What's the force moving electrons against E within copper-zinc

[Tekk]

I have a piece of copper in static condition. Since copper is a conductor, within the copper there is no electric field. Strange comes when I bring this copper in contact with a piece of zinc: in a tiny interval of time, certain amount of electrons would flow from copper to zinc.
I suspect that during this interval, electric field would be created and in order to move electrons against this field, there must be a non-electrical force. So the question is: what's this force?
I have searched this topic and answer are chemical force/potential etc. I would be very appreciated if anyone could give a force which the Newton 3rd Law could apply to, such as the Coulomb force, gravitational force.

 

[Philip Wood]

I offer no answer, but I think you've asked an excellent and quite fundamental question.

 

[Drakkith]

Try this article: http://en.wikipedia.org/wiki/Volta_potential
Edit: And this one: http://en.wikipedia.org/wiki/Galvani_potential

 

[Tekk]

Try this article: http://en.wikipedia.org/wiki/Volta_potential
Edit: And this one: http://en.wikipedia.org/wiki/Galvani_potential

The first article explains why electrons move within zinc-copper interface by introducing energy required for the most energetic electron to escape from a metal (so-called work function). It is said that the most energetic electrons in zinc have more energy than those in copper. And because electrons in zinc love to stay in a low energy state, they flow from zinc to copper. Is my understanding correct?

Besides, the article has not discussed what the energy is and what force is applied when electrons flow from zinc to copper. In the second article you have cited, the energy is called chemical potential. Is it a scalar function? Can I take the gradient of that chemical potential? If yes, does the gradient make sense as some kind of force?