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Electric fields , acceleration

  1. May 28, 2013 #1
    hello , could someone please remind me I have forgotten a few things.

    So the electric field is perpendicular to the conductor and the flow of charge.
    So if i have a pipe and in the pipe flows a conducting gas or a plasma , now around the pipe I have a simple copper wire wound in many many turns like a tesla coil. Now by giving impulses the higher up the coil the stronger the impulse gets by voltage and an electric field is proportional to voltage , now we also know that electric fields can accelerate charged particles , would such an impulse driven into a coil that is wound around a pipe in which a plasma is injected would accelerate the plasma throughout the pipe because the flow of the plasma would be in the same direction as the electric field thus the field would be getting stronger and stronger by every inch because the voltage builds up in such a pulsed coil.

    Now is my thinking correct or not?

    Also if used the same coil but just dc (non impulse ) would it have any effect on the gas or whatever is inside the pipe or not? In terms of accelerating it?
    As long as I think it shouldn't.
  2. jcsd
  3. May 29, 2013 #2

    Simon Bridge

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    "by giving impulses"
    What sort of impulses and what to?

    You are sending a pulse of electric current down a wire, which is wound around a gas-filled pipe.
    The gas is a conductor.

    "the higher up the coil the stronger the impulse gets by voltage"
    If the "top" of the wire is more positive than the "bottom" then the top will have a higher voltage than the bottom. But you are sending a pulse - so this is not the DC case - so the pulse will just get weaker as it travels "up" the wire.

    "an electric field is proportional to voltage"
    The electric field is proportional to the gradient of the voltage.

    I think we can stop there - you will have enough now to reword your description.
  4. May 29, 2013 #3
    Reword or not I may have been a little confusing in the late hours of the night but the thought is simple , I want to understand something about how electric fields react with charged particles , conducting gasses or plasmas. in overall.

    Well imagine a simple tesla coil , it has a large induction coil with lots of turns of wire , they send a pulsed dc like the one that is being sent into smps transformers into the coil, now the voltage builds up as usually before the main coil they have a pulsed dc at around let's say 3kv but at the top there may be 25kv 50kv or more depending on the design and we all know that i guess.

    Now the thing i wanted to understand is if you would put a conducting gas or a plasma through the middle of the coil or the pipe it is wound onto , now the wire is perpendicular to the pipe to 90 degrees now but the electric field is also perpendicular to the conductor so in the case of the 90 degrees wire it would be parallel to the pipe itself and anything flowing into or through it right?
    Now it is is parallel to anything flowing through it like a gas in my case would it also accelerate the gas because of the fact that the field would be weaker at the beginning at the pipe and stronger at the end. If I understand correctly the electric field strength is dependent on the voltage , so in an induction coil the voltage is being raised so should be the field, but it ( the coil) only works in pulsed mode not in static dc , just like a cars ignition coil is driven by pulsed mechanical contacts( in older models)
    So how come the pulse get's weaker if it is made to get stronger sending it through the coil ( and yes by stronger I mean higher voltage lower amps)

    The other thing here would be now if I am right and the gas would be accelerated then how would it "behave" in the pipe because the electric field would get higher due to higher voltage but increase in voltage would also mean a decrease in current and hence the magnetic field is dependent on current it would get weaker right?
  5. May 29, 2013 #4

    Simon Bridge

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    "in overall" represents a couple of years of post-graduate study ... a bit outside what we do here. But I can respond to specific questions.
    Your description was not so much confusing as confused - you appeared to misunderstand some basic concepts in electromagnetism, leading to erroneous conclusions. I had hoped that you would use the corrected ideas to revisit your original example - the act of rewriting the description using the actual physics would have answered a lot of your questions.

    I don't know what you understand by this.
    Perhaps you could supply a reference to what you are talking about.

    I think you should decide if you mean a conducting gas or a plasma since they have different physics.
    OR if you, as I suspect, mean something more general - like some substance composed of many charges which is free to deform or something.

    The magnetic field does this.
    There are no free charges inside the coil to make an electric field - so where would it come from?
    Of course you are using a pulse - so maybe there's radiation? I'll have to check.
    Also the pulse could be very small compared with the circumference of the coil.

    You do not understand correctly - the electric field depends on the gradient of the voltage and not the voltage itself ... you'd have to change the overall shape of the pulse ... does the pulse get sharper as well as higher?

    A pulse travelling along a wire, all other things remaining equal, will lose energy.
    But you are talking about a tesla coil - all other things are not equal.

