A Parallel-Plate Capacitor Stores Charge Q

Multiple Choice

A parallel-plate capacitor stores charge Q.The capacitor is then disconnected from its voltage source,and the space between the plates is filled with a dielectric of constant κ = 2.What is the relationship between the stored energies of the capacitor,where PE_i is the initial stored energy,and PE_f is the stored energy after the dielectric is inserted?​

A) ​PEf = 0.25PE_i
B) ​PEf = 0.5PE_i
C) ​PEf = PE_i
D) ​PEf = 2PE_i