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Charge is relative?

  1. Jun 3, 2009 #1
    Suppose we have a test charge of unknown polarity (just assume)...now we can assume this test charge to have a certain polarity (positive), then relative to this charge, certain charges will positive or negative.

    Then if we assume this same test charge to be negative, then again relative to this charge, things will be negative or positive.

    So what we conclude here is that charge is relative, i.e there is no absolute coordinate system defining what should be negative or positive (yeah, considering electrons are positive, we do have such a coordinate system, but point is if we assume electrons to be positive there's no harm)
  2. jcsd
  3. Jun 3, 2009 #2
    Hi there,

    If you back into the history of science, and look at the foundations of electricity, you would have a very good answer to your question.

    Let's to a bit of history in this post.

    Like you said, eventhough we could look at and electron from very close, nowhere would be written a minus sign on it. Therefore, the polarity objects is purely CONVENTIONAL

    But when electricity was discovered, knowledge about atomic particles was pratically non existant. No one knew that matter was made of electrons, protons, and even less of neutrons. At the time, matter was thought to be like an english plum-pudding (imagine by Dalton). The "matter-sponge" was thought to be surrounded by a pool of electric charges. When an electric field was applied to a metal, a current was created (duh!!!). With the matter theory of Dalton, scientist said that the current is in the direction of the moving positive charges.

    When atoms/atomic particles were discovered, the convention was kept. Therefore, the electrons (which are the moving charges in the current) had to be negatively charged.

  4. Jun 3, 2009 #3


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    Staff: Mentor

    The way we assign positive and negative charge (one way as opposed to the other way) originated with Benjamin Franklin. I've posted the story a few times, most recently here.
  5. Jun 3, 2009 #4
    Sorry for the name confusion, and thank you for pointing it out.
  6. Jun 3, 2009 #5
    Benjamin Franklin may have discovered the battery for storing charge in 1748, but A. Volta was the first to build "piles" of silver and zinc discs (about 1800) with electrolytes and generating electricity reliably. Perhaps if Volta had used silver and zinc, instead of zinc and sliver, the electron would be positively charged.
  7. Jun 4, 2009 #6
    Thank you everyone!

    Assertion's now confirmed.
    Last edited: Jun 4, 2009
  8. Jun 4, 2009 #7


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    Staff: Mentor

    I think "discover" overstates it. Musschenbroek had already invented what became known as the "Leyden jar", a type of capacitor. Franklin simply put several of them in parallel to combine their effect, which seems to be a rather obvious step. In the very first letter (dated July 28, 1747) preserved in his Experiments and Observations on Electricity (1751), he acknowledges Musschenbroek by name:

  9. Jun 4, 2009 #8


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    Um not quite. Dalton's theory was the atomic theory of matter. Stuff is made of atoms. Dalton's atomic model did not explain electricity, or purport to. It explained the 'law of fixed proportions', which is what he initially called it. How matter could combine and recombine from certain 'elements' in fixed proportions - which could be explained if everything was composed of discrete units of the elements. He never purported to know what atoms were made of; it wasn't generally accepted during his lifetime, in fact the atomic model wasn't universally accepted until a century later (~1911). The evidence for atoms (as opposed to 'fixed proportions') only started coming along with Avogadro (~1840).

    Around the same time (~1800) you had Galvani and later, Faraday, who demonstrated there was a connection between electricity and chemistry. At that time, it was hypothesized that electricity was a kind of invisible fluid that permeated all matter - it was not known or believed to be composed of charge-carrying particles.

    (This was after the invisible all-permeating fluids of phlogiston and ether had been debunked, but prior to the debunking of the invisible all-permeating fluid of ether. We've really had a penchant for inventing those things.)

    The 'plum-pudding' model was the first theory of atomic structure, due to J.J. Thompson (~1900), subsequently disproven by Rutherford about ten years later.
  10. Jun 4, 2009 #9
    Sorry it was "Assertion's now confirmed."
  11. Jun 5, 2009 #10


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    Homework Helper

    Charge in the sense of magnitude is absolute, relative to the number of missing or excess electrons on an object. There is an state of zero charge (no missing or extra) electrons, and an maximum limit for negative charge, depending on the material, size, ... of the object. Attempting to increase charge beyond this results in the electrons being ejected from the charged object. I don't know if it's possible to remove all electrons from an object, and I don't know what the maximum positive charge (missing electrons) of an object would be.
  12. Jun 5, 2009 #11
    Just for fun: let us take a long neutral wire with electrical current I. In the laboratory reference frame there is only a magnetic field H around the wire and there is no charge density ρ in it.

    In a reference frame moving along the wire with v, there is an electric field E(v) and there is a charge density in the wire ρ(v). The sign of this density depends on the velocity direction: co- or counter-current observers will see the wire charged positively or negatively.

    Last edited: Jun 5, 2009
  13. Jun 5, 2009 #12
    Are you sure?...this was another problem I was about to post.

    Yes, we have plasma.

    But if this was so, we would have never been ensured that the electron is negatively charged.

    Instead of flipping the charge, we can always assume direction of velocity to make a difference.
  14. Jun 5, 2009 #13
    Plasma is nearly neutral on average. It is normally slightly charged positively depending on the temperature.

    Yes, if you are a student, maybe with time you will learn the four-current jµ Lorentz transformations and you will see that if in one reference frame the wire is neutral (jµ=(0,j)), in a moving reference frames it looks charged (j'µ=(ρ',j')).

  15. Jun 5, 2009 #14


    Staff: Mentor

    Bob's point is that charge (or more specifically the charge density) is the timelike component of the http://en.wikipedia.org/wiki/Four-current" [Broken]. Therefore even after you have chosen a convention for positive and negative charge the amount of charge is also relative to your coordinate system, since it is a component of a four-vector rather than its invariant norm.

    EDIT: Never mind, Bob explained already
    Last edited by a moderator: May 4, 2017
  16. Jun 5, 2009 #15
    We can have a new thread on that.
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