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Thread: cathode ray tube

  1. #1 cathode ray tube 
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    Can somebody please explain what actually happens in cathode ray tube? Is there any current passing through the tube? How does the anode (positive electrode) lost electrons? Is it before the effect of cathode rays? How does the cathode (negative electrode) have bigger amount of electrons? Is it before the whole process?


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  3. #2  
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    http://en.wikipedia.org/wiki/Cathode_ray_tube

    I just typed in "cathode ray tube wiki" the answer is in front of you....


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    Quote Originally Posted by Megabrain
    http://en.wikipedia.org/wiki/Cathode_ray_tube

    I just typed in "cathode ray tube wiki" the answer is in front of you....
    Actually, I couldn't find my answer.
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  5. #4  
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    The anode has a high positive potential, the cathode is covered in strontium (rich in free electrons) and then heated, a cloud of liberated electrons forms close to but above the cathode. The anode then attracts these and they form a stream (so a current does travel between them) electromagnets then deflect the beam to enable it to strike the screen at the desired place.

    If you want to study it in more detail you need to research 'thermionic emission' wiki is bound to have something on it. thermionic valves, crt's klystrons all work on this principle.
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    Quote Originally Posted by Megabrain
    The anode has a high positive potential, the cathode is covered in strontium (rich in free electrons) and then heated, a cloud of liberated electrons forms close to but above the cathode. The anode then attracts these and they form a stream (so a current does travel between them) electromagnets then deflect the beam to enable it to strike the screen at the desired place.

    If you want to study it in more detail you need to research 'thermionic emission' wiki is bound to have something on it. thermionic valves, crt's klystrons all work on this principle.
    And what is the high voltage for?
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  7. #6  
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    The high voltage is there to increase the electrostatic field density thus enticing the eletrons to leave the nice warm cathode they hover above.
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    Quote Originally Posted by Megabrain
    The high voltage is there to increase the electrostatic field density thus enticing the eletrons to leave the nice warm cathode they hover above.
    And it is just voltage, isn't any current from the alternative source?
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  9. #8  
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    Quote Originally Posted by scientist91
    Quote Originally Posted by Megabrain
    The high voltage is there to increase the electrostatic field density thus enticing the eletrons to leave the nice warm cathode they hover above.
    And it is just voltage, isn't any current from the alternative source?
    Voltage causes current. They're part of the same thing.
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  10. #9  
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    You are incorrect, a voltage applied to a resistance, impedance or reactance, will cause a current to flow.

    Voltages can and do exist without currents flowing.

    A current flow (ie movement of electrons) is neccessary to cause an emf resulting in a potential difference (voltage to you!).
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    Quote Originally Posted by Megabrain
    You are incorrect, a voltage applied to a resistance, impedance or reactance, will cause a current to flow.

    Voltages can and do exist without currents flowing.

    A current flow (ie movement of electrons) is neccessary to cause an emf resulting in a potential difference (voltage to you!).
    But what is the role of current (just additional electrons flowing), couldn't the ions of the gas go into the proper electrode (protons to cathode, electrons to anode)?
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  12. #11  
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    my dear boy the tube is a vacuum, what flows is an electron stream, not ions or kettles just electrons, these are accelerated to a high velocity and strike the phosphor coating on the inside of the glass, the kinetic energy of the electron becomes a photon which then hurtles off towards your eye!

    Now I don't mind answering the odd question but you pose too many, you don't debate, and you don't appear to be able to do even the most simple of searches on wiki or read the links that others use. If you ask any more questions I will not give you detailed answers but refer you to wiki.
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    Quote Originally Posted by Megabrain
    my dear boy the tube is a vacuum, what flows is an electron stream, not ions or kettles just electrons, these are accelerated to a high velocity and strike the phosphor coating on the inside of the glass, the kinetic energy of the electron becomes a photon which then hurtles off towards your eye!

