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Is dielectric necessary in capacitors?

  1. Nov 18, 2015 #1
    Why do capacitors need dielectrics (insulators) in between the two plates(conductors)?What if there is no dielectric in capacitors ?Function of a capacitor is " a capacitor stores energy in the form of an electrostatic field between its plates"
    Let's say there is a conductor is between them instead of an insulator I think electrostatic field would be formed in between the plates and hence capacitor will store energy.So,it comes out that what is necessary for formation of capacitor is there has to be some separation in between the two conductors (plates)of capacitor .
    Hence answer of
    Is dielectric necessary in capacitors?
    should be "NO",Right?
     
  2. jcsd
  3. Nov 18, 2015 #2
    Of course not. You can have capacitors where a vacuum exists between the plates.
     
  4. Nov 18, 2015 #3
    While there doesn't need to be a dielectric, there can't just be a conductor between the plates. There needs to be some sort of barrier to stop current from flowing. gracy, if there were a conductor, then the current would just flow from one plate to the other and there would not be any charges built up on either plate.
     
  5. Nov 18, 2015 #4
    I think you've quoted me when you actually intended to quote the OP.
     
  6. Nov 18, 2015 #5
    I meant to add to what you said in response to OP.
     
  7. Nov 18, 2015 #6

    berkeman

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    As has been explained, the correct answer is indeed No.

    *But* why do we use dielectrics in capacitors? They have a purpose that goes beyond just providing insulation between the plates. Can you post the Relevant Equation and explain why dielectric materials are used in capacitors? :smile:
     
  8. Nov 18, 2015 #7
    Seems the term "dielectric just means the 2 plates are held apart, because if they touch then the charge would equalize. So the spacer ( dielectric) needs to be a non-conductor, in other words, an insulator. Of course it doesn't need to be 'stuff" it could be air or as someone pointed out, in space the distance wouldn't even have air molecules between the plates.
    Me thinks this is what you are asking, Mike
     
  9. Nov 18, 2015 #8
    If I found the correct formula that berkeman was referring to, I think that the use of a dielectric increases the amount of charge that can be stored for a given applied voltage. The larger the value of permittivity, ##\epsilon##, the more charge can be stored.

    I could be wrong. I never quite did well with electricity although this is more on the theoretical side which is good. Back in school when we started talking circuits all of the engineering students got excited and the physics students tried to hide in our rooms with our frictionless spheres* and vacuums.

    *unless it is rolling in which case the friction is unbounded, of course.
     
  10. Nov 18, 2015 #9

    berkeman

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    That is correct. :smile:
     
  11. Nov 18, 2015 #10
    berkeman had asked this question to the OP (to help her learn the concept). It wasn't supposed to be answered by us!
     
  12. Nov 18, 2015 #11

    berkeman

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    True, but Gracy was a bit slow in responding, and this poster seemed to learn from the question. I'm okay with it in this case. :smile:
     
  13. Nov 19, 2015 #12

    fresh_42

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    And so was I, since I had been waiting for any answers, too. Although I was hoping for something less obvious. Reading about it left me same as Mike with 'dielectric' being used for any spacer, even vacuum. Additionally in Wiki I've read the following: (my translation)

    "Vacuum capacitors are of advantage at high frequency currents and voltages in the kV range. They are preferably used in radio transmitters with a high output power."

    Why?
     
  14. Nov 19, 2015 #13

    sophiecentaur

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    That reads more like a Dictionary entry than a useful piece of technical description.

    1. They have good insulation properties.
    2. There is very little loss involved - which can be very relevant when dealing with high power RF (little heating of the device)
    3. The low Capacitances of Vacuum Capacitors is appropriagte to high RF frequencies.
    4. They are very reliable
    5. They also look very pretty; like an expensive designer table lamp.
     
  15. Nov 19, 2015 #14

    fresh_42

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    Thank you, esp. for #5
     
  16. Nov 19, 2015 #15
    I only posted the part that you could guess. If you check on the wiki page you can find the pertinent integral that will give a better understanding.
     
  17. Nov 19, 2015 #16
    You are right about the extra charge with the choice of dielectric material and it is not just something that keeps the plates apart
    Both Myself and Gracie have benefited and learned something here

    Best, Mike
     
  18. Nov 19, 2015 #17

    NascentOxygen

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    Somewhere in here you'll find the advantage that a vacuum has of being "self-healing": if there's a flashover between the plates due to momentary excessive voltage, the capacitor is undamaged and immediately again "good to go" like new.
     
  19. Nov 19, 2015 #18

    sophiecentaur

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    It's worth mentioning something of how the dielectric works. Although it is an insulator, it allows displacement of charges in its molecules (polarisation) with a modest Elecric Field across it although the charges cannot actually flow. So, given the E field between just two metal plates, will permit a small amount of polarisation for a given PD (i.e. a low Capacitance). Adding a dielectric will increase the polarisation by a factor of the 'dielectric constant' of the dielectric. The dielectric has allowed charges to appear right next to each plate, rather than the full separation would allow, which is the equivalent of having the plates much closer together. Apart from the small problem of its conductivity, a block of metal would do the same - leaving a tiny gap between this block and the plates. Much higher capacitance but it would only operate at a tiny PD before breakdown.
    When the operating frequency is very low, there are many dielectrics that can be used but. when AC is put across them, the continual reciprocating movement of the charges (polarisation) dissipates energy. This gets worse ( pro rata with frequency). For high frequency Capacitors, more expensive dielectrics, with lower dielectric constants have to be used. Ceramics are used for the highest frequencies. (Vac-caps aren't suitable for PCB mounting!).
    It is an interesting exercise to look in a electronics supplier's catalogue and see how the maximum value of Capacitance for a given construction type, relates to the maximum suitable frequency. For significantly high frequencies, the parasitic Inductance of the Capacitor becomes relevant, too. But that's another story.
     
  20. Nov 19, 2015 #19

    fresh_42

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    So, finally, there is only one more question left behind. :cool:
     
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