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Author Topic: Can an electromagnetic field make photons?  (Read 6786 times)

Offline jaiii

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Can an electromagnetic field make photons?
« on: 11/11/2013 13:45:16 »
Hello.

How to create a photon source using electromagnetic field?

Thank you.
« Last Edit: 11/11/2013 19:21:40 by chris »


 

Offline Pmb

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Re: Photons
« Reply #1 on: 11/11/2013 13:51:45 »
Quote from: jaiii link
How to create a photon source using electromagnetic field?
I don't understand your question. Can you rephrase it for me please? Since a photon is a quantum of the EM field I don't know what you mean by create a photon source using electromagnetic field?

Until I hear your response you can take a look at
http://en.wikipedia.org/wiki/Synchrotron_radiation
 

Offline distimpson

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Re: Photons
« Reply #2 on: 11/11/2013 19:04:17 »
My best guess as to the question,
"An electromagnetic field (also EMF or EM field) is a physical field produced by electrically charged objects."  "Classically, electric and magnetic fields are thought of as being produced by smooth motions of charged objects. For example, oscillating charges produce electric and magnetic fields that may be viewed in a 'smooth', continuous, wavelike fashion." "Experiments reveal that in some circumstances electromagnetic energy transfer is better described as being carried in the form of packets called quanta (in this case, photons) with a fixed frequency."
http://en.wikipedia.org/wiki/Electromagnetic_field

From a lab point of view, if you have a light source and a photo-multiplier tube detector, at high light levels the detector output will be continuous (analog), if you block the light using a pinhole and get to a low enough level the output will be a series of pulses (digital).

The bremsstrahlung process is interesting, accelerating a charged particle and smashing into a target:
"The spectrum has a sharp cutoff at low wavelength, which is due to the limited energy of the incoming electrons. For example, if an electron in the tube is accelerated through 60 kV, then it will acquire a kinetic energy of 60 keV, and when it strikes the target it can create X-rays with energy of at most 60 keV, by conservation of energy. (This upper limit corresponds to the electron coming to a stop by emitting just one X-ray photon. Usually the electron emits many photons, and each has an energy less than 60 keV.) A photon with energy of at most 60 keV has wavelength of at least 21 pm, so the continuous X-ray spectrum has exactly that cutoff, as seen in the graph."

http://en.wikipedia.org/wiki/Bremsstrahlung

Electron-positron annihilation is another way to make a couple of photons. As with pmb, not sure where to start or stop here.

I used to say an accelerated charge radiates, still studying this, maybe non-uniform proper acceleration?

Quotes are from material in the links.
 

Offline jaiii

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Re: Can an electromagnetic field make photons?
« Reply #3 on: 12/11/2013 12:44:38 »
Thank for replies.

I vaguely expressed.
I would need a source of photons with a frequency greater than the frequency of light.

Thank you.
 

Offline JP

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Re: Can an electromagnetic field make photons?
« Reply #4 on: 12/11/2013 14:56:34 »
I would need a source of photons with a frequency greater than the frequency of light.

Why would you need this?  Such sources tend to be quite dangerous because those frequencies of light can damage tissue and cause cancer in the long-term. 
 

Offline alancalverd

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Re: Can an electromagnetic field make photons?
« Reply #5 on: 12/11/2013 15:10:29 »
You can generate x-rays (i.e. photons with higher frequency than visible light) by bending an electron beam in a magnetic field. Not easy to do on a small scale. Depending on the required photon flux and energy I can probably build you a synchrotron about the size of a large car, for about $5M.
 

Offline evan_au

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Re: Can an electromagnetic field make photons?
« Reply #6 on: 12/11/2013 15:49:42 »
The cheapest and simplest source of photons at higher than visible frequencies are the Ultraviolet "Black Light" lamps that were once popular in discos and theme park rides. These tubes can be installed in a normal fluorescent light. For short exposures, they make your white socks glow.

However, long exposures at close range could be dangerous - they could cause sunburn, increase your risk of skin cancer and cause cataracts.

Can you be more specific - what do you want to achieve with this short wavelength source?
 

Offline JP

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Re: Can an electromagnetic field make photons?
« Reply #7 on: 12/11/2013 15:53:11 »
The cheapest and simplest source of photons at higher than visible frequencies are the Ultraviolet "Black Light" lamps that were once popular in discos and theme park rides. These tubes can be installed in a normal fluorescent light. For short exposures, they make your white socks glow.

