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this is my first post in PF, and I really don't know if the level of my question is below, above or just right for this place, so bear with me.

I'm writing a paper on electromigration, and I am the kind of person that wants to understand well every aspect of the subject he is working on. One of the articles I use is Electromigration in metals" by P. S. Ho and T. Kwok. The first equation is:

[tex]Ji = \sum^{n}_{j=1} {Lij Xj}[/tex]

And equation 2:

Ji = T[tex]^{-1}[/tex] [tex]\sum[/tex][tex]^{n}_{j=1}[/tex] Lij [tex]\nabla[/tex] ([tex]\mu[/tex]j + qj[tex]\phi[/tex])

where Lij are phenomenological coefficients correlating the flux of the ith component, to the driving force Xj of the jth component. ([tex]\mu[/tex]j + qj[tex]\phi[/tex]) is the electrochemical potential of the jth component.

My pressing and (to me at least) not-so-trivial question is: what exactly are these components? I can understand that in case of electromigration, the atoms of the lattice could be one, the electrons could be another, and the impurities could be yet more components. That would be fine, if not for the fact that the electrochemical potential of the atoms (of the lattice, for instance) is not the same everywhere along the conductor.

But then again, this equation surely wasn't written so that one would sum across all atoms in the system.

Could someone explain the meaning of "component" in this context?

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# Flux equations question

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