Electromagnetic field theory. Q and A

[69dodge]

Vacuum has a breakdown strength?

Nothing's there to break down.

 

[Walter Wayne]

Mmm, I have to disagree here. An electron moving in a vacuum will, once accelerated, simply keep moving till something intercepts it. If it is inside an electic field, it will keep accelerating till it impacts with the positive elctrode.

Hans

It starts "at rest" on a plate with one potential, and ends at a lower potenital on another plate. So at some point it loses energy. Though now that I think about it, it could also lose the energy by heating the opposite plate, Somewhere we got to get rid of that energy it had.

Walt

 

[jj]

Vacuum has a breakdown strength?

Nothing's there to break down.

Well, let me ask a question, which is easier to strike, an arc in atmosphere, or an arc in 10% of atmospheric pressure, then.

 

[Yahweh]

These are the kinds of questions that I have to ask at 3:00 AM on a Sunday morning...

* People who shape alien-proof helmets out of foil, is there anything remotely sane about that kind of idea. Ideally, no. But for the sake of questioning, will those helmets really keep out the EM radiation that puts thoughts in your heads.

* I am interested in winning the JREF Paranormal Prize. Aside from choking hazards, could it be feasible that my neodymium will allow me to attract objects through my clothing after swallowing it? If not, is there anything I can do to make my neodymium more powerful?

* Followup: If I grind my neodymium into a very fine powder, and inject it into my bloodstream, will it still retain its magnetic strength, thereby allowing me to feign telekinetic powers? Any negative side-affects?

* When I throw refrigerator magnets at my cat, he gets agitated and hides somewhere in the house. So my cat repels from certain types of magnets, is this enough to assume my cat is magnetized?

 

[MRC_Hans]

davefoc :

Q: OK, at what point in the acceleration of the electron is the photon being generated? Ah, a thought: Maybe the thermal motion is causing the electons to transition to higher energy levels within the atom and the transition of the electron to ground state is what actually generates the infrared photon?

A: I have to admit I'm a little in the deep end here. The way I understand it, thermal energy simply radiates away in the form of photons, it needs not invove electrons changing level.

Q: All right, how does the generation of these higher frequency photons relate to the generation of photons in an antenna? Somehow moving the free electrons back and forth in the antenna causes the antenna to emit a series of photons.


Again, the energy is emitted as photons. Theoretically, you need not have a current. If you spin a permanent magnet, it will emit a waveform. As long as a field is stationary, no energy is moved, but if the field is changed, no matter how, the change requires energy to be redistributed, and this happens at the speed of light, and is carried by photons.


Q: What is necessary to cause the antenna to emit just one photon?

A: I assume that would have to be a single electron moving in it.

Q: How many cycles are present in a single photon?

A: I don't think it is directly related to cycles, the energy is more important. I would assume a single cycle generated by a single electron would be a photon, but I'm not sure.

Q: Can a single photon at say a 100MHz be detected?

A: As the energy of a photon is proportional with the frequency (inverse proportional to the wavelength), a 100MHz photon will have very low energy, so I very much doubt it could be detected. It would be way under the noise floor.

Q: What kinds of single photons be detected?

Somewhere from the far infrared and up, they become energetic enough to be detectable.

Hans

 

[MRC_Hans]

It starts "at rest" on a plate with one potential, and ends at a lower potenital on another plate. So at some point it loses energy. Though now that I think about it, it could also lose the energy by heating the opposite plate, Somewhere we got to get rid of that energy it had.

Walt

Mmm, yes. It has a potential energy when near the negative plate, then as it accelerates towards the positive plate, the potential energy is converted to kinetic energy, which is then converted to thermal energy on impact. Basically just like an object falling in gravity.

Hans

 

[MRC_Hans]

Vacuum has a breakdown strength?

Nothing's there to break down.

I don't think break-down strength is the proper term. Vacuum poses no obstacle to an electric current, but charge carriers have to be present. The voltage at which a current starts to flow depends on how much is needed to tear electrons free of the negative electrode, and tis depends on material and temperature. Like in the vacuum-tube, where the hot cathode will emeit electrons even if no potential exists. Thus, even a low voltage will cause a current to flow.

