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Basic Electricity Questions

  1. Jan 3, 2007 #1
    I have a couple of questions about basic electricity concepts. First of all, is the "default" state of things to be neutrally charged, so that their level of conductivity is the only thing that matters. Secondly, I have a question about the common demonstration with a glass rod and a piece of silk cloth that rubs against it to get the glass charged. Is the glass neutrally charged and the cloth neutrally charged too? If so, how would there be any kind of attraction or anything between them? If everything is neutrally charged by "default" then how can there ever be any charges transmitted?I guess more generally, my question is why and how friction causes electrical charge to be transmitted or does it create electrical charge?
     
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  3. Jan 3, 2007 #2

    chroot

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    You are correct that charges tend to neutralize, because a positive charge attracts negative charge until the two are roughly in equilibrium.

    All materials are "seas" of positive charges (protons in atomic nuclei) and negative charges (electrons) in roughly equal numbers. When you rub one material against another, electrons can be pulled off one material and deposited on the other. The result is that one material becomes more strongly negatively charged (the one that captured the electrons), and the other becomes more strongly positively charged (the one that lost the electrons).

    So, although both started with equal numbers of positive and negative charges, the end up with unequal numbers. The net positive charge attracts the net negative charge, and the materials become attracted to one another.

    - Warren
     
  4. Jan 3, 2007 #3
    So when you rub the cloth against the glass, the glass gives some of it's electrons to the cloth and the cloth may give some of it's electrons to the glass but not as many because it is not as good a conductor, so that the glass is more positively charged (has less electrons) than before and the cloth, which recieved more electrons from the glass than it lost in giving to the glass, would have a negative charge, right?

    On a bit of a different note: You said the electrons can be pulled off, I assume this is only possible which valence electrons far from the nucleus, is this why metallic objects with large atoms and many electrons are better conductors, because the electrons are easier to pull off because the electrical charge between them and the nucleus is less?
     
  5. Jan 3, 2007 #4

    SGT

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    In reality, the electrons that are pulled of are free electrons. They are not attached to any atom.
    A good conductor (a metal) has many free electrons. They are free to move under the influence of an electric field.
    An insulator has very few free electrons, so an electric field has little influence. A semiconductor is in between conductors and insulators.
     
  6. Jan 3, 2007 #5

    chroot

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    Yes, the glass becomes positively charged, and the cloth negatively charged.

    All materials have something called a "energy band structure" that defines the specific energies that electrons are allowed to have. Some bands of energies are called "conduction bands," because electrons with such energies move essentially freely from one atom to another. Metals always have at least some electrons in their conduction bands, and this is what makes them good conductors. It's as if all of the metal atoms are collectively sharing these conduction-band electrons.

    Insulators, on the other hand, have no electrons in the conduction band. (They're all in a band of lower energies, called the valence band.) Each electron is rather strongly bound to each atom, and it takes a lot of force (emf) to move electrons from one atom to another.

    - Warren
     
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