A parallel plate capacitor having crosssectional area $A$ and separation $d$ has air in between the plates. Now an insulating slab of same area but thickness $d/2$ is inserted between the plates as shown in figure having dielectric constant $K (=4) .$ The ratio of new capacitance to its original capacitance will be,
NEET 2020, Medium
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$C_{0}=\frac{\varepsilon_{0} A}{d}$

After inserting dielectric

$C=\frac{\varepsilon_{0} A}{(d-t)+\frac{t}{k}}$

$=\frac{\varepsilon_{0} A}{\frac{d}{2}+\frac{d}{8}}$

$=\frac{8 \varepsilon_{0} A}{5 d}$

$=\frac{8}{5} C_{0}$

So, $\frac{C}{C_{0}}=\frac{8}{5}$

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