that infinity is not a real number). It can approach infinity, though.

 

[jedishrfu]

I think its limited by the Planck length when you go toward 0 and limited by the size of the universe as you go toward infinity.

https://en.wikipedia.org/wiki/Electromagnetic_spectrum

In classical physics of the 19th century, it was believed to be continuous going from 0 to infinity.

 

[anorlunda]

I think its limited by the Planck length when you go toward 0 and limited by the size of the universe as you go toward infinity.

https://en.wikipedia.org/wiki/Electromagnetic_spectrum

In classical physics of the 19th century, it was believed to be continuous going from 0 to infinity.

But Wikipedia on Plank length says that there is no proven physical significance of the Plank Length.

Can we say that the lower (practical) limit on wavelength is the upper limit on energy? Whatever emits the photon must conserve energy.

 

[Blop]

Do you mean that it's range is infinite from a whole numbers perspective? Because something's frequency can't literally be 'infinity'

 

[Drakkith]

Do you mean that it's range is infinite from a whole numbers perspective? Because something's frequency can't literally be 'infinity'

Whole numbers? No. Ignoring possible quantum and cosmological effects, the range includes all positive real numbers, whole or not.

 

[anorlunda]

Do you mean that it's range is infinite from a whole numbers perspective? Because something's frequency can't literally be 'infinity'

It is more of a question of semantics. Neither zero nor infinite wavelengths are practically possible. But there is no theoretically defined 0<limit or limit<infinity.

So I think the best way to say it is that the limits are practical, not theoretical.

 

[tech99]

It is more of a question of semantics. Neither zero nor infinite wavelengths are practically possible. But there is no theoretically defined 0<limit or limit<infinity.

So I think the best way to say it is that the limits are practical, not theoretical.

If we uniformly accelerate an electron and then allow it to continue at constant velocity, the radiated E-field would seem to be unidirectional, and hence it must possesses a zero frequency component.

 

[anorlunda]

If we uniformly accelerate an electron and then allow it to continue at constant velocity, the radiated E-field would seem to be unidirectional, and hence it must possesses a zero frequency component.

That's interesting. Can you tell me the difference between a zero-frequency zero-energy photon and no photon at all?

 

[AlphaLearner]

Does a wave exist with 0 frequency? If so, should it be just a wave pulse?

 

[Drakkith]

Does a wave exist with 0 frequency? If so, should it be just a wave pulse?

No, a wave pulse consists of many frequencies that interfere with each other to form the pulse. A wave with zero frequency can't be called a wave at all because nothing is changing. There is no oscillation, no vibration, nothing.

 

[AlphaLearner]

No, a wave pulse consists of many frequencies that interfere with each other to form the pulse. A wave with zero frequency can't be called a wave at all because nothing is changing. There is no oscillation, no vibration, nothing.

Based on a picture in book 'Fundamentals of physics', wave pulse does not look like as you said.

 

[AlphaLearner]

I know topic is Em Waves and brought a picture from mechanical waves. But way of imagining even an Em wave look like this only right?

 

[NTL2009]

In electronics, a "zero frequency" signal would be considered DC. So for EM, would a stationary magnet be analogous?

 

[AlphaLearner]

In electronics, a "zero frequency" signal would be considered DC. So for EM, would a stationary magnet be analogous?

Yea... the first one sounds like a wave can exist with 0 frequency (case of DC) I don't know the second.

 

[Drakkith]

Based on a picture in book 'Fundamentals of physics', wave pulse does not look like as you said.

Sure it does. The different frequencies interfere with each other such that they sum to zero or near zero everywhere outside of the pulse.

In electronics, a "zero frequency" signal would be considered DC. So for EM, would a stationary magnet be analogous?

You can probably think of it like that, but I would still say that a wave with zero frequency isn't a wave at all.

 

[NTL2009]

You can probably think of it like that, but I would still say that a wave with zero frequency isn't a wave at all.

I agree, DC isn't a wave at all, at least not by any definition I can think of, or even just common sense. I was just pointing out a convention, or thinking that I think I've seen, that zero hertz would be thought of as DC (but no longer a 'wave'). I'm pretty sure there is a software front end for a 'wave generator' that would let you set the "frequency" to zero, and apply a DC offset.

Would a stationary magnet be analogous to that thought?

 

[Drakkith]

Would a stationary magnet be analogous to that thought?

To a DC current? I guess you could say they are analogous in the sense that there is no change in the "signal".

