RAJASTHAN PMT Rajasthan - PMT Solved Paper-2008

  • question_answer
    The length of a given cylindrical wire is increased by 100%. Due to the consequent decrease in diameter the change in the resistance of the wire will be

    A)  200%                                   

    B)  100%

    C)  50%                      

    D)         300%

    Correct Answer: D

    Solution :

    Given:\[l=l+100%l=2l\]                 Initial volume = final volume \[ie,\]    \[\pi {{r}^{2}}l=\pi r{{}^{2}}l\] \[\Rightarrow \]               \[r{{}^{2}}=\frac{{{r}^{2}}l}{l}={{r}^{2}}\times \frac{l}{2l}\] \[\Rightarrow \]               \[r{{}^{2}}=\frac{{{r}^{2}}}{2}\] \[\therefore \]  \[R=\rho \frac{l}{A}=\rho \frac{2l}{\pi r{{}^{2}}}\]             \[\left( \because R=\frac{\rho l}{A} \right)\] Thus,\[\Delta R=R-R=4R-R=3R\] \[\therefore \]  \[%\Delta R=\frac{3R}{R}\times 100%\]                 \[=300%\]


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