BCECE Engineering BCECE Engineering Solved Paper-2001

  • question_answer
    The masses of three wires of copper are in the ratio 1 : 2 : 3 and their lengths are in the ratio 3 : 2 : 1. The ratio of their resistances is:

    A)  3 : 2 : 1                                

    B)  1 : 2 : 3

    C)         27 : 6 : 1              

    D)         1 : 6 : 27

    Correct Answer: C

    Solution :

    Resistance of wire \[R=\frac{\rho l}{A}\]                 or            \[R\propto \frac{{{l}^{2}}}{Al}\]                 or            \[R\propto \frac{{{l}^{2}}}{V}\]                 or            \[R\propto \frac{{{l}^{2}}d}{m}\] (where\[d\]is density and \[m\] is mass) or            \[R\propto \frac{{{l}^{2}}}{m}\] Here,     \[{{l}_{1}}:{{l}_{2}}:{{l}_{3}}=3:2:1\] \[{{m}_{1}}:{{m}_{2}}:{{m}_{3}}=1:2:3\] \[\therefore \] \[{{R}_{1}}:{{R}_{2}}:{{R}_{3}}:\,\,:\,\frac{{{l}_{1}}^{2}}{{{m}_{1}}}:\frac{{{l}_{2}}^{2}}{{{m}_{2}}}:\frac{{{l}_{3}}^{2}}{{{m}_{3}}}\] or            \[{{R}_{1}}:{{R}_{2}}:{{R}_{3}}:\,\,:\,\frac{9}{1}:\frac{4}{2}:\frac{1}{3}\] or            \[{{R}_{1}}:{{R}_{2}}:{{R}_{3}}:\,\,:\,27:6:1\]


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