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Capacitor question

2021 · 26 Feb · Shift 1 · Q44
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  5. /2021 · 26 Feb · Shift 1 · Q44

Capacitor question

2021 · 26 Feb · Shift 1 · Q44

JEE MainPhysicsCapacitorMCQ+4 / −1
Consider the combination of 2 capacitors C1 and C2 with C2 > C1, when connected in parallel, the equivalent capacitance is 154{{15} \over 4}415​ times the equivalent capacitance of the same connected in series. Calculate the ratio of capacitors, C2C1{{{C_2}} \over {{C_1}}}C1​C2​​.
  1. A
    1511{{15} \over {11}}1115​
  2. B
    No Solutions
  3. C
    2915{{29} \over {15}}1529​
  4. D
    154{{15} \over {4}}415​
View written solutionFree

Correct answer: B

  1. Let the capacitors be C1C_1C1​ and C2C_2C2​, with C2>C1C_2 > C_1C2​>C1​.

  2. Equivalent capacitance in parallel: Cp=C1+C2C_p = C_1 + C_2Cp​=C1​+C2​

  3. Equivalent capacitance in series: Cs=C1C2C1+C2C_s = \frac{C_1C_2}{C_1 + C_2}Cs​=C1​+C2​C1​C2​​

  4. Given that the parallel equivalent is 154\dfrac{15}{4}415​ times the series equivalent: Cp=154CsC_p = \frac{15}{4}C_sCp​=415​Cs​

Substitute the formulas: C1+C2=154(C1C2C1+C2)C_1 + C_2 = \frac{15}{4}\left(\frac{C_1C_2}{C_1 + C_2}\right)C1​+C2​=415​(C1​+C2​C1​C2​​)

  1. Multiply both sides by C1+C2C_1 + C_2C1​+C2​: (C1+C2)2=154C1C2\left(C_1 + C_2\right)^2 = \frac{15}{4}C_1C_2(C1​+C2​)2=415​C1​C2​

  2. Let x=C2C1x = \frac{C_2}{C_1}x=C1​C2​​ Then C2=xC1C_2 = xC_1C2​=xC1​.

Substitute: (C1+xC1)2=154C1(xC1)\left(C_1 + xC_1\right)^2 = \frac{15}{4}C_1(xC_1)(C1​+xC1​)2=415​C1​(xC1​) C12(1+x)2=154xC12C_1^2(1+x)^2 = \frac{15}{4}xC_1^2C12​(1+x)2=415​xC12​

Cancel C12C_1^2C12​: (1+x)2=154x(1+x)^2 = \frac{15}{4}x(1+x)2=415​x

  1. Expand and simplify: 1+2x+x2=154x1 + 2x + x^2 = \frac{15}{4}x1+2x+x2=415​x Multiply by 4: 4+8x+4x2=15x4 + 8x + 4x^2 = 15x4+8x+4x2=15x 4x2−7x+4=04x^2 - 7x + 4 = 04x2−7x+4=0

  2. Solve the quadratic: x=7±49−648=7±−158x = \frac{7 \pm \sqrt{49 - 64}}{8} = \frac{7 \pm \sqrt{-15}}{8}x=87±49−64​​=87±−15​​

The discriminant is negative, so there is no real solution.

Since capacitance values must be real and positive, this situation is impossible.

  1. Therefore, the correct option is: B: No Solutions\boxed{\text{B: No Solutions}}B: No Solutions​
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