A parallel plate capacitor with square plates is filled with four dielectrics of dielectric constants K1,K2,K3,K4 arranged as shown in the figure. The effective dielectric constant K will be
A
K=(K1+K2)(K3+K4)2(K1+K2+K3+K4)
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B
K=(K1+K4)(K2+K3)2(K1+K2+K3+K4)
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C
K=(K1+K2)(K3+K4)K1+K2+K3+K4
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D
K=(K1+K3)(K2+K4)K1+K2+K3+K4
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Solution
The correct option is DK=(K1+K3)(K2+K4)K1+K2+K3+K4
As C1 and C3 are in parallel, C13=C1+C3=(K1+K3)⎡⎢
⎢
⎢⎣ϵ0L2×Ld/2⎤⎥
⎥
⎥⎦ C24=C4+C2=(K4+K2)⎡⎢
⎢
⎢⎣ϵ0L2×Ld/2⎤⎥
⎥
⎥⎦
As combination of C13&C24 are in series Ceq=C13C24C13+C24=(K1+K3)⎡⎢
⎢
⎢⎣ϵ0L2×Ld/2⎤⎥
⎥
⎥⎦(K4+K2)⎡⎢
⎢
⎢⎣ϵ0L2×Ld/2⎤⎥
⎥
⎥⎦(K1+K3)⎡⎢
⎢
⎢⎣ϵ0L2×Ld/2⎤⎥
⎥
⎥⎦+(K4+K2)⎡⎢
⎢
⎢⎣ϵ0L2×Ld/2⎤⎥
⎥
⎥⎦ Ceq=(K1+K3)(K2+K4)K1+K2+K3+K4ϵ0L2d…(i)
Now if Ceq=Keqϵ0L2d....(ii)
on comparing equation (i) to equation (ii), we get Keq=(K1+K3)(K2+K4)K1+K2+K3+K4