Why does Saran Wrap become charged?

  1. Hi

    I'm trying determine the electric charge configuration of my saran wrap.


    When it's wrapped up like this, I feel no electric field around it.

    But when I pull it I feel an E-field around the sheet i just pulled.

    At first I thought that the sheet was just polarized with net 0 charge but I've determined that this isn't true.

    Could someone draw/explain to me the charge configuration of pulled vs non-pulled saran wrap?
  2. jcsd
  3. Pythagorean

    Pythagorean 4,596
    Gold Member

    I think a confounding factor is that saran wrap has adhesives in it. Residues, resin, powders, etc. That allow it to ashere to itself:


    it does act like a capacitor and can acquire charge easily, as well, and it probably has minimal charge on it when its manufactured and packaged (as part of a clean manufacturing process), but separating adhesive forces from electromagnetic ones is not trivial to me.
  4. collinsmark

    collinsmark 2,178
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    Gold Member

    What a wonderful question. I love it! :smile:

    For some reason or another, it seems that the sheet pulled off from the roll has a different electron affinity than the rest of the roll. How could this be?

    I speculate that stretching has something to do with it.

    Plastic is made up of long, stretchy molecules. When you pull off a sheet of wrap, two things happen:
    • The sheet of wrap that you are pulling is stretched, at least temporarily.
    • The part of the sheet that is touching the roll is stretched, thus it moves (rubs) against the rest of roll until separation occurs.

    So the question is, does the stretching of the plastic change its electron affinity?

    Experimentation might help here. iScience, you seem to have some method of measuring electric fields. Try measuring the field around a sheet of wrap that was pulled off with a lot of stretching (like you would normally do). Next, pull off a sheet as gently as possible, in attempt to minimize the stretching when separating from the roll. Is there a difference in the resulting electric field around the sheets? :wink:

    [Edit: if my speculation is correct, the sheet that was temporarily stretched more (when pulled off the roll) will cling better.]
  5. davenn

    davenn 3,464
    Science Advisor
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    2014 Award

    Keep in mind this effect happens with all rolls of tape :smile"

    and its the same process when walking over a carpet then getting a zap when touching a door knob
    This static charge generation is called tribocharging. when the 2 surfaces that are in contact become separated
    some electrons are moved and this causes an imbalance of charge

    Last edited: Aug 14, 2014
  6. davenn

    davenn 3,464
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    No, I think you will find that is true. there is an initial 0 net charge throughout the material on the roll
    Its the act of separating the sheet that generates a charge imbalance as I described in my previous post

  7. I found a reference to this in the first edition of Chabay, Matter and Interactions a while ago. The chemists speculate that the polymer strands actually get torn apart at weak bonds, leaving charged "pieces" of molecule left over. It is apparently a topic of current research (or was whenever that book was written).
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