the capacitance of parallel plate air capacitor is 15 microfarade and distance between its plate is 6 mm . a copper plate is 3mm is inseted between two plate symmetrically . capacitance will be

Dear student,

When a conductor is inserted inside a capacitor, the air inside the capacitor, that is, the space where the electric field resides, shrinks. Hence effectively the width of the capacitor is decreased.

Initially, the width of region of air inside the capacitor was.
After inserting copper plate of width 3mm, the width of region of air decreases to 3mm.

Since, the capacitance on insertion of the copper plate doubles. Hence new capacitance=.