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Baryons have charges that are the result of a polynomial calculation of their building blocks (quarks)'s fractional charges. But what gives these quarks electric charges? What interactions do they have with photons? And what about leptons, like muons and electrons? How do they get their -1 charge? And then, how do positrons and anti-muons get +1 charge? Same goes for quarks: how does up-antiquarks get -2/3 charge? Or I may want to formulate my question this way:

How does any elementary particle interact with the EM-Force to get an electric charge?

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I think the question has no answer. It is the other way around, the elementary particles have charges and one can find/model how the EM force works(photon) and interacts with the particles. The concept of "force" is not useful at the elementary particle level. Force is an emergent phenomenon, macroscopically. One can consider the Standard Model of particles as axiomatic for the theory of how the world works microscopically.en.wikipedia.org/wiki/Standard_Model –  anna v Mar 18 '13 at 9:18
    
@annav "One can consider the Standard Model of particles as axiomatic"... only if one is not doing research in particle physics ;) –  Michael Brown Mar 18 '13 at 9:24
    
@MichaelBrown true, but the questioner is not doing research in particle physics. Ultimately it is a "why", and physics only answers how, based on data and model assumptions. –  anna v Mar 18 '13 at 9:35
    
@annav True. I did the best I could by answering why within the context of the best theory we have today. Hopefully the OP understands that all scientific knowledge is provisional. Granting that I don't think a "why" question should be off limits. –  Michael Brown Mar 18 '13 at 9:40

1 Answer 1

Electric charge is just a measure of the strength of a particle's interaction with the electromagnetic field (i.e., photons). Particles don't obtain a charge from the the field. Saying that a particle has a given charge is the same as saying it interacts with the electromagnetic field with a certain strength. Just a shorthand way of saying it.

The overall sign and scale of charge is a convention but once you fix one, say the charge of the electron = -1, all of the others are fixed by comparing their relative strengths and knowing that "like charges repel, opposite charges attract."

Antiparticles always have the opposite sign charge as the corresponding particles. The rule is very simple, but the reason for the rule is fairly subtle, having to do with combining relativity with quantum mechanics and causality.

The reason why quarks have fractional charges 2/3 and -1/3 relative to the electron (edit: actually positron - that sign convention again) is presently unknown, although for complicated reasons (quantum field theory anomaly cancellation) it turns out to be necessary for the consistency of the Standard Model. In unified field theories these charge relationships are a fundamental feature of the theory, generally required at a deep level by the structure of the symmetries at very high energies, but so far none of these models has experimental support.

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protected by Qmechanic Mar 5 at 18:15

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