Hans

 

[MRC_Hans]

Yahweh:
These are the kinds of questions that I have to ask at 3:00 AM on a Sunday morning...

Q: People who shape alien-proof helmets out of foil, is there anything remotely sane about that kind of idea. Ideally, no. But for the sake of questioning, will those helmets really keep out the EM radiation that puts thoughts in your heads.

A: No. While tin foil helmets do block, or at least attenuate, certain (mainly very short) wavelengths EM radiations, the wavelength of "brainwaves" is very long, and a tin foil hat wil have little or no effect on them.

The reason for the absence of thoughts in the heads of tin foil hat bearers is something else.

Q: I am interested in winning the JREF Paranormal Prize. Aside from choking hazards, could it be feasible that my neodymium will allow me to attract objects through my clothing after swallowing it? If not, is there anything I can do to make my neodymium more powerful?

A: the body does not interfere with stationary magnetic fields, so it is theoretically possible to have a magnet inside your body, which can hold iron objects to your skin. I would, however, not recommend swallowing it, since the distance to the skin would be too high. Implanting magnets right under the skin would be much more effective (and probably safer).

Q: Followup: If I grind my neodymium into a very fine powder, and inject it into my bloodstream, will it still retain its magnetic strength, thereby allowing me to feign telekinetic powers? Any negative side-affects?

A: I don't know what the chemical properties of neodynium is, but I suspect you would soon find out. However, it wouls not serve your purpose, because the powder would become disorganized, so the little grains, being tiny individual magnets, would cancel each other out, giving no overall field.

Q: When I throw refrigerator magnets at my cat, he gets agitated and hides somewhere in the house. So my cat repels from certain types of magnets, is this enough to assume my cat is magnetized?

A: Cats are intrinsically paranormal.

Hans

 

[MRC_Hans]

Well, let me ask a question, which is easier to strike, an arc in atmosphere, or an arc in 10% of atmospheric pressure, then.

Arcs form easier in low pressure, but it is not a proportional function, so you cannot infer that current flows freely in a vacuum.

The difference is that as long as there is air present, there is also charge carriers in form of molecules that can be ionized, so the voltage required to form an arc depends partly on how much voltage it takes to ionize the air, partly on how much resistance the air poses. As the first is roughly constant, and the second falls with lower pressure, arcing happens easier in thin air.

A hard vacuum, however, contains no molecules to ionize, so here the carriers must be torn loose from the electrodes.

Hans

 

[davefoc]

MRC_Hans,
Thank you for starting this thread.

This may be a little off the topic of this thread but I am still mulling over the nature of photon creation in an antenna.

I suspect that your answer about the creation of infrared photons might be wrong. It seems like the basis of Plank's derivation of the equation for black body radiation (I don't mean to imply I understand this) was based on the idea that the radiation was quantized. I am thinking the reason it was quantized was because it was created by electron changing states in discrete ways.

OK, skip the above if you wish, it may be off the subject of the thread. Back to the antenna emitting photons.

Suppose you had an omnidirectional antenna connected to a very low power, high frequency (say microwave) source. Might one be able to dectect the quantization of the radiation because the photons would be transmitted in random directions and there would be so few of them that the random nature of the transmission could be detected by the nature of the reception in a closely placed receiving antenna?

What I am trying to resolve in my mind with the above question is the disconnect I have between my understanding of high frequency photons and low frequency photons. The low frequency photons seem to be generated when a free electron is accelerated. So does the acceleration of a single electron produce one photon? Does it go off in a random direction?

Hmm, as I think about this I realize that it might not be possible to accelerate just one electron in an antenna. As the signal goes through the antenna lots of electrons are accelerated no matter how small the signal.

So consider this: An electron suspended in a vacuum between two conductive plates. Presumably when we apply a signal to the two plates the electron moves back and forth. Is a single photon being emitted from the electron for each movement of the electron in one direction or is the number of emitted photons proportional to the size of the movement. I suspect the latter because the frequency of the photons I suspect is equal to the frequency of the applied signal but the power emitted (and therefore the number of photons emitted) is proportional to the size of the applied signal and therefore the size of the movement.

My apologies if the above questions are not on the subject you intended for the thread. Please feel free to ignore them or to make fun of me.

