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Surface Area

  1. Feb 9, 2015 #1
    If I crumple and unfold a piece of foil, am I increasing it's surface area? Since it's malleable, if I step on it with golf spikes, without completely piercing it, will it gain surface area?
     
  2. jcsd
  3. Feb 9, 2015 #2
    For part one, you certainly increase the surface area per unit area of extent, but it isn't clear to me if you increase the total surface area. Probably yes, by a very small amount, due to the stretching that occurs along the outer side whenever you fold the foil. For part two, yes.
     
  4. Feb 9, 2015 #3
    Thank you very much. Why do they not use this method with ceramic or film capacitors?
     
  5. Feb 9, 2015 #4
    ceramic is not malleable. AFAIK, ultracapacitors do have very high surface area materials inside the electrolyte, but I don't know anything about the manufacturing process.
     
  6. Feb 9, 2015 #5
    Well I mean the conductors, not the dielectrics. Film capacitors are metalized so that might be hard to do. But ceramics have solid electrodes I believe. Why don't they increase their surface area?
     
  7. Feb 9, 2015 #6
    Samson
    Can you elaborate why you want to extend the surface area of the film capacitors? :)
     
  8. Feb 9, 2015 #7
    To increase the capacitance of my homemade caps. A lot of the things I'm doing needs very specific specs that I don't find available. Also, I enjoy making them.
     
  9. Feb 9, 2015 #8

    Bystander

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    You're looking at the volume of the dielectric, its surface area times thickness, not a fractal surface area of the plates when calculating capacitance.
     
  10. Feb 9, 2015 #9
    Wow! thats great ! and i think its really fun doing that indeed.
    I suggest use foil as an alternative capacitance for your homemade caps. It is more durable and easy to expand
    compared to ceramic. :D
     
  11. Feb 9, 2015 #10
    No, I'm looking at overlapping plate surface area.

    Is it no manipulable? If that's the case, why all the fuss over high surface area materials? Why do they etch the aluminium in electrolytic capacitors?
     
  12. Feb 9, 2015 #11
    I'm using foil and glycerol right now. I tried activated carbon but I've just made a messy method of powder coating things black.
     
  13. Feb 9, 2015 #12

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    It is not manipulable in the sense you want. If the dielectric spacing is small compared to the scale of the surface irregularities, and you can "nest" your enhanced surface area plates like cupcake pans, it's worth the effort. Scaling each layer differently to roll sheets of cupcake pans isn't worth the engineering and fabrication headaches.
    Electrolytics aren't really capacitors --- they're batteries with low capacities and high discharge rates, and electrode surface area does make a difference.
     
  14. Feb 9, 2015 #13
    I see. Let me clarify, high surface area electrodes in capacitors have no impact on the capacitance if all other things are equal?
     
  15. Feb 9, 2015 #14

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    Not in the sense of using Kaiser "Quilted" foil rather than just plain old Reynold's wrap. You lose more in spacing the dielectric than you gain in area; you also lose the ability to roll layers since they no longer "nest."
     
  16. Feb 9, 2015 #15
    Im more interested in very small imperfections. For example, if I etch my foil with hcl, will this increase the capacitance?
     
  17. Feb 9, 2015 #16

    sophiecentaur

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    If you just make the surface of the electrodes 'rough' you may be increasing the area but you are also increasing the spacing between some parts of them and that must decrease the overall capacitance, I think. If you make 'spikes' then you would need to produce matching spikes in dielectric and the other plate, which would be difficult - especially at home because of the possible tolerances. The clever thing about good conventional capacitor design is to make the spacing and dielectric thickness as uniform as possible because it is the point of least separation that determines the maximum operating voltage.
    If you want to make good home made (tubular) Capacitors then you need very flat foil with no burrs on the cut edges and a very uniform thickness dielectric. Also, you have to wind them (Swiss Roll construction) uniformly tightly and exclude any dust or dirt. I have given students the task of producing Capacitors for a homework task and they mostly are just not careful enough to get good results - internal shorts are very common. If you are very careful, though, you should be able to achieve something worth while. Good luck.
     
  18. Feb 9, 2015 #17
    I have carbon aerogel composite papers and I'll be using a liquid dielectric, glycerol. If I am able to prevent shorting, do you thing the increased surface area of the electrodes would outperform the same setup with aluminium foil?
     
  19. Feb 11, 2015 #18
    I've been thinking and thinking and drawing diagrams, and have a few points I'd like to address.

    1. Roughing the surface of an electrode reduces mutual repulsion between charges stored on it's surface; when you look at it in the perspective of a newton's cradle.


    2. Increasing the surface area of an electrode 100 times through etching is possible. Even if you used the furthest distance between the two plates as your dielectric thickness, the distance won't increase proportionally to the surface area.

    3. If sharp points < flat surfaces < round objects: in terms of charge storage. Is it because the electric field lines are most scattered in a round object? Porous materials will have more directions the field lines can propagate in; but, will it manifest in less strain for the same amount of charge?

    I have an experiment I'm planning to do when the materials come in. I'm going to use foil tape and glycerol, wound into a capacitor. Then, I'll make another with the same amounts of tape. I'm going to etch the electrodes of the second capacitor with either white vinegar or low strength hcl. They will be wound on spoils to limit the glycerol from leaking. I might also submerge them in glycerol. After measuring their diameter; and noting the amount of glycerol used, I'll test their capacitance.

    Does anyone see any shortcomings with this experiment or have any parameters I should add? I'll only be able to do this once this pay period.
     
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