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A wire P has half the diameter and half the length of a wire Q...

Physics
JAMB 1985

A wire P has half the diameter and half the length of a wire Q of similar material. The ratio of the resistance of P to that of Q is
  • A. 8 : 1
  • B. 4 : 1
  • C. 2 : 1
  • D. 1 : 1
  • E. 1 : 4
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Correct Answer: Option C
Explanation
Resistance \(\alpha\) \(\frac{length}{area}\)

\(\frac{R_Q}{R_P}\) = \(\frac{L_QA_P}{L_PA_Q}\)

LQ = 2LP

DQ = 2DP

AP = \(\pi\)\(\frac{(2D_P)^2}{2}\)

= \(\pi\)DP

AQ = \(\pi\)\(\frac{(D_P)^2}{2}\)

= \(\frac{1}{4}\)\(\pi\)DP

therefore, \(\frac{R_Q}{R_P}\) = \(\frac{2L_P \times \pi D_P}{L_P \times \frac{1}{4} \pi D_P}\)

= 2 : 1

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JAMB CBT Mobile App 2025 - Free Download
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