9)
A practical way to get around the problem (of keeping Q undisturbed
in the presence of q) is to hold Q to its location by unspecified forces This may look strange but actually this is what happens in practice When we are considering the electric force on a test charge q due to a
charged planar sheet (Section 1 14), the charges on the sheet are held to
their locations by the forces due to the unspecified charged constituents
inside the sheet