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Homework Help: Electrostatics - Which object has the (+) charge and which one has the (-) charge?

  1. Jul 28, 2009 #1
    I'm doing an electrostatics lab and I'm getting a bit confused.

    The object of the lab is to establish an electrostatic series.

    First, I must choose a reference material, like plastic wrap. Then I charge this reference material by rubbing it in wool. Now, just by looking at any other triboelectric series I can see that wool is more electropositive than plastic wrap, therefore, the plastic wrap gains a negative charge.

    Then, I have to take another object, like a balloon, and rub it in wool so that it gains a charge. Then I have to move the balloon close to the reference material and notice a reaction. Both objects attract! Again, by looking at an electrostatic series, I can see that a balloon is more electronegative than wool, which means it too gains a negative charge.

    However, the negatively charged reference material and the negatively charged balloon attract. I know that with respect to each other, one must be more positive than the other.

    However, my question is, how do I know which one it is? (without having to a look at an already developed electrostatic series; since this defeats the whole purpose of the lab).

    Thanks a lot.
  2. jcsd
  3. Jul 28, 2009 #2
    Re: Electrostatics - Which object has the (+) charge and which one has the (-) charge

    The definition of which charge is "positive" and which is "negative" is completely a matter of convention ("What's in a name?"), so at some point, if you want to know which has the positive charge and which has the negative, you're going to have to consult some reference to find out what the convention is.

    On the other hand, you can always look at the interaction between two object to determine if they have the same charge or opposite charges (if they attract or repel each other). Here we are dealing with something that has physical significance and not just a naming convention.

    In recap: you should be able to find relative charges (same vs. opposite) based on your experiment with no reference to outside sources, however, to find the absolute charge (positive vs. negative) you will have to look something up.

    NB: This is my golden post! 42 posts for cipher42...this means (life, the universe, and) everything to me!:wink:
  4. Jul 28, 2009 #3
    Re: Electrostatics - Which object has the (+) charge and which one has the (-) charge

    I understand what you're saying, and I should state, that in my lab, it is given that polymers rubbed in wool will acquire a negative charge; thus my reference material. However, what I'm confused about is the fact that I rubbed plastic wrap in wool, and because plastic wrap is more electronegative it gains electrons which gives it an overall negative charge.

    Now, when I do the same to the balloon, the balloon is more electronegative than the wool so again, it takes electrons from the wool and it too has an overall negative charge.

    so if I use the law that states "opposite charges attract and like charges repel" I notice that the balloon and the plastic wrap attract which to me doesn't make sense in terms of positive or negative charge since they both should have a negative charge.

    I've read up on triboelectricity, and it seems that this whole concept is not really about positive and negative charges, it seems to be more about electron transfer.

    Therefore, two substance can technically have an overall like charge, but one substance may be more electronegative than the other, meaning it will still attract to consume more electrons from the other.
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