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Clarification of where energy is stored in a capacitor?

  1. Nov 3, 2015 #1

    davenn

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    hi guys

    This query comes about after the comment by BillyT
    in post#22 in this thread
    https://www.physicsforums.com/threads/energy-in-a-capacitor.840145/page-2

    is what he is saying correct, or only for the very specific situation he is involved with
    and it doesn't apply to every day capacitors ?

    everywhere I read tells me that the energy is stored in the electric field
    eg ....
    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html

    meaning that the dielectric ... plastic, paper, air, vacuum etc etc just happens to be within the electric field
    The electric field is permeating the dielectric but it is where the energy is stored, rather than specifically in the dielectric ?

    just trying to clarify my understanding :smile:

    cheers
    Dave
     
  2. jcsd
  3. Nov 3, 2015 #2

    sophiecentaur

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    I think the explanation is a mechanical one. When there is a strong field across the plates of a capacitor, there can be strong electrostrictive forces. This will squeeze the polarised material of the dielectric and change the Capacitance of the Capacitor. When the supply is disconnected, the dielectric can creep back to its original width and this will cause a voltage to appear across the plates.
    I remember visiting the Culham labs in around 1964 (University Physics Soc. visit, iirc, and being shown the vast capacitor bank associated with the Zeta project. They mentioned this effect at the time.
    (Strange, recalling the Zeta experiment today, in the light of the latest Fusion Project News)
     
  4. Nov 3, 2015 #3

    davenn

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    Thanks for responding :smile:
    That sort of implies that it is applicable in like BillyT is working with .... high voltages and large value caps
    So in that case, whilst there is the possibly primary energy stored in the electric field, there is also some mechanical energy stored in the dielectric ... as he said, "like a tensioned spring"

    I would still assume that tis isn't really a factor in everyday smaller caps in general electronics
    where voltages, capacitances and resulting electric fields are so much smaller and any stored
    mechanical energy in the dielectric would be miniscule ?


    Dave
     
  5. Nov 3, 2015 #4

    davenn

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    googling Zeta Project, gave a zillion hits to some American scifi cartoon hahaha

    changing it to zeta project experiments, gave this wiki link
    https://en.wikipedia.org/wiki/ZETA_(fusion_reactor)

    interesting ... wasn't something I was aware of


    Dave
     
  6. Nov 3, 2015 #5

    sophiecentaur

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    That wiki link reads about right. When we were shown it, zeta was sitting in a dusty corner of a lab with loads of wires hanging off it. People were pretty gloomy about the possible time scale for Fusion at the time. But now . . . . . . .?
     
  7. Nov 4, 2015 #6

    NascentOxygen

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    I think this unexpected retention of charge does apply to capacitors in the high tension side of TV sets, especially.
    Phew! The lengths one must go to to block unnecessary, unwanted & uncalled-for images!

    Also, I recall a youtube clip showing the dismantling of a charged parallel plate capacitor into three apparently-uncharged parts then reassembling it and demonstrating it was still charged. Someone drew attention to the clip here on the P F forums a few years back. I confess I could not understand it. I expected that each plate, once isolated, would be shown to be carrying excess charge, but it seemed not so. Maybe it was bogus?

    I'll see whether I can find it.
     
    Last edited: Nov 4, 2015
  8. Nov 4, 2015 #7

    sophiecentaur

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    I have seen the demo with a Leyden Jar. The Leyden jar was from a Whimshurst Machine and there was a healthy 'crack' when it was re-assembled. We didn't really get the significance at the time.
     
  9. Nov 4, 2015 #8
    I think this is the video you are thinking about:
    http://video.mit.edu/watch/dissectible-capacitor-3540/
    However, the site was down when I tried to view it.

    Here is another YouTube video that demonstrates the same experiment:
     
  10. Nov 5, 2015 #9

    ogg

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    As a chemist I can tell you that there are no such things as ghosts, no action at a distance, and no disembodied electric fields. Some sort of matter stores/generates a field - unless you're talking about propagation of electromagnetic radiation. The "energy" (which you surely know is an abstract category of things, not a specific thing itself) is stored in the separation of charge as well as any secondary effects (such as physical deformation, polarization of the insulator, etc.) Since plate capacitors are only one type, "where" the energy is stored depends on what kind of capacitor you have. I'm certainly no expert in this field, but in general physicists tend to ignore the "stuff" which generates fields, and concentrate on the fields - a prime example is the Sun's magnetic fields (sunspots, mass ejections, etc.) where what is carrying the fields is mostly ignored (in the more elementary treatments). Last I heard, it is the polarization of the insulator which "holds" most of the energy of a capacitor...I think the problem with a conductive plate holding it is that, as you know, the charge will be mostly on its surface, which means that the size scales with the square root of charge (area) compared to the cube root (volume) in a insulator...roughly speaking. I say "think" because I'm really outside my area of competence. I used to be involved in evaluating dielectric breakdown - the failure of a material due to one of several mechanisms. The first two things we (chemists) wanted to know about a material (in that case it was compounded plastic) was what voltage it took for the surface to conduct and second, what voltage it took to get a bulk failure (conduction through the bulk of the dielectric, usually a "pinhole"). Take this all with a grain of salt, as its been a long time since I've worked on this stuff, memory fails. My "words of advice" are: first understand the materials involved before you ignore them.
    If you think about it, the slow (low) conduction of charge in an insulator (dielectric) means that discharge will also be slow, hence the need to wait before working with capacitors and any electronics which may contain capacitors. CRT TVs were notorious killers, back when guys tried to diagnose and fix them diy.
     
  11. Nov 5, 2015 #10

    NascentOxygen

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    And when the dielectric is a vacuum?
     
  12. Nov 5, 2015 #11

    davenn

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    exactly
    so unless anyone can come up with info to state that the energy is stored in the dielectric rather than the electric field
    (other than the extreme case BillyT was mentioning, where is a signif amount stored in the dielectric as well)
    I will stick with the electric field being the primary storage medium and a dielectric just happens to be within the field
    ( and yes from what I have read the dielectric does affect the amount of energy stored :smile: )


    Dave
     
  13. Nov 5, 2015 #12

    davenn

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    Thanks for that video TurtleMeister :)
    hadn't seen that one before
     
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