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Charge question made me shut

  1. Jun 6, 2012 #1
    Charge question made me shut !!!

    My younger sister just started learning about electrons and charge associated with it, while she was studying , she asked me "what is a charge " and this question made me shut !! i wasn't able to answer (we might say that its a property but that's not a good answer at all) .
    So do we have any definition of charge, like for mass, we can say its just same as energy stored in a different form (E=mc^2). do we have any such relation of charge with energy or something..... if we have then that relation can be used to combine gravitation and electrodynamics, and hence GUT can be developed ????
  2. jcsd
  3. Jun 6, 2012 #2
    Re: Charge question made me shut !!!

    Electrical potential energy in electrostatics = Voltage * Charge

    Unlike the definition of c^2, voltage is not a universal constant, so with charge and energy, it's not really the same kind of fixed relationship that you see with mass and energy.

    Keep in mind though that this is only one of many components of the total energy in existence.

    This is far away from actually developing a GUT.
  4. Jun 6, 2012 #3
    Re: Charge question made me shut !!!

    There is no answer. It's an inherent property to the particle, just like mass is. We measure mass by it's coupling to a gravitational field, and we measure charge by it's coupling to an electromagnetic field.

    You can describe the properties of charge, and how charges interact, but asking what it is is rather meaningless.
  5. Jun 6, 2012 #4
    Re: Charge question made me shut !!!

    The O.P.'s sister asks, "What is a charge?"

    The answer can be simply stated:
    "It is a measure of how strong it reacts to an electric field."

    And if she asks, "What is an electric field?"

    You reply, "It's a force of nature."

    And if she asks, "What is a force of nature?"

    You reply, "Anything that causes motion."

    Then give examples to explain what makes electrical forces different from magnetic and gravitational ones.

    Of course, the more curious the person is, expect more questions to follow.
  6. Jun 6, 2012 #5


    Staff: Mentor

    Re: Charge question made me shut !!!

    Well, there is one way that you could make a similarity between energy and charge (but not related to E=mc^2). One of the most important theorems in physics is Noether's theorem, it states that for every conserved quantity there is a symmetry in the fundamental law of nature (the Lagrangian). Most of modern physics is based on the idea that the most fundamental defining characteristics are these symmetries.


    One symmetry of the laws of nature is that they don't change over time. That leads to the conservation of energy. So energy is the thing which is conserved due to time invariance.

    Another symmetry of the laws of nature is that they don't change if you call a different voltage your 0 volts (gauge invariance). That leads to the conservation of charge. So charge is the thing which is conserved due to gauge invariance.
  7. Jun 7, 2012 #6
    Re: Charge question made me shut !!!

    How old is your sister? Does she know about electrostatics (rubbed piece of plastic attracts stuff, etc.)?
    It's simply not known what charge is. All that is known is how charge interacts (electric/magnetic fields) and that charge is carried by electrons and protons. But that's about it.
  8. Jun 8, 2012 #7
    Re: Charge question made me shut !!!

    Hi Zubeen,

    Maybe your sister is unusually wise. You might very well get an uncountable number of responses that tell what one can observe about a charge, but your sister might only be satisfied with a direct answer to what it is.

    To understand what a charge is from its historical origins, you first need to be acquainted with the concept and effect of polarization. The most simple picture is that particles with a net positive amount of electricity will move as far as needed from particles with a net negative amount of electricity in order to balance the force on them that is provided by electric and magnetic fields. This assumes that the particles are wholly or partially free to move (they almost always have some type of tension restraining their movement).

    A charge then, was defined long ago by Clausius Mossotti as a discontinuity in that polarization. If you think about it, the discontinuity is necessary so that the EM force has something to push or pull against and therefore allow the polarization to happen. Mossotti's concept and definition was taken up by Faraday and later by J. C. Maxwell to whom we owe the bulk of our understanding of the dynamic effects of electricity and magnetism.
  9. Jun 8, 2012 #8


    User Avatar
    Science Advisor

    Re: Charge question made me shut !!!

