Haryana PMT Haryana PMT Solved Paper-1999

  • question_answer
    A fuse wire of radius 0.2 mm blows when current flowing is 5 A. For what value of current another fuse wire of same material but of radius 0.3 mm will blow?

    A)  9.2 A                                    

    B)  4.5 A

    C)  6.2 A                                    

    D)  12.2 A

    Correct Answer: A

    Solution :

                    For a fuse wire maximum safe current i is  related to radius r-through \[{{i}^{2}}\propto {{r}^{3}}\]       or   \[i\propto {{r}^{3/2}}\]                 So,          \[\frac{{{i}_{2}}}{{{i}_{1}}}={{\left( \frac{{{r}_{2}}}{{{r}_{1}}} \right)}^{3/2}}={{\left( \frac{0.3}{0.2} \right)}^{3/2}}={{\left( \frac{3}{2} \right)}^{3/2}}\]                 or            \[{{i}_{2}}={{\left( \frac{27}{8} \right)}^{\frac{1}{2}}}\times {{i}_{1}}=\sqrt{\frac{27}{8}}\times 5\]                 So,          \[{{i}_{2}}=9.18\approx 9.2A\]


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