What opposing force must be overcome by charges flowing in electric current?

Answer 1

resistance

It depends on what you mean by overcome. Resistance is usually what most people will say when what they actually mean is impedance. The difference between the two depends on the situation. For a steady flowing current (read flow of charges) that is not changing with time, then the two are the same. For a changing current then the two are distinctly different from each other as resistance is only a part of impedance, reactance making up the second part.

Reactance can be classified into two types...capacitive and inductive reactance.

At a microscopic scale, like if you could shrink yourself down and watch an individual "charge" then the answer becomes more complicated. In this case the answer is...NOTHING! The charge has no force to overcome if it was isolated on it's own in a vacuum in no electromagnetic field. That's not a practical situation and there is usually some type of material medium that the charges are bound to...like a copper wire. That's not always the case, but I'll assume that's the question your asking since it's a very common situation.

In the copper wire case, the answer is collisions and electromagnetic fields. In the direct current case, the charges are colliding with each other and the copper atomic lattice. At normal temperatures the lattice is vibrating violently and traveling charges approach the lattice sites closely enough to be influenced by their localized electromagnetic fields. This causes the charges to change velocity and direction. They also can collide with each other since there are a huge number of them even in the small pieces of copper.

So here you hit back against a basic definition of electric charge, that which is influenced by an electromagnetic field and you find the answer in that definition. The opposing force that must be overcome by charges flowing in electric currents is the force of electromagnetic fields.