Q.

A parallel-plate capacitor of capacitance 40 μF is connected to a 100 V power supply. Now the intermediate space between the plates is filled with a dielectric material of dielectric constant K = 2. Due to the introduction of dielectric material, the extra charge and the change in the electrostatic energy in the capacitor, respectively, are          [2025]

1 2 mC and 0.2 J  
2 8 mC and 2.0 J  
3 4 mC and 0.2 J  
4 2 mC and 0.4 J  

Ans.

(3)

C=40 μF, C'=kC=80 μF

V = 100 V

Q = CV = 4 mC

Q'=C'V=8 mC

 Extra charge, Q = 4 mC

U1=12CV2, U2=12C'V2

U=12(C'C)V2=12×40×104×106=0.2 J