    Now I don't mind answering the odd question but you pose too many, you don't debate, and you don't appear to be able to do even the most simple of searches on wiki or read the links that others use. If you ask any more questions I will not give you detailed answers but refer you to wiki.
    Sorry, man. I read my textbook. It says that positive ions striking the cathode produce cathode rays. I don't know what to think anymore.
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    If you can scan the page in and print it on the screen I'd be happy to comment but it's really hard to imagine that anyone other than a scotsman (JLB) would come up with such an absurdity. Sounds like you are mixing crt's up with electrolysis where ions do move to the cathode.
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    Quote Originally Posted by Megabrain
    You are incorrect, a voltage applied to a resistance, impedance or reactance, will cause a current to flow.

    Voltages can and do exist without currents flowing.

    A current flow (ie movement of electrons) is neccessary to cause an emf resulting in a potential difference (voltage to you!).
    My bad. I didn't mean that a voltage invariably implies a current. I just wanted scientist91 to stop reifying all these linked concepts as though they were separate, detachable entities.

    Scientist91, perhaps before we attempted to address any more of your queries, you could give us an idea of what your level of understanding is regarding physics and electronics? It might help us better tailor our responses to your needs.
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    Here is the picture.

    Note: Where, there is "excited", it should be "accelerated". Sorry for the mistake and the bad quality.
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  17. #16  
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    errr yes... just one thing, that is not the usual representation of a crt,

    just to make sure we are talking on the same lines do you have a picture of the device?

    A typical monochroome crt has two heater connections, a Kathode, control grid, focus electrode and 2 or three anodes so it has a lot more than 2 connections.

    Here's a page showing many different types of crt's

    http://www.sphere.bc.ca/test/crts.html
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    Quote Originally Posted by Megabrain
    errr yes... just one thing, that is not the usual representation of a crt,

    just to make sure we are talking on the same lines do you have a picture of the device?

    A typical monochroome crt has two heater connections, a Kathode, control grid, focus electrode and 2 or three anodes so it has a lot more than 2 connections.

    Here's a page showing many different types of crt's

    http://www.sphere.bc.ca/test/crts.html
    Yes, there are two more pictures.
    But still can't understand the whole process step by step. It seems like messed up. Ions, electrons, electrodes.
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  19. #18  
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    ok what you seem to be showing there is a very simplified example of the most primitive or earliest example of crt, called a Maltese cross crt


    http://fas.harvard.edu/~scidemos/Qua...eCrossCRT.html

    Try that for a minute and see if it makes sense. and here is one in action.


    http://uk.youtube.com/watch?v=Xt7ZWEDZ_GI
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    Quote Originally Posted by Megabrain
    ok what you seem to be showing there is a very simplified example of the most primitive or earliest example of crt, called a Maltese cross crt


    http://fas.harvard.edu/~scidemos/Qua...eCrossCRT.html

    Try that for a minute and see if it makes sense. and here is one in action.


    http://uk.youtube.com/watch?v=Xt7ZWEDZ_GI
    So the collision of the electrons, cause the cathode rays, right? And what happens with the protons, like it is stated in my textbook? I can't understand what they want to say with the neutral molecules, are the neutral molecules, the molecules of the gas inside the tube? What is the role of the gas?
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  21. #20  
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    There is no gas in a crt otherwise the electrons would strike the gas, ionise it and never reach the screen, if your book says there is gas in a crt or that ions are flying around in it then throw the book away!

    Electrons leave the cathode and are attracted to the anode by the high voltage there causing an electrostatic field. the electrons strike the phosphor atoms which are coated on the glass screen and cause the screen to emit photons.

    Please believe me there is no gas in a crt tube and it is only electrons that are fired from the electron gun.

    Protons do not leave either the cathode or anode in normal operation that would mean the physical mass would change.
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  22. #21  
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    Megabrain, the Wikipedia article that you posted the link to earlier says this
    The electron gun accelerates not only electrons but also ions present in the imperfect vacuum (some of which result from outgassing of the internal tube components). The ions, being much heavier than electrons, are deflected much less by the magnetic or electrostatic fields used to position the electron beam. Ions striking the screen damage it; to prevent this the electron gun can be positioned slightly off the axis of the tube so that the ions strike the side of the CRT instead of the screen. Permanent magnets (the ion trap) deflect the lighter electrons so that they strike the screen. Some very old TV sets without an ion trap show browning of the center of the screen, known as ion burn. The aluminum coating used in later CRTs reduced the need for an ion trap.
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  23. #22  
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    The ratio is so small as to be negligible, outgassing was a problem in early sets due to poor quality material. The theory of operation does not include ions, which is only a problem for a crt designer, the book mentioned above seems to have given the impression that ions are at least a significant part of the operation. Since they are dealing with a maltese cross crt (which has no iontrap) I did not feel the need to go that deep, but yes I do recognise it. As the last line of my previous post indicates that "Protons do not leave either the cathode or anode in normal operation ". Outgassing and 'cathode stripping' are not the norm.

