Why can't we integrate over a charge $Q$? I've just started self-studying electricity and magnetism, and I am learning to calculate charge over line/surface/volumes. The standard procedure consists of changing the integrating variable from dq to space, the reason being that "we cannot integrate over a charge".
For example, take this tutorial https://physics.nfshost.com/textbook/06-Integration/01-Introduction.php

In particular, we don't know what the limits of integration are so we can't come up with a number: generally speaking, we know how to find the limits of integration if we're integrating over space (i.e. $x$) or time ($t$), but not over some other quantity like charge.

I don't get this. Why can't we integrate over a charge? It seems obvious to me that $ \int_0^Q dq = Q$, where $Q$ is the total charge of a rod.
What am I missing?
 A: I actually wrote a long rant about this recently here. As it turns out, $dq$ is just a placeholder for either $\lambda dl$ in one dimension, $\sigma dA$ is two dimensions, and $\rho dV$ in three dimensions. One advantage is that they have a well-defined form. $dV$ can be found through the Jacobian determinant. Likewise, $dA$ and $dl$ have well-defined forms from the theory of surface integrals and line integrals, respectively.
For really simple cases (where the integrand is constant and your geometry is flat), you can get away with integrating over $dq$. But if your charge distribution is curved or variable over space, then the $dq$ usage breaks down and stops making sense, in my opinion. There is also a problem with the limits of integration: what does it even mean to integrate from $q=0$ to $q=Q$?
In reality, this usage (sometimes) works because you use $q$ to parameterize your line/surface/volume integral. But there's no real advantage to this, since $dq$ obfuscates the meaning of your integral (and their integration limits) and you lose the advantage of having well defined differentials.
Personally, I wish that textbook authors would stop using $dq$ in basic E&M. It obfuscates a simple integral for no apparent reason.
