NEET Sample Paper NEET Sample Test Paper-93

  • question_answer
    The masses of the 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 resistance is

    A) 1: 3: 5  

    B)        5: 3: 1

    C) 1: 25: 125         

    D)        125: 15: 1

    Correct Answer: D

    Solution :

    [d] \[R=\frac{\rho l}{\pi {{r}^{2}}}.\]But \[m=\pi {{r}^{2}}\,ld\] \[\therefore \]\[\pi {{r}^{2}}=\frac{m}{ld}\] \[\therefore \]\[R=\frac{\rho {{l}^{2}}d}{m},\]\[{{R}_{1}}=\frac{\rho {{l}_{1}}^{2}d}{{{m}_{1}}},\]\[{{R}_{2}}=\frac{\rho {{l}_{2}}^{2}d}{{{m}_{2}}}\] \[{{R}_{3}}=\frac{\rho {{l}_{3}}^{2}d}{{{m}_{3}}}\] \[{{R}_{1}}:{{R}_{2}}:{{R}_{3}}=\frac{{{l}_{1}}^{2}}{{{m}_{1}}}:\frac{{{l}_{2}}^{2}}{{{m}_{2}}}:\frac{{{l}_{3}}^{2}}{{{m}_{3}}}\] \[{{R}_{1}}:{{R}_{2}}:{{R}_{3}}=\frac{25}{1}:\frac{9}{3}:\frac{1}{5}=125:15:1\]


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