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Charging two plates to the *same* charge, using a voltage source?

  1. Dec 6, 2012 #1
    Here's the scenario:
    I need two plates that start off in proximity (a few mm apart), and I would like to vary the charge in these plates to achieve a repulsion force. Ideally it would be varied by a voltage source, so that I have fine control over the repulsion.

    But as it happens, it is not a simple matter of connecting the two plates to the same potential of a voltage supply. The scenario would work if I wanted to achieve attraction, by connecting one plate to positive and the other to ground. But in my case, no charging would occur.

    So how could I charge two plates to the same charge and vary it at will, without fancy manoeuvres like taking it far away, charging by capacitance, disconnecting from voltage, and bringing it closer again?
     
  2. jcsd
  3. Dec 6, 2012 #2
    Would something like this work at all?

    3MMWR.png
     
  4. Dec 7, 2012 #3

    vk6kro

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    This is the principle of the gold leaf electroscope and the electrostatic voltmeter.

    A capacitor has two plates but one of them has two parts that can move relative to each other.

    As the capacitor is charged, the two movable parts of one plate repel each other (since they have the same charge) and they move apart.

    This movement increases with increasing voltage on the capacitor, because the part that is moving is lifted against gravity, so greater force results in greater lifting.
     
  5. Dec 7, 2012 #4
    So if I took only the + terminal of a variable lab power supply (say, in the kV, or whatever is necessary) and contacted it to the topside of a gold leaf electroscope, and then varied it, would the gold leaves dance around accordingly?
     
  6. Dec 7, 2012 #5

    vk6kro

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    Yes, that is exactly what happens, but you have to connect the metal case of the electroscope to the other side of the power supply.

    Your power supply needs to be able to produce more than 1000 volts to get a reasonable deflection.

    I have calibrated a moving vane electroscope to use it as a voltmeter.
     
  7. Dec 8, 2012 #6

    sophiecentaur

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    What is the purpose of the outer pair of plates in that diagram?
     
  8. Dec 8, 2012 #7
    In that setup the outer plates would actually be attracted towards the center and not repelled.
    What exactly do you need this for? Can't you use electric attraction instead or even better magnetic attraction/repulsion?
     
  9. Dec 8, 2012 #8
    Wouldn't it be easier to simply make (or use) a pair of opposing solenoids? You could scavenge the parts from old loudspeakers or something.

    Edit: Even easier would be to just use one solenoid and mount it perpendicular to the axis you want the plates to move on, then connect it to both plates via levers/gears.
     
  10. Dec 9, 2012 #9

    sophiecentaur

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    Firstly, the way the charge distribution is drawn on the diagram is wrong - the unbalanced charges would nearly all be on the inner two plates because the local field in the gap is higher than anywhere else.
    Secondly, the argument that the 'like charges' would be 'attracted to' each other is analogous to saying that the can of beans on a shelf is 'attracted to' the shelf, rather than both of them being attracted to the centre of the Earth. Any gravitational attraction (or electrical attraction, in the original model) will be finite but negligible. The net electric force between the upper plates will be one of repulsion.
     
  11. Dec 9, 2012 #10

    sophiecentaur

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    To get a high repulsive force between two plates, you need a big charge on them.
    To get as much charge as possible on the plates you can position them near a ground plane. The attractive effect of the ground plane will not be significant (for small angles) but the ground plane will increase the Capacitance of the system and Q = CV.

    Alterrnatively, you could put grounded plates on each side of your suspended plates and significantly increase the electric forces as you will get attraction plus repulsion.
     
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