(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

It comes from example 3.2 in griffith's 3rd edition electrodynamics book

A point charge q is situated a distance a from the center of a grounded conducting sphere of radius R. Find the potential outside the sphere.

2. Relevant equations

3. The attempt at a solution

I can follow the example, but it assumes that there is another point charge q' = -(R/a) * q, where q' is a distance b from the center (b = R^2/a) and q is a distance a from the center or a distance a-b from q'

My question is... where does it get such an assumption? Or, how does one know to chose such an arbitrary expression? I'm mostly interested in the relationship between the two charges... and how the charge q' (on the inside) is determined.

Thanks for your help

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# Homework Help: I don't understand an assumption the textbook made ?

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