Dielectric permittivity

  1. Hi , I would like to know if there is any material , a dielectric like the one used in a capacitor , whose dielectric permittivity could be altered , like increased or decreases , I'm not talking about older type capacitors which had plates on a rotor that could be adjusted so that one could change the plate area.
    I mean more like a material which is a dielectric but whose dielectric permittivity could be changed by some interaction , electric, magnetic maybe ?
     
  2. jcsd
  3. Simon Bridge

    Simon Bridge 14,479
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  4. Jano L.

    Jano L. 1,213
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    Dielectric constant depends on temperature, so in principle by controlling temperature one can vary the dielectric constant. Does not seem to be too practical though. There are also artificial dielectrics, that can be manufactured with prescribed permittivity, but I think these are quite big and are used not in static fields, but in oscillating EM waves.

    In principle, one could prepare material with desired permittivity ##\epsilon## by mixing powders of two kinds with lower and greater permittivity ##\epsilon_1##, ##\epsilon_2##, but the mixing ratio and compression used would most probably need to be found empirically.
     
  5. Ok , let me take a wild guess as a question , is or would it be possible to make a dielectric whose permittivity amplitude is so large that using it in a capacitor for example I could go from a few nF to as high as a few uF and maybe more ? To achieve noticeable charge flow to and from the capacitor as it would be charging up and down ?
     
  6. Simon Bridge

    Simon Bridge 14,479
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    you want a relative permittivity of 3-4 orders of magnitude ... you could just look it up?
    http://en.wikipedia.org/wiki/Relative_permittivity
    ... i.e. Barium Titanate @ 120C has ##\small \epsilon_r=10000##.
    That would make a 1nF cap into a 10000nF=10μF cap.

    Also see conjugated polymers.

    But you don't need to do that to notice the "charge flow" (i.e. electric current) as the capacitor charges and discharges.

    Did you have a specific application in mind?
     
  7. ok thanks , I see Simon , well the thing I had in mind is, I have a capacitor in a circuit attached to a power supply in series with a transformer , I would need to change the permittivity of that dielectric so that I could change the capacitance of the capacitor which would make the charge flow back and forth to the capacitor , in other words a capacitor which also acts like a charge pump , I hope you understand what I mean, like in a transistor you control the current flow with an electric field or a smaller current flow , here you control the charge flow with varying the dielectric so that the amount of charge would vary on the plates.
     
  8. Simon Bridge

    Simon Bridge 14,479
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    You can get that effect (a charge pump) more easily, and cheaper too, by other means.
    (i.e. - the PSU is a "charge pump" ... or do you mean one of these?)
    So what is the application?
     
  9. well , thanks for the link , I wasn't aware of such devices , yet I see they too need a transistor to do the switching part , I imagined that if one could make the right dielectric material for such a purpose one could have a all in one capacitor charge pump which could be used in a circuit with a transformer as a power converter , that was kinda the application.
    I mean that would be the simplest device in it's very basis requiring only two capacitors and a transformer.
     
  10. Simon Bridge

    Simon Bridge 14,479
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    You'd still need something to do the switching ... in your case, how does the dielectric material know when to change it's permitivity?
     
  11. yes that is the thing, well I would need some switches that would change the permittivity of the material if possible at all of course, but then at least those switches wouldn't serve as the main current paths, in fact there would be no main current path at all , rather current formed from charges flowing back and forth , what do you think is the idea even sound?
     
  12. sophiecentaur

    sophiecentaur 13,292
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    You actually want a variable C Capacitor, I think.
    If you could accept a small value for you capacitance then a Varactor diode would do it. You would need to operate at high frequency, though.

    One point has to be made, though. If you want to get a voltage increase by changing the Capacity, that will fundamentally involve putting energy into the system. Work needs to be done to reduce the Capacitance when there is a given charge on the Capacitor. Halve the capacitance and you will need to double the energy stored. This energy has to come from somewhere.
    E = CV2/2
    and Q = CV
    both apply. V changes inversely with C, for constant Q
    In the light of this, are you sure you want to proceed?
     
  13. Yes I know that if I want the same power transferred with smaller capacitance the frequency would need to go up and vice versa for larger capacitance.

    So are you saying that if I would have this capacitor with variable dielectric , upon varying the dielectric constant I would get what ? Charge flow or voltage rise? Or both?
    Im speaking about a circuit case not a free standing capacitor?