 

[MRC_Hans]

I certainly won't make fun of you. The question is interesting, but I simply don't know the answer. I'll file it in the "bug the experts" drawer .

Hans

 

[Walter Wayne]

I suspect that your answer about the creation of infrared photons might be wrong. It seems like the basis of Plank's derivation of the equation for black body radiation (I don't mean to imply I understand this) was based on the idea that the radiation was quantized. I am thinking the reason it was quantized was because it was created by electron changing states in discrete ways.

In the case of black body radiation, while some of it is is generated by electrons changing states, it is by no means the main contributor. I am told that if one solves shroedinger's equations for the situation where you thermally excite bulk tungsten until the black body radiation spectral output is centered in the visible spectrum (i.e turn on a light bulb) that the main contributor is actually phonons (lattice vibrations) falling back to lower states and emitting phtons as they do this, and not the excited electrons emitting stuff.

Walt

 

[69dodge]

davefoc, it's a bit confusing to think about an individual accelerating electron as emitting individual photons. If we want to analyze the electromagnetic field from a quantum mechanical point of view, by talking about quantized photons rather than continuous field strength, then we ought to analyze the electron quantum mechanically as well, by talking about the different quantum states it might be in and the various transitions it might make between them rather than by talking about its continuous acceleration. An electron doesn't really have an acceleration in the classical sense, if we examine it closely enough to detect the emission of individual photons; it jumps discontinuously from a high-energy state to a lower-energy state.

 

[davefoc]

69Dodge,
Thanks for your reply.

We may be getting into an area that I am just not going to understand without a more disciplined study of the issue, but I would like to try to at least get a qualitative idea about the situation.

First, does an electron undergoing acceleration produce photons. For instance if an electron is falling in a gravitational field are there photons being emitted? Discontinuously perhaps?

Is it the acceleration of electrons which is responsible for EM emmisions in an antennae? Or is it the EM wave which is being transmitted to the antennae via a transmission line of some sort really the deal here? The motion of the electrons in the antennae is just an artifact of sending an EM wave into the antennae. At higher frequencies current doesn't actually flow through the antenna particulary I would think antennaes that are just horns attached to wave guides. Maybe it was the acceleration of electrons that produced the photons that are being carried by the waveguide in this case?

OK, are we sure that a low frequency RF signal is really made up of individual photons? One view of this situation might be that individual high frequency photons exist because of the way that they are generated but that individual low frequency photons don't exist because of the way they are generated (completely uninformed speculation there). Could we in theory at least get far enough away from an RF emitter such that we begin to see a kind of quantization of the signal? I wonder if the Arecibo antenna is sensitive enough to detect any kind of an effect like this.

 

[Walter Wayne]

69Dodge,
Thanks for your reply.

We may be getting into an area that I am just not going to understand without a more disciplined study of the issue, but I would like to try to at least get a qualitative idea about the situation.

First, does an electron undergoing acceleration produce photons. For instance if an electron is falling in a gravitational field are there photons being emitted? Discontinuously perhaps?

Acceleration can produce or absorb photons, depending on the starting or ending momentum of the electron. I am not sure about the case of electrons in a gravitational field. I believe if it is in free fall it is considered a straight line and doesn't radiate photons. I could be wrong on that part.

Is it the acceleration of electrons which is responsible for EM emmisions in an antennae? Or is it the EM wave which is being transmitted to the antennae via a transmission line of some sort really the deal here? The motion of the electrons in the antennae is just an artifact of sending an EM wave into the antennae. At higher frequencies current doesn't actually flow through the antenna particulary I would think antennaes that are just horns attached to wave guides. Maybe it was the acceleration of electrons that produced the photons that are being carried by the waveguide in this case?

In any transmission system (waveguide or transmission line) the energy is carried both in and out of the conductors. Because of the skin effect, at higher frequencies one tends to get less energy in the conductor, but it still there. This is also true of an antenna, some of the energy travels along the conductor, and some in the dielectric (usually air). The majority of the power emitted will come from the EM wave, just because that is where the majority of the power usually is.

The EM wave and and the electron motion is reciprocal. Each can cause the other.

