A parallel plate capacitor has plates of area A separated by distance ' d  ' between them. It is filled with a dielectric which has a dielectric constant that varies as k ( n ) = k ( 1 + α n )   where '  x ' is he distance measured from one of the plates.

If ( α d ) < < 1  , the total capacitance of the system is best given by the expression:

Option 1 - <p><span class="mathml" contenteditable="false"> <math> <mfrac> <mrow> <mrow> <mi mathvariant="normal">A</mi> <mi mathvariant="normal">K</mi> <msub> <mrow> <mrow> <mi>ε</mi> </mrow> </mrow> <mrow> <mrow> <mn>0</mn> </mrow> </mrow> </msub> </mrow> </mrow> <mrow> <mrow> <mtext> </mtext> <mi mathvariant="normal">d</mi> </mrow> </mrow> </mfrac> <mfenced separators="|"> <mrow> <mrow> <mn>1</mn> <mo>+</mo> <mfrac> <mrow> <mrow> <mi>α</mi> <mi mathvariant="normal">d</mi> </mrow> </mrow> <mrow> <mrow> <mn>2</mn> </mrow> </mrow> </mfrac> </mrow> </mrow> </mfenced> </math> </span></p>
Option 2 - <p><span class="mathml" contenteditable="false"> <math> <mfrac> <mrow> <mrow> <mi>A</mi> <msub> <mrow> <mrow> <mi>ε</mi> </mrow> </mrow> <mrow> <mrow> <mn>0</mn> </mrow> </mrow> </msub> <mi>K</mi> </mrow> </mrow> <mrow> <mrow> <mi>d</mi> </mrow> </mrow> </mfrac> <mfenced separators="|"> <mrow> <mrow> <mn>1</mn> <mo>+</mo> <mfrac> <mrow> <mrow> <msup> <mrow> <mrow> <mi>α</mi> </mrow> </mrow> <mrow> <mrow> <mn>2</mn> </mrow> </mrow> </msup> <msup> <mrow> <mrow> <mi>d</mi> </mrow> </mrow> <mrow> <mrow> <mn>2</mn> </mrow> </mrow> </msup> </mrow> </mrow> <mrow> <mrow> <mn>2</mn> </mrow> </mrow> </mfrac> </mrow> </mrow> </mfenced> </math> </span></p>
Option 3 - <p><span class="mathml" contenteditable="false"> <math> <mfrac> <mrow> <mrow> <mi mathvariant="normal">A</mi> <mi mathvariant="normal">K</mi> <msub> <mrow> <mrow> <mi>ε</mi> </mrow> </mrow> <mrow> <mrow> <mn>0</mn> </mrow> </mrow> </msub> </mrow> </mrow> <mrow> <mrow> <mtext> </mtext> <mi mathvariant="normal">d</mi> </mrow> </mrow> </mfrac> <mo>(</mo> <mn>1</mn> <mo>+</mo> <mi>α</mi> <mi mathvariant="normal">d</mi> <mo>)</mo> </math> </span></p>
Option 4 - <p><span class="mathml" contenteditable="false"> <math> <mfrac> <mrow> <mrow> <mi mathvariant="normal">A</mi> <msub> <mrow> <mrow> <mi>ε</mi> </mrow> </mrow> <mrow> <mrow> <mn>0</mn> </mrow> </mrow> </msub> <mtext> </mtext> <mi mathvariant="normal">K</mi> </mrow> </mrow> <mrow> <mrow> <mtext> </mtext> <mi mathvariant="normal">d</mi> </mrow> </mrow> </mfrac> <mfenced open="[" close="]" separators="|"> <mrow> <mrow> <mn>1</mn> <mo>+</mo> <msup> <mrow> <mrow> <mfenced separators="|"> <mrow> <mrow> <mfrac> <mrow> <mrow> <mi>α</mi> <mi mathvariant="normal">d</mi> </mrow> </mrow> <mrow> <mrow> <mn>2</mn> </mrow> </mrow> </mfrac> </mrow> </mrow> </mfenced> </mrow> </mrow> <mrow> <mrow> <mn>2</mn> </mrow> </mrow> </msup> </mrow> </mrow> </mfenced> </math> </span></p>
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1 Answer
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5 months ago
Correct Option - 1
Detailed Solution:

Capacitance of element

Capacitance of element, C ' = K ( 1 + α x ) ε 0 A d x

1 C ' = 0 d ? d x K ε 0 A ( 1 + α x )

1 C = 1 K ε 0 A α l n ? ( 1 + α d )

Given: α d ? 1

1 C = 1 K ε 0 A α α d - α 2 d 2 2 ; 1 C = d K ε 0 A 1 - α d 2

C = K ε 0 A d 1 + α d 2

 

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Now, using junction analysis
We can say, q? + q? + q? = 0
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x = 36/11, q? = 36 (5)/11 = 180/11
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Physics Electrostatic Potential and Capacitance 2025

Physics Electrostatic Potential and Capacitance 2025

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