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Electrostatic charge of Scotch tape

  1. Jan 22, 2009 #1
    The other day I ran across a writeup for a general physics experiment (it might have come out of a high school AP course - I'm not sure) about using charged Scotch tape to separate charges for the purpose of measuring the charge through a torque equilibrium setup. The technique was to put a piece of tape, sticky side down, on a surface, put another piece, sticky side down, on top of the first, and then rip off the top piece and use it to make a crude electroscope.

    So, I understand how this works (it requires a fair amount of hand-waving and beard-muttering to say this is a good example of Coulomb's Law, but whatever...) but I came up short as to why the writer thought it plainly obvious that the top piece would be negatively charged. I thought it might just be a slip till I found another similar writeup from a University I won't name stating pretty much the same thing.

    What am I missing? What is there about Scotch tape that makes it so clear (notice I avoided any pun about transparent) that the top piece would be negative? Besides, I can't get one to be negative.
  2. jcsd
  3. Jan 22, 2009 #2
    It's made of plastic and most plastics acquire a negative charge, when they do acquire a charge:


    Be grateful: you got a cheerful roll.
  4. Jan 23, 2009 #3
    But both the top and bottom pieces are plastic. I'm still puzzled. But your joke about the cheerful roll got me thinking. Would this be different with the older cellophane tape? I have no idea where to find that, but I'll give it a try. Thanks.
  5. Jan 23, 2009 #4
    Two pieces of the same material can easily acquire different amounts of the same charge and regard each other as opposite in charge. As can two or more different parts of the same piece of material. They know the top piece will be negative simply because it's plastic. (That doesn't mean it will automatically be more negative than the bottom piece).

    I am pretty sure the point of using tape is to insure the contact between the first piece and the surface you stick it to is as uniform as possible, and the same for the second piece and the first piece. That way, when you pull the second piece up the charge it acquired from contact with the first piece should be as evenly distributed over the length of the tape as it reasonably can be here.

    The charge you give an insulator stays right where you put it: it doesn't spread out by itself the way it would on a conductor. One thing to consider is that your handling of the tape after you pull it up might be upsetting the uniformity of the charge distribution creating a situation where some parts of the tape are less negative than others resulting in an overall neutrality. You'll see from the list at the link that your hands tend to be positive and that touching the tape anywhere with your fingers is going to change the charge at the point of contact.

    Since glass tends to acquire a positive charge it might enhance the result if you stick the first piece to a sheet of glass (got an old picture frame?). That will also optimize flatness, hence uniformity of contact.
  6. Jan 23, 2009 #5
    No, actually you provided the clue in your first post. I found some old style cellophane tape (you remember, the kind that turns yellow, gets brittle, and then falls off). It does acquire a negative charge pretty consistently. It's the new, transparent tape that acquires a positive charge, usually small. Whoever wrote that lab procedure I read must have written it many years ago. Perhaps they never thought there would be magic tape? Thanks for the idea!
  7. Jan 23, 2009 #6
    You must have two different materials to transfer charge. One is tape; one is glue.
  8. Jan 23, 2009 #7
    Part of the mechanism for sandwich wraps to work is that when the plastic is pull away from the roll the static electricity, a negative charge, helps it to stick to the relatively positive containers etc.
  9. Jan 24, 2009 #8
    If I pull a piece of magic tape off the roll, say 6 inches long, it seems already to be charged up and will cling to my hand. Either side. Both the sticky and smooth side are electrostatically attracted to my hand. I assume from the triboelectric series my hand is positive.
  10. Jan 24, 2009 #9
    We might start with some basics. Given two uncharged insulators, the total charge adds up to zero after separation. The charge on one will increase as much as the other decrease. If one is charged positive, the other is charged negative.

    One side of the magic tape is a layer of glue. The other side is plastic, so the two materials being separated are different at their interface.

    The charge on the free end of the tape will be due to its glue side. Though the unbalanced charge remains on the glue side, the electric field surrounds the tape on both sides.

    You haven't rubbed your hands on the tape to transfer charge, so where your hand stand in the triboelectric series doesn't matter. But your body is a conductor. As you move the tape
    close to your hand it attacts the opposite charge in your body to your hand.
  11. Jan 24, 2009 #10
    Thanks much, Phrak. I was discounting the glue and forgetting I can also function as a conductor
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