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Capacitor question

2025 · 24 Jan · Shift 1 · Q63
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  5. /2025 · 24 Jan · Shift 1 · Q63

Capacitor question

2025 · 24 Jan · Shift 1 · Q63

JEE MainPhysicsCapacitorMCQ+4 / −1
A parallel plate capacitor was made with two rectangular plates, each with a length of l=3 cml=3 \mathrm{~cm}l=3 cm and breath of b=1 cm\mathrm{b}=1 \mathrm{~cm}b=1 cm. The distance between the plates is 3μ m3 \mu \mathrm{~m}3μ m. Out of the following, which are the ways to increase the capacitance by a factor of 10 ? A. l=30 cm, b=1 cm, d=1μ ml=30 \mathrm{~cm}, \mathrm{~b}=1 \mathrm{~cm}, \mathrm{~d}=1 \mu \mathrm{~m}l=30 cm, b=1 cm, d=1μ m B. l=3 cm, b=1 cm, d=30μ ml=3 \mathrm{~cm}, \mathrm{~b}=1 \mathrm{~cm}, \mathrm{~d}=30 \mu \mathrm{~m}l=3 cm, b=1 cm, d=30μ m C. l=6 cm, b=5 cm, d=3μ ml=6 \mathrm{~cm}, \mathrm{~b}=5 \mathrm{~cm}, \mathrm{~d}=3 \mu \mathrm{~m}l=6 cm, b=5 cm, d=3μ m D. l=1 cm, b=1 cm, d=10μ ml=1 \mathrm{~cm}, \mathrm{~b}=1 \mathrm{~cm}, \mathrm{~d}=10 \mu \mathrm{~m}l=1 cm, b=1 cm, d=10μ m E. l=5 cm, b=2 cm, d=1μ ml=5 \mathrm{~cm}, \mathrm{~b}=2 \mathrm{~cm}, \mathrm{~d}=1 \mu \mathrm{~m}l=5 cm, b=2 cm, d=1μ m Choose the correct answer from the options given below:
  1. A
    C only
  2. B
    A only
  3. C
    B and D only
  4. D
    C and E only
View written solutionFree

Correct answer: D

  1. For a parallel plate capacitor,

C=ε0Ad=ε0lbdC = \varepsilon_0 \frac{A}{d} = \varepsilon_0 \frac{lb}{d}C=ε0​dA​=ε0​dlb​

So capacitance is proportional to

C∝lbdC \propto \frac{lb}{d}C∝dlb​

  1. Original capacitor:

l=3 cm,b=1 cm,d=3 μml=3\text{ cm}, \quad b=1\text{ cm}, \quad d=3\,\mu\text{m}l=3 cm,b=1 cm,d=3μm

Thus,

lbd=3×13=1\frac{lb}{d} = \frac{3\times 1}{3} = 1dlb​=33×1​=1

So for the new capacitor to have capacitance increased by a factor of 101010, we need

l′b′d′=10×lbd\frac{l'b'}{d'} = 10 \times \frac{lb}{d}d′l′b′​=10×dlb​

Since the original ratio is 111, we simply need

l′b′d′=10\frac{l'b'}{d'} = 10d′l′b′​=10

(Here l,bl,bl,b are in cm and ddd in μ\muμm, so we compare factors relative to the original setup.)

  1. Check each case:

Case A

l=30, b=1, d=1l=30,\ b=1,\ d=1l=30, b=1, d=1

lbd=30×11=30\frac{lb}{d} = \frac{30\times 1}{1} = 30dlb​=130×1​=30

Increase factor =30=30=30, not 101010.

So A is not correct.

Case B

l=3, b=1, d=30l=3,\ b=1,\ d=30l=3, b=1, d=30

lbd=3×130=0.1\frac{lb}{d} = \frac{3\times 1}{30} = 0.1dlb​=303×1​=0.1

This is one-tenth of original, not ten times.

So B is not correct.

Case C

l=6, b=5, d=3l=6,\ b=5,\ d=3l=6, b=5, d=3

lbd=6×53=10\frac{lb}{d} = \frac{6\times 5}{3} = 10dlb​=36×5​=10

This is exactly 101010 times the original.

So C is correct.

Case D

l=1, b=1, d=10l=1,\ b=1,\ d=10l=1, b=1, d=10

lbd=1×110=0.1\frac{lb}{d} = \frac{1\times 1}{10} = 0.1dlb​=101×1​=0.1

Not 101010 times.

So D is not correct.

Case E

l=5, b=2, d=1l=5,\ b=2,\ d=1l=5, b=2, d=1

lbd=5×21=10\frac{lb}{d} = \frac{5\times 2}{1} = 10dlb​=15×2​=10

This is exactly 101010 times the original.

So E is correct.

  1. Therefore, the correct cases are:

C and E\boxed{C \text{ and } E}C and E​

  1. Match with given options:

Option D = C and E only.

So the correct option is

D\boxed{\text{D}}D​

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