Or pulling scotch tape apart (in a vacuum): http://en.wikipedia.org/wiki/Triboluminescence
 

Offline jaiii

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Re: Can an electromagnetic field make photons?
« Reply #8 on: 12/11/2013 17:43:56 »
I would like these photons as propulsion for spacecraft using rotating magnetic field.
 

Offline evan_au

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Re: Can an electromagnetic field make photons?
« Reply #9 on: 12/11/2013 20:27:24 »
You can certainly use photons for spacecraft propulsion - each photon has a small momentum which can produce an accelerating force for a spacecraft.

The challenge here is that generating each photon requires energy - and you might need the output of a nuclear power plant to produce enough energy to produce a reasonable thrust from the photons. This combination does not give you a good power-to-weight ratio.

It may be better to generate the photons from a source which is not on the spacecraft, eg the Sun, or ground-based lasers. There are some solar sail experiments underway now to evaluate the effectiveness of this method.
 

Offline Pmb

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Re: Can an electromagnetic field make photons?
« Reply #10 on: 13/11/2013 17:15:12 »
I would like these photons as propulsion for spacecraft using rotating magnetic field.
There's really no such thing as a rotating magnetic field. All too often people make the mistake of thinking that electric and magnetic fields move, and they can't. This misunderstanding comes from the fact that there are sometimes useful to think of the fields as moving such as in visualizing magnetic field lines "cutting" across a conductor.

To see how you can get into trouble with moving field lines think of a rotating magnet. In the past some physicist took this moving field lines literally and tried to apply it to the action of the field of a rotating magnet. This implies that at certain distaces from the magnet the magnetic field would be moving faster than the speed of light. One astrophysicist thought that meant that the action of the rotating magnet didn't apply to distances that far away and called that distance the "light cylinder". He was trying to apply this to the magnetic field of a rotating neutron star. When one trieds to calculate the divergence of the field when one attempts to do it this way an error resuts.
 

Offline spartaman64

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Re: Can an electromagnetic field make photons?
« Reply #11 on: 15/11/2013 03:01:31 »
Im guessing you need einstein's photoelectric effect
 

Offline SimpleEngineer

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Re: Can an electromagnetic field make photons?
« Reply #12 on: 15/11/2013 10:07:08 »
I would like these photons as propulsion for spacecraft using rotating magnetic field.
There's really no such thing as a rotating magnetic field. All too often people make the mistake of thinking that electric and magnetic fields move, and they can't. This misunderstanding comes from the fact that there are sometimes useful to think of the fields as moving such as in visualizing magnetic field lines "cutting" across a conductor.

To see how you can get into trouble with moving field lines think of a rotating magnet. In the past some physicist took this moving field lines literally and tried to apply it to the action of the field of a rotating magnet. This implies that at certain distaces from the magnet the magnetic field would be moving faster than the speed of light. One astrophysicist thought that meant that the action of the rotating magnet didn't apply to distances that far away and called that distance the "light cylinder". He was trying to apply this to the magnetic field of a rotating neutron star. When one trieds to calculate the divergence of the field when one attempts to do it this way an error resuts.

So if you rotated a magnet, the field wouldn't move? Sound like dodgy science to me, I can rotate a bar magnet as many times as  i like and I dont break the laws of physics.. the north pole can be pointed south, and its still the north pole of the magnet, the south pole can be pointed east, and its still the south pole.. and my junior school experiment with iron filings show that the layout of the magnetic field changes as i rotate it..

Does my junior school magnet experiment show that magnetic fields can move? Plus I worked on generators for long enough to know that moving the magnetic forces through conductors has been a solid way of making electricity for quite some time.

I have rotating magnetic fields.. I feel sorry for those who dont.

But I am willing to accept that the motion of the field does not correspond to classical theories of motion
 

Offline jaiii

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Re: Can an electromagnetic field make photons?
« Reply #13 on: 16/11/2013 15:26:03 »
Hello.

I want to use rotating magnetic fields, therefore, that the working fluid (gas, ions, Photon) spun and created a centripetal force.

By.
 

Offline yor_on

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Re: Can an electromagnetic field make photons?
« Reply #14 on: 17/11/2013 12:02:03 »
Interesting stuff. What happens then when we rotate a bar magnet? As Pmb poits out, either the 'field' created reach out indefinitely and then, assuming that it is 'created' (action and reaction/classical locality) through a bar magnets orientation, must do ftl at some circumference. Or it is not unlimited reach, and then the 'cut of' should be before the rotation/circumference reaches 'c'. Or its orientation 'light up a pre-existing field' :) But it will make electricity, as SE points out?
 

