Conducting slab between the plates of parallel plate capacitor

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cnh1995
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From the above equations it is evident that it is better to use Metalic slab instead of Insulator,
If you look at the mechanism of charge storage, you can see it is achieved by polarization of the dielectric. Polarization implies no conduction and hence, charges are trapped on the plates. If the dielectric is replaced by a metallic slab, there will be conduction and no charge will be stored on the plates. In dielectrics, the opposing field due to polarization is less than the original applied field, hence you get a non-zero net (but reduced) electric field in the dielectric. To bring the net field in the dielectric back to the original applied magnitude, more charge is needed on the plates, which is provided by the source in the circuit. When the dielectric is replaced by a conducting slab, the net electric field becomes 0 i.e. conductor produces an opposing field equal to the applied field. This means, if you were to bring the net field back to the original applied magnitude, you'd need an infinite amount of charge, which means it will never happen practically. If such a capacitor is placed in a circuit, it will simply conduct and no charge will be stored. Hence, practically it is no longer a capacitor.
 

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