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Concepts Of Physics MCQ Edition [Volume 2]
Exam
Concepts Of Physics MCQ Edition [Volume 2]
Subject
Physics
Chapter
Capacitors

Concepts Of Physics MCQ Edition [Volume 2] · Physics · Capacitors

A parallel-plate capacitor features a plate area of 100 cm^2 and a...

A parallel-plate capacitor features a plate area of 100 cm2100 \text{ cm}^2 and a plate separation of 1.0 cm1.0 \text{ cm}. A glass plate (dielectric constant 6.06.0) with a thickness of 6.0 mm6.0 \text{ mm} and an ebonite plate (dielectric constant 4.04.0) are placed one over the other to completely fill the space between the capacitor plates. Determine the new capacitance.

Options

  1. A22 pF22 \text{ pF}
  2. B44 pF44 \text{ pF}
  3. C46 pF46 \text{ pF}
  4. D88 pF88 \text{ pF}

Correct answer

B. 44 pF44 \text{ pF}

Step-by-step solution

Given plate area A=100 cm2=10−2 m2A = 100 \text{ cm}^2 = 10^{-2} \text{ m}^2 and separation d=1.0 cm=10−2 md = 1.0 \text{ cm} = 10^{-2} \text{ m}.

The space between the plates is completely filled with two dielectric slabs. The thickness of the glass plate is t1=6.0 mm=6×10−3 mt_1 = 6.0 \text{ mm} = 6 \times 10^{-3} \text{ m} with dielectric constant K1=6.0K_1 = 6.0.

The thickness of the ebonite plate is t2=d−t1=1.0 cm−0.6 cm=0.4 cm=4×10−3 mt_2 = d - t_1 = 1.0 \text{ cm} - 0.6 \text{ cm} = 0.4 \text{ cm} = 4 \times 10^{-3} \text{ m} with dielectric constant K2=4.0K_2 = 4.0.

The equivalent capacitance of the parallel-plate capacitor with two dielectric slabs in series is given by:

C=ε0At1K1+t2K2C = \dfrac{\varepsilon_0 A}{\dfrac{t_1}{K_1} + \dfrac{t_2}{K_2}}

Substituting the given values:

t1K1=6×10−36=10−3 m\dfrac{t_1}{K_1} = \dfrac{6 \times 10^{-3}}{6} = 10^{-3} \text{ m}

t2K2=4×10−34=10−3 m\dfrac{t_2}{K_2} = \dfrac{4 \times 10^{-3}}{4} = 10^{-3} \text{ m}

t1K1+t2K2=10−3+10−3=2×10−3 m\dfrac{t_1}{K_1} + \dfrac{t_2}{K_2} = 10^{-3} + 10^{-3} = 2 \times 10^{-3} \text{ m}

Now, calculating the capacitance:

C=8.85×10−12×10−22×10−3C = \dfrac{8.85 \times 10^{-12} \times 10^{-2}}{2 \times 10^{-3}}

C=4.425×10−11 FC = 4.425 \times 10^{-11} \text{ F}

C=44.25 pF≈44 pFC = 44.25 \text{ pF} \approx 44 \text{ pF}
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