OK, are we sure that a low frequency RF signal is really made up of individual photons? One view of this situation might be that individual high frequency photons exist because of the way that they are generated but that individual low frequency photons don't exist because of the way they are generated (completely uninformed speculation there). Could we in theory at least get far enough away from an RF emitter such that we begin to see a kind of quantization of the signal? I wonder if the Arecibo antenna is sensitive enough to detect any kind of an effect like this.

Low frequency RF signals are still photons. In theory we could get far enough away from an antenna to observe quantum effects.

I'd like to know when the contacted Pioneer 10 awhile back what the received power was like, it must have been a tiny signal.

Walt

 

[MRC_Hans]

I think one of the problems is that we are trying to understand the photon as an elementary particle, so we keep loking for its minimal size. Now I realize I'm going out on a limb here, but I the way I see it, the photon is a secondary quantum particle. The photon is released by a quantum event, which means that it comes in quantum steps, but the size of those steps depend on the primary event. Therefore photons come in an indefinite range of energies. The Eart's magnet field changing poles on a geological timescale emits photons, but their wavelength is a considerable fraction of the size of a galaxy, and thus the energy is very low. A gamma burst emits photons, which are some of the most energetic we know of.

When we are looking for the photon emitted by a moving electron, we need to look at the quantum nature of the electron. We tend to (or at least I tend to) imagine a moving electron as a tiny dot drifting along from A to B at some variable speed, but it is not. Heisenberg tells us that we cannot know both the speed and the position of the electron. At some point we can realize it has changed position, at which time it has emitted a photon. I think this is about the closest we can get.

Hans

 

[MRC_Hans]

Mmmm, aren't there some more questions on a more elementary level ? Roger?

Hans

 

[Kumar]

I think one of the problems is that we are trying to understand the photon as an elementary particle, so we keep loking for its minimal size. Now I realize I'm going out on a limb here, but I the way I see it, the photon is a secondary quantum particle. The photon is released by a quantum event, which means that it comes in quantum steps, but the size of those steps depend on the primary event. Therefore photons come in an indefinite range of energies. The Eart's magnet field changing poles on a geological timescale emits photons, but their wavelength is a considerable fraction of the size of a galaxy, and thus the energy is very low. A gamma burst emits photons, which are some of the most energetic we know of.

When we are looking for the photon emitted by a moving electron, we need to look at the quantum nature of the electron. We tend to (or at least I tend to) imagine a moving electron as a tiny dot drifting along from A to B at some variable speed, but it is not. Heisenberg tells us that we cannot know both the speed and the position of the electron. At some point we can realize it has changed position, at which time it has emitted a photon. I think this is about the closest we can get.

Hans

Mr. Hans,

Does it mean that science is bit unclear in understanding the photons?

In physics, the photon is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by quantum electrodynamics (QED) which is the oldest part of the Standard Model of particle physics.

In layman's terms, photons are the building blocks of electromagnetic radiation: that is, a photon is a "particle" of light, although, according to quantum mechanics, all particles, including the photon, also have some of the properties of a wave.

Properties
All electromagentic radiation, from radio waves to gamma rays, is quantised as photons: that is, the smallest amount of electromagnetic radiation that can exist is one photon whatever its wavelength, frequency, energy, or momentum. Photons are fundamental particles. Their lifetime is infinite, although they can be created and destroyed. Photons are commonly associated with visible light, which is actually only a very limited part of the electromagnetic spectrum. Photons have zero invariant mass but a definite finite energy. Because they have energy, the theory of general relativity states that they are affected by gravity, and this is confirmed by observation.
http://en.wikipedia.org/wiki/Photon

 

[MRC_Hans]

Mr. Hans,

Does it mean that science is bit unclear in understanding the photons?

No, it means that I am a bit unclear in understanding photons. You see, I'm not a quantum scientist.

You cannot say that the understanding of photons is unclear, beause photons are defined by science, but obviously, QM is a very complex field, and not everything about it is understood.

Hans

 

[Kumar]

No, it means that I am a bit unclear in understanding photons. You see, I'm not a quantum scientist.

You cannot say that the understanding of photons is unclear, beause photons are defined by science, but obviously, QM is a very complex field, and not everything about it is understood.

Hans

"QM is a very complex field, and not everything about it is understood."

Yes, few concepts are still pending to understand.

I edited & added some details about photons in my previous post.