Question

A dielectric is inserted into a capacitor while the charge on it is kept constant. What happens to the potential difference and the stored energy?

• A. The potential difference decreases and the stored energy increases
• B. Both the potential difference and the stored energy increase
• C. The potential difference increases and the stored energy decreases
• D. Both the potential difference and the stored energy decrease
• E. Both the potential difference and the stored energy remain the same

Answer 1

Answer: (D)

When a dielectric is placed in an electric field then an internal electric field is produced in it due to polarisation of dielectric . This internal electric field inside the dielectric is in opposite direction of the field between plates of capacitor , as a result of this effective electric field between plates decreases , hence the potential difference between plates because ,

$E=\Delta V/\Delta r$ ,

where $\Delta r$ is the distance between plates , which is constant .

Now potential energy of a capacitor is given by ,

$U=\left(1/2\right)q\Delta V$ ,

charge is given constant , therefore due to decrease in potential difference $\Delta V$ , potential energy $U$ also decreases .