If a thin metal foil of same area is placed between the two plates of a parallel plate capacitor of capacitance C, then new capacitance will be :
We have
C=ε0Ad
After placement of plate of equal area in between two plates we have two capacitors in series.
Value of each capacitor
C1=ε0Ad/2
C1=2ε0Ad=2C
Equivalent capacitance
Ceq=2C×2C2C+2C
Ceq=C