Net Electrostatic Field inside a Conductor is $\vec{0}$ I've heard an explanation for this, and my professor wasn't really able to clarify my questions, so I was hoping someone could help:
Suppose there is an electrostatic field $\vec{E_{net}}\ne\vec{0}$, then the electrons on/in the conductor would be free to move, and then $\vec{E}_{net}$ wouldn't be an electrostatic field. Contradiction.
Question: It seems like we are considering the electrons as both the source and test charges, because electrostatic field means that the source charges are stationary (and the contradiction is that the electrons are moving), but this supposed $\vec{E_{net}}$ is also moving the electrons. How is this different from just having a collection of source charges? If you have a collection of stationary charges, and you're trying to find the electrostatic field, then by the argument above, any supposed electrostatic field would cause these charges to move, and so this would contradict the field being electrostatic.
 A: Classical electrodynamics generally makes a continuity approximation for bulk materials. We're not interested on variation in the fields at the scale of the distances between atoms, so we just average them away.1
With that approximation, the conduction electrons are acted on by the mean field. They also contribute to the mean field but we treat the two bits separately because there are so many conduction electrons that we can pretend that they are a continuous fluid instead of a set of discrete parts.2
Finally the "zero field in a conductor" argument (and related ones like "field perpendicular to the surface") are contingent on having reached a quiescent state (sometimes called the "steady state"). Anytime things change there will be a (usually brief) period when this is not true. That is referred to as the "transient behavior".

1There are situation in which that trick is not appropriate, but until you have a strong foundation you don't have the tools to look closely at them.
2This is often a good approximation even at the microscopic scale because at that scale the electrons need to be treated with the (quantum) methods of condensed matter physics.
A: When you talk about a conductor, you really mean perfect ohmic conductor. In an ohmic material is true that
$$
\vec E = \frac{1}{\sigma} \vec J
$$
Where $\sigma$ is the conductivity of the material, in a perfect conductor you have $\sigma \to \infty$, so
$$
\vec E = 0
$$
inside the material.
