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JEE Main 202621 JanuaryMorning Shift
Exam
JEE Main
Year
2026
Date
21 January
Shift
Morning Shift
Subject
Physics
Chapter
Capacitance

JEE Main 2026 · 21 January · Morning Shift · Physics · Capacitance

A parallel plate capacitor has capacitance C , when there is vacuum...

A parallel plate capacitor has capacitance CC, when there is vacuum within the parallel plates. A sheet having thickness (13)rd\left(\frac{1}{3}\right)^{\mathrm{rd}} of the separation between the plates and relative permittivity KK is introduced between the plates. The new capacitance of the system is :

Options

  1. A4KC3K−1\frac{4 K C}{3 K-1}
  2. B3CK2(2K+1)2\frac{3 C K^{2}}{(2 K+1)^{2}}
  3. C3KC2K+1\frac{3 K C}{2 K+1}
  4. DCK2+K\frac{C K}{2+K}

Correct answer

C. 3KC2K+1\frac{3 K C}{2 K+1}

Step-by-step solution

Original capacitance: C=ε0AdC = \frac{\varepsilon_0 A}{d}
After inserting dielectric of thickness d3\frac{d}{3}, system becomes two capacitors in series.
Vacuum region: C1=ε0A2d/3=3C2C_1 = \frac{\varepsilon_0 A}{2d/3} = \frac{3C}{2}
Dielectric region: C2=Kε0Ad/3=3KCC_2 = \frac{K\varepsilon_0 A}{d/3} = 3KC
Equivalent capacitance: 1Cnew=1C1+1C2=23C+13KC=2K+13KC\frac{1}{C_{new}} = \frac{1}{C_1} + \frac{1}{C_2} = \frac{2}{3C} + \frac{1}{3KC} = \frac{2K+1}{3KC}
Cnew=3KC2K+1C_{new} = \frac{3KC}{2K+1}
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