    Leaving aside the details for a moment - lets say you could align an electric field with the long axis of a tube of charges, and you could make the field stronger at one end than the other, and you pulsed the fieid.

    The particles would behave like water in a tub when you slosh it back and forth.
    The exact details depend on the timing and shape of the pulses - i.e. short pulses that travel along the length of the tube would be like running your hand up and down the water in a tub while pulses which apply to the whole tube at once would be like rocking the tub. The pulse being higher at one end than the other just means you rock the tub more towards one end than the other or push the water harder as you reach the end.
  6. May 31, 2013 #5
    Ok I usually have a tendency when I word a question I have the thought in mind but then i just mess up.
    in a coil i would get not only electric but also magnetic field which together is electromagnetic.
    The question then rather is is it possible to get only electric field and use it for accelerating charged particles that have been under pressure already when they were put into that tube but under pressure is not enough acceleration is also needed.

    In other words , wherever you control charged particles or whatever with a electromagnetic field you have to use alot of current to make the field strong and that requires pretty big power supply and consumption.But an electric field also affects charged particles or conductive gas or whatever is there but the field is dependent on the gradient as you say of voltage so is it possible to accelerate particles in a tube with electric field ?
    I guess the problem would be that an electric field also comes with charge flowing and magnetic field which would create an opposite effect in the coil around the tube if the electric field would want to accelerate things that are parallel to it then the magnetic field would try to slow them down because the B field is perpendicular to the E field so I would literally have to cancel the one to get the effect from the other right?
    It would be great to use only the electric field as it requires not current but voltage to be strong.

    Excuse me for you probably expect a quite more intelligent question from me but im a little confused about this.
  7. May 31, 2013 #6

    Simon Bridge

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    It is possible to accelerate charged particles with an electric field.
    You can do this despite the associated B field. Gas discharge tubes work this way - exploiting the photon emission when the atoms recombine. Electrons and protons get pulled in opposite directions by the electric field gradient, when the field is relaxed they recombine.

    A common way to generate an electric field is to just use two plates - the shape of the plates shapes the field.

    A Tesla coil can generate very high voltages, but you realize that it is actually two coils right?
    The one normally located on top is toroidal. It makes it difficult to figure out what you mean in your earlier posts. Have you ever built one of these?
  8. Jun 1, 2013 #7
    well as long as tesla coils , i was thinking about the main coil , i know it has two circuits, the primary which hits the spark gap and then the secondary which is usually wound on the vertical pipe which steps up the voltage many times because of induction.
    I was thinking about this pipe , well maybe you could explain what is the difference between just two metal plates at a certain dc potential and a gas between them and a coil like the one i just talked and a gas inside it.
    Wouldn't the gas get accelerated differently in the pipe coil setup than in the metal plates one? Not to mention that the coil around a pipe would use a dc pulse just like a tesla coil uses , because as long as I know a spark gap (the old way) or semiconductor sent dc pulse makes induction in the coil and in such a case the E field isn't as strong at the input of the coil as it is at the output right?

    now with the plates there would be no use of pulsing and the E field would be uniform in every area of the plate right?,

    well my scenario is more like you take a compressed gas that has been under high pressure then let it escape through a pipe that has a coil around it and the coil's job is to accelerate the already fast going compressed gas , is that even realistic?

  9. Jun 1, 2013 #8

    Simon Bridge

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    The geometry of the fields around a tesla coil is not simple. You'd have to crunch the numbers for a specific situation.

    The plates let you work with the electric field without worrying about the magnetic one so much. The geometries can be as simple as you like too.

    That depends on how you wanted the gas to get shoved about.

    depends on the geometry of the plate and what you needed the E field to do.

    So I think this is the crux of the matter - what is it that you are hoping to achieve?
  10. Jun 2, 2013 #9
    Well what is it that I am trying to achieve is a hard question even for me myself but for now it's just a theoretical model in my head let's just say that there is a source of compressed (highly) gas and it needs to be "channeled" through small pipes , many ones because then it would be easier to confine the gas and stop it from touching the pipe walls , loosing speed etc.

    So I though of electric fields as the ones that would do in confining the gas + accelerating it to even higher velocities than a simple compressed gas through a small pipe could achieve.
    Electric fields instead of magnetic ones because as I said earlier if my reasoning is correct electric field would be strong enough without requiring a large power consumption as in the magnetic field case. Am I right?

