Question

why dont we use vaccume as medium instead of the air in the parallel plate capacitor ?

Answer 1

We don't use vacuum instead of air in the parallel plate capacitor because air is a dielectric medium.Use of any dielectric medium between the parallel plates of the capacitor leads to rise in the value of capacitance of the capacitor.
Its known that Capacitance 1 = Q/V =Kε₀A/d where K is the dielectric constant of the dielectric medium between the two parallel plates of the capacitor.ε₀ is the permitivity of free space.'A' is the cross sectional area of the plates and 'd' is the separation between the two plates.
Further, its also true that Capacitance 2 = ε₀A/d when there is vacuum or air between the two plates of capacitor.The dielectric constant for air is 1.0009.
By dividing the first equation with the second , we get :
Capacitance1=K times Capacitance 2.
Thus using air as a medium between the two plates of the parallel plate capacitor increases the capacitance of the capacitor by a factor of 1.0009.