I was reading "point charge in the presence of a charged, insulated, conducting sphere" from Jackson's Classical Electrodynamics.
Let initially the sphere had on its surface $Q$. When $q$ is brought at $y$ from the center of the sphere, it must provide force to the charges residing on the surface owing to its electric field. However, he immediately jot down that only $q' = -\frac{a}{y}q$ will experience force from $q$ & that $q'$ would balance the electrostatic force of $q$; thus $(Q - q')$ will spread uniformly on the surface as this suffers no external force.
If we wish to consider the problem of an insulated conducting sphere with total charge $Q$ in the presence of a point charge $q$, we can build up the solution for the potential by linear superposition. In an operational sense, we can imagine with a grounded conducting sphere(with its charge $q'$ distributed over its surface). We then disconnect the ground wire & add to the sphere an amount of charge $(Q - q')$. ... .To find the potential, we merely note that the added charge $(Q - q')$ will distribute itself uniformly over the surface since the electrostatic forces due to point charge $q$ are already balanced by the charge $q'$.[...]
I'm confused how really $q'$ 'balanced' the electric field of $q$. I thought only a same magnitude of charge can balance the field of an equal but opposite charge exactly at the mid-point between them. So, I couldn't understand how/why $q'$ balanced the field of $q$. Was there no field of $q$ anymore, after $q'$ 'balanced' its field?? Can anyone explain me why & how $q'$ cancels or "balances" the field of $q$ even it is not of the same magnitude as that of $q$? Does all the field of $q$ vanish after being balanced by $q'$??
If I bring a charge near a system of $q$ & $q'$, it still gets force from the configuration even if the field of $q$ is balanced by $q'$ at the middle. So, why doesn't the charge $Q - q'$ experience force from $q$ despite its field got balanced by $q'$??
