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# A cylindrical layer of homogeneous dielectric with dielectric constant K is introduced in a cylindrical capacitor so that a gap of width d is filled in between them. Radius of plate $R > > d$. If the capacitor is connected to battery of EMF V, the force pulling the dielectric inside the capacitor is :

$(A)\;\frac{\in _0 (k-1) \pi RV^2}{d} \\ (B)\;\frac{\in _0 (k-1)V^2 R}{\pi d} \\ (C)\; \frac{\in _0 (k-1) V^2}{2d} \\ (D)\;none$

Use formula $F= -\large\frac{du}{dx}$
Where $U= \large\frac{q^2}{2c}$
We can treat cylindrical capacitor as a parallel plate capacitor as $d < < R$
=>$\large\frac{\in _0 (k-1) \pi RV^2}{d}$
Hence A is the correct answer.