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Current Electricity question

2023 · 12 Apr · Shift 1 · Q55
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Current Electricity question

2023 · 12 Apr · Shift 1 · Q55

JEE MainPhysicsCurrent ElectricityMCQ+4 / −1
A wire of resistance 160 Ω160 ~\Omega160 Ω is melted and drawn in a wire of one-fourth of its length. The new resistance of the wire will be
  1. A
    640 Ω640 ~\Omega640 Ω
  2. B
    40 Ω40 ~\Omega40 Ω
  3. C
    16 Ω16 ~\Omega16 Ω
  4. D
    10 Ω10 ~\Omega10 Ω
View written solutionFree

Correct answer: D

  1. Use the resistance formula

    For a wire, R=ρLAR = \rho \frac{L}{A}R=ρAL​ where ρ\rhoρ is resistivity, LLL is length, and AAA is cross-sectional area.

  2. Apply conservation of volume

    When the wire is melted and redrawn, its material and hence its volume remain the same: AL=A′L′A L = A' L'AL=A′L′

    Given the new length is one-fourth of the original: L′=L4L' = \frac{L}{4}L′=4L​

    So, A′⋅L4=ALA' \cdot \frac{L}{4} = A LA′⋅4L​=AL A′=4AA' = 4AA′=4A

  3. Find the new resistance

    The new resistance is R′=ρL′A′=ρL/44A=ρL16A=R16R' = \rho \frac{L'}{A'} = \rho \frac{L/4}{4A} = \rho \frac{L}{16A} = \frac{R}{16}R′=ρA′L′​=ρ4AL/4​=ρ16AL​=16R​

  4. Substitute the original resistance

    R′=16016=10 ΩR' = \frac{160}{16} = 10~\OmegaR′=16160​=10 Ω

  5. Match with the options

    Therefore, the new resistance is: 10 Ω\boxed{10~\Omega}10 Ω​

    So the correct option is D.

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