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Interesting youtube video

  1. Aug 15, 2013 #1
    Ok so I came up to a very interesting and both practical youtube video.Now I must say i understand by the description and the video how it basically works , so the question is not as much about the workings of this electrostatic phenomenon as rather, can somebody please draw a equivalent electrical schematic to the device seen in the video and post it in this thread , like with capacitors etc where they are needed, as this device with the cans looks kinda similar to an LC series circuit or something like that , Ok any commentary much appreciated.

    By the way here is the link to the video

     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Aug 15, 2013 #2

    sophiecentaur

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    Pity there are hardly any CRT TVs around any more. (Not really!!!)

    What is happening there is that the charge on the front of the screen causes the right hand can to become negatively charged. The tab becomes polarised (some electrons are repelled from the right had can and they end up on the left hand side - leaving the right hand side positively charged) and it is attracted to that can. Once it touches the can, it becomes negatively (net) charged because the positive charges are canceled by some of the electrons from the screen. It is then repelled from that can and attracted to the grounded can. When it hits the grounded can, the surplus electrons flow onto the can and the tab is, once more, attracted to the negative can. And so on. A current is flowing, in tiny bursts, to discharge the screen.
    There is not a simple analogy with an LC circuit because the oscillation is not SHM. The time for the cycle will depend mainly upon the mass of the tab and the strength of the field between the two cans. The fact that the tab bounces off the cans will raise the frequency somewhat.
     
  4. Aug 15, 2013 #3
    Why is that a pity ? Well aside from the fact they use more materials to make and are more dangerous after used up , they are kinda cool , ofcourse as long as it is an old tv set which you watch some hours in the evening not the computer screen (God I hated the computer CRT's :D, one year was enough , I'm glad it was also the time LCD came around in pretty cheap prices)


    Ok speaking about the CRT and those cans , I don't get something , well yes I know that in a CRT the screen is the anode and the electron lamp which accelerates the electrons is the cathode so we could say the screen is negatively charged , hence also the foil placed infront of it is at the same charge , but then there rises the question why does the current flow through those cans if one can which is connected to the tv is negatively charged and the other can is roughly speaking grounded?
    How do you get a current flow from - to ground which basically in dc should be both at the same potential?
    Ok one chance that comes to mind is that the filament emitted cathode accelerated electrons which have some (how much was it about 25, 30kv?) of potential strike the screen and leave some leftover charge on the screen which the big anode wire attached to the back of the tube where the lead coating is just cannot "suck" away , so the potential on the screen is abit higher than that of a human body or ground.So there is some charge that can flow to the ground via the cans? Is that the case?

    Also I would like to ask , can we consider this buildup charge on the front end of the screen and on the foil attached to it as a dc voltage at a given potential?
    If so then that's why I asked about the analogy with the LC circuit , as in a LC circuit if you connect it to a steady dc supply you only get like one half cycle and then the oscillations die out and never continue because the dc supply keeps everything steady and doesn't allow for that so we use some switches with given frequency to give the circuit a boost at the right moments , but if this setup works the way it does I thought maybe there is an analogue which I could build from some radioparts which would do the same thing - oscillate with a steady dc supply without any mechanical or electrical semiconductor switches? Just wondering .
     
  5. Aug 15, 2013 #4

    sophiecentaur

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    It's only a pity that more people can't do this experiment any more!
    You have things the wrong way round about anode and cathode. The cathode of a CRT is where the electrons initially come from ("cathode rays"???) There is a high voltage positive connection just behind the face of the screen that acts as an anode. When the set is turned off (as mentioned in the video), that high voltage is removed and some electrons just hang around on the screen and, afaik, the glass is polarised - leaving negative charges on the surface at a relatively high negative potential with respect to ground.
    Some of this (negative) charge gets to the foil and can find its way, in 'spoonfulls' via the tab and the two cans, to ground.

    There are many circuits which let charge through, in pulses, from a DC supply. It's what most electronics does, in fact (DC power in and AC /sound/ etc power out). Take a simple solid state or even mechanical buzzer, for instance. The metal tab is just a mechanical way of achieving the same thing.
     
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