    I guess it's down to how deep he wants to go but from his previous questions I think he is a relative beginner in which case he is reading the wrong book and the book is wrong to overcomplicate the Maltese cross crt

    But thanks for that I had not rea the article! I assumed I knew what it would say! (but assume can make an ASS out of U and ME :wink:
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  24. #23  
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    Quote Originally Posted by Megabrain
    The ratio is so small as to be negligible, outgassing was a problem in early sets due to poor quality material. The theory of operation does not include ions, which is only a problem for a crt designer, the book mentioned above seems to have given the impression that ions are at least a significant part of the operation. Since they are dealing with a maltese cross crt (which has no iontrap) I did not feel the need to go that deep, but yes I do recognise it. As the last line of my previous post indicates that "Protons do not leave either the cathode or anode in normal operation ". Outgassing and 'cathode stripping' are not the norm.

    I guess it's down to how deep he wants to go but from his previous questions I think he is a relative beginner in which case he is reading the wrong book and the book is wrong to overcomplicate the Maltese cross crt

    But thanks for that I had not rea the article! I assumed I knew what it would say! (but assume can make an ASS out of U and ME :wink:
    Ok, I also want to ask you, how the metal bars (the electrodes) are turned into anode and cathode. How we put excess of electrons in the cathode, and how lack of electrons, in the anode?
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  25. #24  
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    As I said the cathode is made from a material (or coated in a material) which has 'free electrons' THis does not mean it has more than it should have at rest, only that electrons from it's atoms can easily be drawn away. In order to get a surplus or a shortage of electrons you need only apply a voltage. The positive terminal will be defficient and the negative will have a surplus, think of a battery, when you charge it the applied voltage causes a current to flow this 'moves' the electrons to the negative terminal.
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    Quote Originally Posted by Megabrain
    As I said the cathode is made from a material (or coated in a material) which has 'free electrons' THis does not mean it has more than it should have at rest, only that electrons from it's atoms can easily be drawn away. In order to get a surplus or a shortage of electrons you need only apply a voltage. The positive terminal will be defficient and the negative will have a surplus, think of a battery, when you charge it the applied voltage causes a current to flow this 'moves' the electrons to the negative terminal.
    So the current pushes the electrons from the anode and they travel to the cathode (so the cathode will have excess of electrons and the anode less electrons), right?
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  27. #26  
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    Current flows into the positive terminal of the power supply (or battery) through the battery, out of the negative terminal and along the wires to the cathode, they are then attracted (through electrostatic attraction) through the vacuum towards the anode, strike the anode to cause photons to be emitted and then flow along the wire back to the positive of the power supply. That is the complete path the electrons take in the Maltese cross crt.
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    Quote Originally Posted by Megabrain
    Current flows into the positive terminal of the power supply (or battery) through the battery, out of the negative terminal and along the wires to the cathode, they are then attracted (through electrostatic attraction) through the vacuum towards the anode, strike the anode to cause photons to be emitted and then flow along the wire back to the positive of the power supply. That is the complete path the electrons take in the Maltese cross crt.
    Ok, but how the anode (positive terminal), lost electrons, so it became positive?
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  29. #28  
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    Because it is connected to the power supply positive, you must accept this for now and believe it later when you have more knowledge you will understand better.
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    Quote Originally Posted by Megabrain
    Because it is connected to the power supply positive, you must accept this for now and believe it later when you have more knowledge you will understand better.
    The electrons are attracted by the field of the power supply positive pole?
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  31. #30  
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    For simplicity's sake yes.
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  32. #31  
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    Quote Originally Posted by Megabrain
    For simplicity's sake yes.
    Ok, thank you very much for the help. I very appreciate your efforts to help me. Thanks again.
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  33. #32  
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    Ok, I am glad we got there in the end!
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