Offline acsinuk

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Re: Can an electromagnetic field make photons?
« Reply #15 on: 05/12/2013 09:59:01 »
If scientists cant work out how a rotating magnet causes an electric current to flow in a conductor then we need some new physics to prioritize the conservation of the balance of electric charge within a set volume of magnetized space!
CliveS
 

Offline A Davis

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Re: Can an electromagnetic field make photons?
« Reply #16 on: 08/12/2013 00:39:50 »
Faraday observed the laws of Science, why a rotating magnetic field induces an electric field isn't really known, one accepts that it does, and use the result in electric motors, transformers, radios etc. A light bulb creates photons from electricity but why it should act like a white body radiator is again another fact of science.
 

Offline Pmb

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Re: Can an electromagnetic field make photons?
« Reply #17 on: 08/12/2013 02:50:11 »
Quote from: A Davis
Faraday observed the laws of Science, why a rotating magnetic field induces an electric field isn't really known,....
Whoa there. Where did you get the idea that nobody knows why a rotating magnet (or rotating magnet field) induces an electric field? I know why it does as do many other physicists. I'll explain why later. I'm curious as to why you think this first.

Also there's no such thing as a rotating magnetic field.
 

Offline evan_au

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Re: Can an electromagnetic field make photons?
« Reply #18 on: 08/12/2013 10:47:55 »
Quote
there's no such thing as a rotating magnetic field.
I think that the arrangement of macroscopic coils inside a synchronous motor, when fed by a three-phase sinusoidal voltage, is able to produce a reasonable facsimile of a smoothly rotating magnetic dipole (or quadrupole or higher-order rotating magnetic fields).

Satellite designers sometimes use magnetic torquers to control the orientation of a satellite in low Earth orbit; its not obvious how you could use such fields to provide a linear force for a spacecraft, even when a suitable external magnetic field is available (eg the magnetic field of Earth, Jupiter, the Sun or the Galaxy).
« Last Edit: 08/12/2013 19:41:53 by evan_au »
 

Offline A Davis

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Re: Can an electromagnetic field make photons?
« Reply #19 on: 09/12/2013 01:01:04 »
I know that e=B L v and have done calculations using the equation, Maxwell took this and other results to produce his wave equation. When one asks oneself what is a magnetic field and why is it different from an electric field, I can only use maths to give an answer, I've never seen either of them only the effects they produce and why can't one exist without the other, no E without  H . I decided a long time ago to accept that this is how nature works and it's strange and beautiful.
 

Offline acsinuk

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Re: Can an electromagnetic field make photons?
« Reply #20 on: 11/12/2013 17:28:07 »
Yes, it is strange and beautiful and I accept that to have energy we must have E volts per metre at right angles to an area of flux H per square metre of magnetized space.
This magnetism is normally produced by a loop of current in an alternator making 3D electricity.
 One could argue that magnets can exist quiescently and capacitors could hold a static DC voltages but if they move with respect to each other then we will have energy.
CliveS
 

Offline A Davis

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Re: Can an electromagnetic field make photons?
« Reply #21 on: 12/12/2013 00:51:51 »
I did digress from the post, an electric rocket already exists and is known as an ION rocket, a gas is ionised and accelerated using an electron gun small ones are used in satellites. I don't know if one can build one big enough for Jaiii. There is still a magnetic field inside a capacitor acsinuk one cannot remove the stored electrons magnet field, they do align.
« Last Edit: 12/12/2013 00:56:07 by A Davis »
 

Offline acsinuk

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Re: Can an electromagnetic field make photons?
« Reply #22 on: 13/12/2013 15:27:26 »
Interesting!  Conceptually, I can imagine that the electrons in a fully charged capacitor are dormant and aligned magnetically. However, to produce energy the current flow must produce a magnetic loop flux to move at right angles to the voltage field. There is no problem for jaiii to fire charged gas ions from a satellite to correct its position as Newtons laws of action and reaction force being equal and opposite will apply.
CliveS
 

Offline A Davis

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Re: Can an electromagnetic field make photons?
« Reply #23 on: 15/12/2013 20:01:22 »
I like the word interesting, thank you acsinuk. The current flow occured when the capacitor was charged, energy was being produced then.
 

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Re: Can an electromagnetic field make photons?
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