This is not a duplicate question for many similar questions, I have tried finding an answer to this everywhere but to no avail. So here is my problem: Why is Earth's potential taken to be zero?
Now I know that electric potential is not an absolute quantity, it's only the change in potential that is significant so we can set a reference anywhere according to our convenience. And that's what I thought too, we simply set Earth's potential to be zero instead of infinity for convenience.
Now here's the issue: my professor gave us the following problem:
"there are two thin concentric conducting shells of radius $a$ and $b$ ($a<b$), and the inner shell is given a charge $q$ on its outer surface (charge induction will take place on outer shell) and also the outer shell is now earthed. Find the final charges on the shells".
For the solution, he says that the potential of the outer shell is to be made zero as it is earthed. Using this and the fact that the potential on the surface of a shell is given by KQ/r, where Q is the charge on shell and r is the radius, he finds the solution.
Now my problem is that isn't the formula for the potential of a shell on its surface (i.e., KQ/r) derived with the assumption that the reference is set at infinity? That means my professor is setting the reference at ground level but using the formula derived with reference at infinity and still getting the correct answer? I think I am surely missing something. There are some questions in the textbook that have similar questions and use similar methods. What seems to be the issue here?