    Also Simon if we have gone this far I would like to ask would a gas feel pressure exerted on it if the gas would be between two spheres that would be charged to a high but the same potential like +. Like sphere one (biggest) then comes the gas then sphere two (inner one) but I guess in the second sphere there would be no electric field as it would forma sort of faraday cage , so the only use of e field confinement is when you put something between two charges spheres not in a charged sphere right?
  11. Jun 2, 2013 #10

    Simon Bridge

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    Oh I see - you want to look up electromagnetic bottle and electromagnetic containment.
    If you open the bottle, or container, at one end you get a nozzle that gas can squirt through.

    You also get this sort of thing in particle accelerators.

    Gas confined by an electromagnetic field does experience pressure - just like your fingers d when you try to push two north poles together.
  12. Jun 2, 2013 #11
    So two spheres both at the same +v potential and a gas between them would exert a pressure (electric field) on the gas ?

    By the way electromagnetic bottle shows up googles first page on results that consist of some foreign company products or myth debunking or conspiracy theory sites of the "project rainbow" Philadelphia thing. :D
    Last edited: Jun 2, 2013
  13. Jun 2, 2013 #12

    Simon Bridge

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    You could describe the effect on the bulk (charged) gas in terms of a pressure distribution on the gas.

    The anode and cathode do not have to be spheres.
    In your case, it is not helpful if they are.

    Google tailors search results according to your search history ;)
    Mine gives a bunch of scientific papers first.

    Start with:
    ... with the usual caveats about wikipedia.
  14. Jun 3, 2013 #13
    ""Google tailors search results according to your search history ;)""

    Good one there Simon :D:D But not my case because i frequently clear my search history and cookies so that my browser is clean of unnecessary information.
    I think it's rather the internet is full of crap.

    Well you have been quite a good help for me but I still am open to this one question and I think I know the answer myself but just in case , what does a gas experience when placed between two , let's say plates that are at the same polarity and potential above ground, like two metal plates at for example +100v dc and a gas between them, would the gas be repelled from the plates?
    Now if it would I guess not the same goes for the other state of gas -plasma then the ions would run into the - charged plates right?
    Last edited: Jun 3, 2013
  15. Jun 3, 2013 #14

    Simon Bridge

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    That's right - the the ions get attracted to the opposite charged plate.

    There's a cute experiment you can do with two plates kept at a constant potential difference and you let a pendulum (conducting bob on insulating string) swing between them ... if the bob touches one plate it'll fly to the other one, hit it, and get yanked back - repeat as long as the power is switched on.

    If you had two positively charged plates then they would repel the positive charges in the gas - but those ions would get squeezed out the sides. You could make a positive charged box - but the field inside is zero - you may as well just have a neutral pressure container. You have to be tricky about electromagnetic confinement.

    note: I saw the Philadelphia project stuff about hit #9 and hit #10 had something to do with MK-Ultra and UFOs. The internet is, indeed, full of rubbish.
  16. Jun 3, 2013 #15
    Ok but when I would make a positively charged box the field inside would be zero but then make another box bigger than the first one and put it around the smaller first one now charge them at the same + potential and put a gas between them , now the field between them isn't zero anymore right? It would be the same as the spheres I was talking about.

    Now an interesting question , is there any formula which I could use to get the approximate pressure I could sustain off the walls of the boxes to the voltage needed for the field to be of sufficient strength to hold the gas "confined"
  17. Jun 3, 2013 #16

    Simon Bridge

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    The field between them is just the field due to the inner box - you may as well not have the outer one.

    The pressure of an ideal gas trapped in an EM bottle is determined the same as if it were trapped in a regular bottle. You work out the (classical) volume off the charge and the kinetic energy. Where the potential energy exceeds the kinetic energy is the "wall".
  18. Jun 6, 2013 #17
    I was thinking of making a new post but that would be a waste of resources as the question is from "this series"

    Now I have attached an image and the question goes like in which cases the electric field is outside the conductor and in which cases on both sides?
    Now the conductor in this case is a pipe with an open middle and the red color is a gas inside of it.

    Now in the case B the field is only outside , the inside is a faraday cage type of structure as it blocks the field there , but how about case A ? where the side in the conductor pipe is open at one point?

    Also how about the more complicated structure at the other photo ? Is the field on both sides of it too or not? So would the gas feel a pressure in the inside of it.

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  19. Jun 8, 2013 #18
    Simon? :)
  20. Jun 8, 2013 #19

    Simon Bridge

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