Physics · Electrostatic Potential

A parallel plate capacitor is charged to a potential \( V \) by a battery. The battery is then disconnected and a dielectric slab of dielectric constant \( K \) is completely inserted between the plates. If the initial electrostatic energy stored was \( U_0 \), what is the work done by the external agent in inserting the slab slowly?

1. A
   \( U_0 (K - 1) \)
2. B
   \( U_0 (\frac{1}{K} - 1) \)
3. C
   \( -U_0 (1 - \frac{1}{K}) \)
4. D
   \( -U_0 (1 - \frac{1}{K^2}) \)