 

[AlphaLearner]

Is the frequency in electric current is caused due its patterned flow in a conductor, like in AC current, energy flow half - cycle up and then half cycle down creating to and fro motion treating as wave but not the actual frequency at which electrons vibrate when the disturbance/energy flow through conductor... Am I right anywhere? And what's the difference between a signal and a wave?

Sure it does. The different frequencies interfere with each other such that they sum to zero or near zero everywhere outside of the pulse.

Thanks for clarification, so there is frequency even in wave pulse.

 

[Drakkith]

Is the frequency in electric current is caused due its patterned flow in a conductor, like in AC current, energy flow half - cycle up and then half cycle down creating to and fro motion treating as wave but not the actual frequency at which electrons vibrate when the disturbance/energy flow through conductor... Am I right anywhere?

The details of the electric current is a bit complicated. A simple explanation is that electrons are always whizzing about in all directions and current flow is the net flow of electrons in a direction. The frequency of this net flow is the rate of the oscillation in it. The electrons themselves aren't vibrating back and forth at this frequency.

And what's the difference between a signal and a wave?

Well, I'd say that in the context of current flow, the signal is the measurement of the voltage or current flow at any particular moment in time, regardless of its properties. The behavior of the signal can be described as wave-like when it behaves a certain way, namely that there is a repeating pattern that a wave equation can be applied to.

Thanks for clarification, so there is frequency even in wave pulse.

That's right. Mathematically, any pulse can be broken down into the waves composing it by using a Fourier Transform.

 

[AlphaLearner]

electrons are always whizzing about in all directions and current flow is the net flow of electrons in a direction.

Does that mean electrons literally displace and flow?
The frequency of this net flow is the rate of the oscillation in it. The electrons themselves aren't vibrating back and forth at this frequency.[/QUOTE]
Rate of oscillation in what? The whole bunch of flowing electrons? That whole bunch move back and forth as they flow just like a ship sailing back and forth in harsh waters? Then some people say frequency as cycles/sec. Can it be anywhere linked to this?

 

[AlphaLearner]

limited by the size of the universe as you go toward infinity.

Since particles in EM wave are too small and a wave of size of universe means each particle should show enormous displacement stably without disturbance, I think it is impossible.
According to De Broglie, all such microscopic particles travel in wave pattern showing wave nature in motion. Definitely, such a particle can't travel the universe without possessing even some wave pattern.
Hence frequency tending 0 and wavelength tending to ∞ in impossible. That's my idea.

I think so person who began this thread would have got his answer, we are unnecessarily going off - topic and wasting time.

 

[Drakkith]

Does that mean electrons literally displace and flow?

The electrons in the conduction band of a conductor are constantly moving about within the conductor in random directions and velocities. The electric field of an AC voltage source merely gives this random motion a small net velocity. In other words, more electrons move move one way past a point in a wire over time than in the other direction leading to a net flow of current in the circuit.

Rate of oscillation in what? The whole bunch of flowing electrons? That whole bunch move back and forth as they flow just like a ship sailing back and forth in harsh waters? Then some people say frequency as cycles/sec. Can it be anywhere linked to this?

The net flow is the sum of all the different velocities and it is this net flow that oscillates in direction and magnitude. Any single electron is not oscillating back and forth.

 

[davenn]

The electrons themselves aren't vibrating back and forth at this frequency.

I have always understood that they are
and to confirm my thoughts I had to go google searching and found at least 4 sites that confirm that the electron/charge IS oscillating back and forward at the freq of concern

do you have something to the contrary ?Dave

 

[AlphaLearner]

The electrons in the conduction band of a conductor are constantly moving about within the conductor in random directions and velocities. The electric field of an AC voltage source merely gives this random motion a small net velocity. In other words, more electrons move move one way past a point in a wire over time than in the other direction leading to a net flow of current in the circuit.
The net flow is the sum of all the different velocities and it is this net flow that oscillates in direction and magnitude. Any single electron is not oscillating back and forth.

Thanks for help, Understood what you have said and satisfied.
But when I was referring a book named https://en.wikipedia.org/wiki/Concepts_of_Physics (Published only within India) but still has World - Class standards.
In it he told to imagine flowing electric current as a people standing in a queue at box office for movie tickets and just told, If the person behind the queue pushes the person in front of him, he falls on another and like that whole queue gets knocked down. Does he mean electrons won't flow but that disturbance flow?

 

[Drakkith]

I have always understood that they are
and to confirm my thoughts I had to go google searching and found at least 4 sites that confirm that the electron/charge IS oscillating back and forward at the freq of concern

do you have something to the contrary ?

Well, my textbook, Semiconductor Physics and Devices, by Donald A. Neamen, gives a short explanation of electric current on pg. 74 and describes drift current as the summation of all the individual electrons velocities, each of which is much larger than the drift velocity that gives rise to current.

