JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2007

  • question_answer
        An electric current is passed through a circuit containing two wires of the same material, connected in parallel. If the lengths and radii of the wires are in the ratio of 4/3 and 2/3, then the ratio of the currents passing through the wire will be

    A)  3                                            

    B)  1/3

    C)  8/9                                       

    D)  2

    Correct Answer: B

    Solution :

                    Since, voltage remains same in parallel, so, \[i\propto \frac{1}{R}\] \[\Rightarrow \]               \[\frac{{{i}_{1}}}{{{i}_{2}}}=\frac{{{R}_{2}}}{{{R}_{1}}}\]                 \[\frac{{{i}_{1}}}{{{i}_{2}}}=\frac{\rho {{l}_{2}}/{{A}_{2}}}{\rho {{l}_{2}}/{{A}_{1}}}\]                         \[\left( \because R=\frac{\rho l}{A} \right)\] \[\Rightarrow \]               \[\frac{{{i}_{1}}}{{{i}_{2}}}=\frac{{{l}_{2}}}{{{l}_{1}}}\times {{\left( \frac{{{r}_{1}}}{{{r}_{2}}} \right)}^{2}}\]                    \[(\because A=\pi {{r}^{2}})\] \[\Rightarrow \]               \[\frac{{{i}_{1}}}{{{i}_{2}}}=\frac{3}{4}\times {{\left( \frac{2}{3} \right)}^{2}}\] Hence,  \[\frac{{{i}_{1}}}{{{i}_{2}}}=\frac{1}{3}\]


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