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NEET 2026
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NEET
Year
2026
Subject
Physics
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Chapter

NEET 2026 · Physics · Chapter

A parallel plate capacitor has a capacitance C_0 when there is air...

A parallel plate capacitor has a capacitance C0C_0 when there is air between its plates, which are separated by a distance dd. Two dielectric slabs are inserted between the plates, parallel to them. The first slab has a thickness of d2\dfrac{d}{2} and a dielectric constant of 44. The second slab has a thickness of d4\dfrac{d}{4} and an unknown dielectric constant KK. The remaining space between the plates is filled with air. If the new equivalent capacitance of the system becomes 2C02C_0, what is the value of KK?

Options

  1. A88
  2. B44
  3. C22
  4. D0.50.5

Correct answer

C. 22

Step-by-step solution

Initial capacitance is C0=ε0AdC_0 = \dfrac{\varepsilon_0 A}{d}.

When dielectric slabs are inserted, the equivalent capacitance is given by C=ε0At1K1+t2K2+t3K3C = \dfrac{\varepsilon_0 A}{\dfrac{t_1}{K_1} + \dfrac{t_2}{K_2} + \dfrac{t_3}{K_3}}.

The thicknesses of the regions are t1=d2t_1 = \dfrac{d}{2}, t2=d4t_2 = \dfrac{d}{4}, and the remaining air space is t3=d−(d2+d4)=d4t_3 = d - \left(\dfrac{d}{2} + \dfrac{d}{4}\right) = \dfrac{d}{4}.

Substituting the given values:

C=ε0Ad/24+d/4K+d/41C = \dfrac{\varepsilon_0 A}{\dfrac{d/2}{4} + \dfrac{d/4}{K} + \dfrac{d/4}{1}}

C=ε0Ad(18+14K+14)C = \dfrac{\varepsilon_0 A}{d \left(\dfrac{1}{8} + \dfrac{1}{4K} + \dfrac{1}{4}\right)}

C=C038+14KC = \dfrac{C_0}{\dfrac{3}{8} + \dfrac{1}{4K}}

Given that C=2C0C = 2C_0, we have:

2C0=C038+14K2C_0 = \dfrac{C_0}{\dfrac{3}{8} + \dfrac{1}{4K}}

38+14K=12\dfrac{3}{8} + \dfrac{1}{4K} = \dfrac{1}{2}

14K=12−38=18\dfrac{1}{4K} = \dfrac{1}{2} - \dfrac{3}{8} = \dfrac{1}{8}

4K=8⇒K=24K = 8 \Rightarrow K = 2

Answer: 22
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