Several wikipedia articles also support this. See the following links:
https://en.wikipedia.org/wiki/Drift_velocity
https://en.wikipedia.org/wiki/Electric_current#Metals

Also, see page 3 here: http://alan.ece.gatech.edu/ECE3080/Lectures/ECE3080-L-7-Drift - Diffusion Chap 3 Pierret.pdf

The average instantaneous velocity is extremely large. One of wiki's articles gives a velocity of roughly 10⁶ m/s. In comparison, drift velocity is on the order of cm/s or less.

 

[AlphaLearner]

@Drakkith shows a strong evidence that electrons actually flow. Their movement seems back and forth but actually they are moving almost randomly like gas molecules inside a chamber. Probably for our level, the author may have made it simple stating 'You don't have a need for that level of understanding how current flows'. But truth must be known some day...

And if electrons flow, won't the atoms in conductor turn unstable and disintegrate/collapse?

 

[AlphaLearner]

And I think its a better idea to start a new thread and discuss much upon this.

 

[Drakkith]

In it he told to imagine flowing electric current as a people standing in a queue at box office for movie tickets and just told, If the person behind the queue pushes the person in front of him, he falls on another and like that whole queue gets knocked down. Does he mean electrons won't flow but that disturbance flow?

I believe that disturbance would be drift current.

We're getting pretty far off topic though. If you have more questions about electric current I recommend make a new thread.

Edit:

And I think its a better idea to start a new thread and discuss much upon this.

Indeed.

 

[AlphaLearner]

I recommend make a new thread Indeed.

On it.

 

[AlphaLearner]

Any discussions regarding topic being discussed in few above threads must be further continued in this new thread. Here
Sorry for going off - topic. This thread has been already answered.

 

[davenn]

Well, my textbook, Semiconductor Physics and Devices, by Donald A. Neamen, gives a short explanation of electric current on pg. 74 and describes drift current as the summation of all the individual electrons velocities, each of which is much larger than the drift velocity that gives rise to current.

Several wikipedia articles also support this. See the following links:
https://en.wikipedia.org/wiki/Drift_velocity
https://en.wikipedia.org/wiki/Electric_current#Metals

Also, see page 3 here: http://alan.ece.gatech.edu/ECE3080/Lectures/ECE3080-L-7-Drift - Diffusion Chap 3 Pierret.pdf

The average instantaneous velocity is extremely large. One of wiki's articles gives a velocity of roughly 106 m/s. In comparison, drift velocity is on the order of cm/s or less.

hang on ... they are all about DC currents, Ohms law and drift velocities ... I have no problem with any of that
and none of those 3 references deny that in an AC (RF) signal that the electrons oscillate back and forward about their general position
if fact I saw no obvious reference to an AC signal regardless of freq
There may well still be a general drift of electrons in an AC (RF) circuit I'm not 100% sure, I have never seen anything to back that up
maybe some one can confirm or deny it. Even if there is, it doesn't mean that in an AC signal the electrons are not oscillating about a point at a given freq be it 50/60Hz mains or in a 10 GHz microwave RF circuit

 

[AlphaLearner]

In it he told to imagine flowing electric current as a people standing in a queue at box office for movie tickets and just told, If the person behind the queue pushes the person in front of him, he falls on another and like that whole queue gets knocked down.

This analogy is mainly given for studying wave motion in part one of his series.

 

[Drakkith]

There may well still be a general drift of electrons in an AC (RF) circuit I'm not 100% sure, I have never seen anything to back that up
maybe some one can confirm or deny it. Even if there is, it doesn't mean that in an AC signal the electrons are not oscillating about a point
at a given freq be it 50/60Hz mains or in a 10 GHz microwave RF circuit

I haven't seen anything that says that drift current only happens in DC circuits, and unless the electric field set up by the voltage source can completely counteract random thermal motion that is supposedly on the order of 1,000 km/s then I can't see how individual electrons would oscillate at all.

 

[anorlunda]

At most frequencies, there is no difference between AC and DC electron drift.

For simplicity, think of a square wave AC instead of sinusoidal. DC current flows one direction for a while and then the other direction for a while. Normal models of DC electron drift apply. That holds true for all frequencies say from DC (or say 0.00001 hertz) to an high where the wavelength of an AC cycle at approximately 0.8c is comparable to the length of the wire. I don't know the Mhz or Ghz for the upper limit, but it is high. Only above that high frequency limit might it be appropriate to think of electrons vibrating, and at which RF effects begin to show.