    You reply, "It's what affects charges".
  10. Jun 9, 2012 #9
    Re: Charge question made me shut !!!

    Ya OK but, its just the consequence of the presence of charge, the voltage is just as energy due to unit charge, so since the definition of potential is itself based on charge .... it doesn't hence enables us to define charge.
  11. Jun 9, 2012 #10
    Re: Charge question made me shut !!!

    No but know we can say mass as just the energy in a different form, and saying this even, we wont harm any law of physics......
  12. Jun 9, 2012 #11
    Re: Charge question made me shut !!!

    First thing, i don't want to answer my sister, she wont understand any thing because its beyond her scope. what here matters is, what i myself understand about a charge nd therefore, i framed up this question.

    second thing, how can we explain it on the basis of Electric field ?? The field itself origins due to static or dynamic charges. And i am asking about the a word in Electric field's definition , the charge !!!
  13. Jun 9, 2012 #12
    Re: Charge question made me shut !!!

    No one will be able to answer exactly what a charge is in a material sense without first describing all charge as being the consequence of some kind of motion which confers the properties of "positive" or "negative" to them. Otherwise, it is impossible to conceive of a reason why charge is what it is or behaves the way it does.
  14. Jun 9, 2012 #13


    User Avatar
    Science Advisor

    Re: Charge question made me shut !!!

    Charge and electric field can only be defined together, because neither concept makes sense on its own. We observe that certain objects affect each other in a certain way. We call those objects "electric charges" and the mathematical model that quantifies their effect "electric field".
  15. Jun 9, 2012 #14
    Re: Charge question made me shut !!!

    It seems far too easy to confuse observables with the structural entities that observables depend on. In both Maxwellian electrodynamics and Quantum Mechanics fields play an auxiliary role and are derivable from potentials. There are observables associated with charges, such as their motion which are dependent on the variation of the fields. But there is no indication that the existence of a charge, it's persistence and it's quantization in any way whatsoever depend on the fields, is there?
  16. Jun 10, 2012 #15
    Re: Charge question made me shut !!!

    So does that means that there are chances of further development of electrodynamics ??
    Because mass, earlier , before relativity was defined as something that converts force into acceleration and after relativity , mass got a new definition.
    Can there be also some precise definition for charge still waiting to be revealed ???
    And is the effort being made to reveal this definition ???
  17. Jun 10, 2012 #16


    Staff: Mentor

    Re: Charge question made me shut !!!

    The modern definition is the one I described in post 5. That definition is sufficiently precise that the theory based on it can accurately predict and account for all observed behaviours of charge and EM interactions. So, no more precise definition will be needed until some new observations are discovered which are not accounted for by the current definition.
  18. Jun 10, 2012 #17
    Re: Charge question made me shut !!!

    I doubt your little sister will be able to understand beyond "its a property, just like mass", but if she's adventurous, she wud've found it out on her own.
  19. Jun 11, 2012 #18
    Re: Charge question made me shut !!!

    OK so what observations led to the development of formula of E=mc^2 ?
    Actually, about this formula, I've heard that there's no derivation for it.
    Einstein just himself suggested this formula and later on the formula was found to be experimentally correct !!!!!!
    is it all true ?
  20. Jun 11, 2012 #19
    Re: Charge question made me shut !!!

    Yes, but it seems to be a difficult area to get experimental information on. Theoretically, the Dirac equation for the electron predicts that its charge travels within a very small region at the speed of light (Zitterbewegung). That means it's decoupled from the position of the center of mass of an electron. The Dirac equation makes some extremely accurate predictions but of course no theoretical model matches all results and is perfectly consistent with all theoretical principles.
  21. Jun 11, 2012 #20


    Staff: Mentor

    Re: Charge question made me shut !!!

    All relativistic formulas, including E=mc^2, can be traced to two key postulates:
    1) the principle of relativity
    2) the invariance of the speed of light


    No. There are many derivations.
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