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Current Electricity question

2022 · 28 Jul · Shift 1 · Q56
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Current Electricity question

2022 · 28 Jul · Shift 1 · Q56

JEE MainPhysicsCurrent ElectricityMCQ+4 / −1
A wire of resistance R1 is drawn out so that its length is increased by twice of its original length. The ratio of new resistance to original resistance is :
  1. A
    9 : 1
  2. B
    1 : 9
  3. C
    4 : 1
  4. D
    3 : 1
View written solutionFree

Correct answer: A

  1. Use the resistance formula

    For a wire, R=ρLAR = \rho \frac{L}{A}R=ρAL​ where:

    • ρ\rhoρ = resistivity (constant for same material)
    • LLL = length
    • AAA = cross-sectional area
  2. Interpret the statement

    “Its length is increased by twice of its original length” means the new length becomes: L′=L+2L=3LL' = L + 2L = 3LL′=L+2L=3L

  3. Use conservation of volume

    When a wire is drawn out, its volume remains constant: AL=A′L′AL = A'L'AL=A′L′

    Since L′=3LL' = 3LL′=3L, A′=ALL′=AL3L=A3A' = \frac{AL}{L'} = \frac{AL}{3L} = \frac{A}{3}A′=L′AL​=3LAL​=3A​

  4. Find the new resistance

    Original resistance: R1=ρLAR_1 = \rho \frac{L}{A}R1​=ρAL​

    New resistance: R2=ρL′A′=ρ3LA/3=ρ9LA=9R1R_2 = \rho \frac{L'}{A'} = \rho \frac{3L}{A/3} = \rho \frac{9L}{A} = 9R_1R2​=ρA′L′​=ρA/33L​=ρA9L​=9R1​

  5. Compute the ratio

    R2:R1=9:1R_2 : R_1 = 9 : 1R2​:R1​=9:1

  6. Check options

    • A: 9:19:19:1 ✅
    • B: 1:91:91:9 ❌
    • C: 4:14:14:1 ❌
    • D: 3:13:13:1 ❌

Therefore, the correct answer is A.

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