CET Karnataka Medical CET - Karnataka Medical Solved Paper-2013

  • question_answer
    Masses of three wires of copper are in the ratio of \[1:3:5\]and their lengths are in the ratio of \[5:3:1\]. The ratio of their electrical resistances is

    A)  \[1:3:5\]           

    B)  \[5:3:1\]

    C)  \[1:15:125\]

    D)  \[125:15:1\]

    Correct Answer: D

    Solution :

    : Mass of wire, M = Volume \[\times \]Density \[=Al\times d\] or  \[A=\frac{M}{ld}\] Resistance of wire, \[R=\frac{\rho l}{A}=\frac{\rho l}{(M/ld)}\] or \[R=\frac{\rho {{l}^{2}}d}{m}\] Since all the three wires are made up of same material, therefore p and d remain the same for all the three wires. Hence, \[{{R}_{1}}:{{R}_{2}}:{{R}_{3}}=\frac{l_{1}^{2}}{{{M}_{1}}}:\frac{l_{2}^{2}}{{{M}_{2}}}:\frac{l_{3}^{3}}{{{M}_{3}}}=\frac{{{5}^{2}}}{1}:\frac{{{3}^{2}}}{3}:\frac{{{1}^{2}}}{5}\] \[=25:3:\frac{1}{5}=125:15:1\]


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