    As in a single no circuit capacitor charged to some value , varying the dielectric would cause voltage increase decrease because the charges have nowhere to go so if the field is weakened they need to set up the voltage to increase it back to it's pre vaied condition right?
    But does that happen in a circit also ? because in a circuit the weakened field makes the charges on one plate repel more as they are of the same polarity but instead of setting the field back to keepthemselves on the plates they now have a path to flow to , maybe even more , another empty capacitor to charge up , so would they flow away and the result would be a cap with less capacitance and less voltage or no?
    I had this argument with a guy who studies math and electronics and he too didn't knew the answer to the case in a circuit , we both only agreed to the no circuit case.

    Hope to solve this one , as I learned quite a bit from the last thread, thanks sophie.
     
  14. sophiecentaur

    sophiecentaur 13,292
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    I am just saying that Q=CV and E = CV2/2 will always apply. You get different results, according to how the capacitor is connected, of course but varying the capacitance will involve having a PD change or a flow of charge. Any PD change represents a change in stored energy and any flow of charge will involve some energy transfer somewhere else in the circuit. For this exercise to achieve anything (in the broadest sense), energy will need to be put into making this change of C. If you can't identify the mechanism (it may be a bit subtle in some circuits - like charging and discharging a battery) then you have to keep digging until you can find it. You clearly want to sort this out so stick with it!!!! You should break off for the New Year festivities, perhaps. :wink:
     
  15. Nah , the festivities are kinda boring and they have a very low capacitance in my eyes :D
    Even though I will drink some vine and look at some fireworks, hopefully not the ones that come out of a test circuit.Anyway they both smell " made in china"

    so here is the circuit , what do you think , in such a circuit changing the permittivity 180 degrees out of phase would achive charge flow back and forth ?
    I attached a schematic I just did in paint, excuse me for the quality and the blurred lines, I guess the vine is stronger this year
     

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  16. sophiecentaur

    sophiecentaur 13,292
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    What did you want the circuit to do?
    I presume that you want to cary the C's and, thus, vary the charge at each end of the transformer primary, resulting in an induced AC on the secondary. The 'detail' of how you will vary the C's is missing, of course and that would be the difficult bit.
    It doesn't appear to be achieving anything.
    It's a non-starter as a 'power supply' circuit because it would be, necessarily, very inefficient, for the reasons I gave, involving the inherent power loss. The Power to drive it would all come from the circuit driving the diodes. There is no DC path from the (DC) Battery ("+") so it can't deliver any power. The power supply is just providing 'bias'. A power oscillator circuit would be more applicable for driving the transformer.
     
  17. Simon Bridge

    Simon Bridge 14,479
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    If the energy for changing the permittivity does not come from the "main current path" where does it come from?

    This is my concern here - you appear to be describing a perpetual motion machine!

    Take care:
    You have to do work changing the capacitance by changing the permittivity just like when you mechanically change the capacitance by sliding some plates around.
    Do you understand this?
     
  18. sophiecentaur

    sophiecentaur 13,292
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    He seems to be ignoring this!
     
  19. Ok this was a very rough picture of what I mean , possibly the constant +ve supply kills any charge oscillation that might occur if the capacitors would be varied in capacitance one by one , one after the other.

    It might only look like I'm talking about a PPM machine because I am not aware of all the mistakes I could or have made , I definitely don't think in terms of PPM , I know enough physics to know that doesn't work.
    But why perpetual motion ? I was just thinking that, well you can run a transformer with a current that runs back and forth and increases and decreases in strength periodically just like an AC sine or square wave , but the same could be done by moving charges which is the same current back and forth using the attraction of repulsion of the electric field , rather than a direct current path,
    Is it just me or I kinda thought that changing or varying field both electric and magnetic can transfer energy too , by the way that is how the transformer works as it too doesn't have a current path nowhere through it's core.

    Ok , can you tell me sophie what exactly I am ignoring because it seems im not aware of it as if I would I wouldn't repeat it.
     
  20. sophiecentaur

    sophiecentaur 13,292
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    As I have told you, the energy can only come from the mechanism that changes the values of the Capacitors. You cannot produce power without putting some in and you have not specified how you will do it.
     
  21. Simon Bridge

    Simon Bridge 14,479
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    What he said - that's the pmm part.
    It's the most important bit.

    Example:
    Say you rig a waterwheel to turn a crank that pushes a dielectric into and out of the space between the plates of a capacitor. This will make the capacitance vary periodically - when the wheel is in a water-flow.

    Put it in a box labelled DVC (driven variable capacitor).
    This box will have two electrical terminals for hooking to an electric circuit along with separate inflow and outflow attachments for power ... in this example they would be hose attachments.

    Now the box can have anything that varies the capacitor in it ... whatever it is does not matter, the external configuration will be the same. (It may be that the power-flow stuff has a different physical shape though.)

    What's important is how the capacitance varies with time and understanding that you need an external source of energy to drive it.
     
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