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Question about dielectrics in capacitors

  1. May 10, 2014 #1
    How come an insulator is put in between two capacitors? I realize it is to polarize the atoms and create a smaller electric field and hence smaller electric pressure/voltage and increase the capacitance. But why not use a conductor? Pretty much all the charge would cancel. OR am I missing a huge detail in that insulators actually TouCH the capacitors? If so then that explains why you couldn't use a conductor (since the charge would cancel) but why not use a conductor but just have it not touch either capacitor? Isn't that still polarization and the electric field become smaller? Please elucidate. Thanks
  2. jcsd
  3. May 10, 2014 #2


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  4. May 10, 2014 #3

    But why not use a conductor, but just not have it touch the plates of the capacitor? That way no "leakage" but an effectively smaller electric field
  5. May 10, 2014 #4
    The question seems pertinent to the definition of a capacitor "simply two conducting plates separated by some distance". Addition of another conductor in between them will be tantamount to splitting the capacitor into two capacitors such that:
    Voltage rating of the each new capacitor was halved (if the introduced plate was of negligible thickness). Practically it will be reduced even more.
    Capacitance of the individual capacitor would be doubled due to reduction of the new separation to half of the original one.
    Now visualize these two "new capacitors" as if they were connected in series and see if they are different from the original capacitor i.e. before introduction of the mid plate. They are the same if the mid plate is of "zero" thickness. Otherwise it has degraded by all means - leaving no benefit to do that

  6. May 10, 2014 #5

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  7. May 10, 2014 #6
    My pleasure
  8. May 10, 2014 #7
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  9. May 10, 2014 #8


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    In principle, you could fill the dielectric space with a conductor - leaving a minuscule air gap. That would produce a very high value of capacitance but a maximum operating voltage which would be ridiculously small, because the gap would break down so easily. Using an appropriate material as a insulator (dielectric) means that the ready polarisation of the dielectric allows a charge build up right next to each of the capacitor plates without voltage breakdown. So, in many respects, the dielectric gives the effect of a much smaller gap.

    There is a compromise between Max Operating Voltage and Max Obtainable Capacitance, for any particular technology and allowable physical sizes for the Capacitor. Electrolytic capacitors are operated with a continuous offset voltage across them which deposits (electrolytically) a very thin layer of dielectric between the plates, giving a much higher Capacitance than if you were to use plastic / mica / paper etc.. See also the construction of SuperCapacitors (loads of Google Hits).
  10. May 10, 2014 #9

    What is voltage breakdown exactly? How does it occur?
  11. May 11, 2014 #10


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