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JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2008

  • question_answer
        A 30 y 90 W lamp is to be operated on a 120 V DC line. For proper glow, a resistor of ......\[\Omega \] should be connected in series with the lamp.

    A)  40                                         

    B)  10

    C)  20                                         

    D)  30

    Correct Answer: D

    Solution :

                    Resistance of the lamp \[{{R}_{0}}=\frac{{{V}^{2}}}{P}\] \[=\frac{{{(30)}^{2}}}{90}=10\,\Omega \] Current in the lamp                 \[i=\frac{30}{10}=3\,\Omega \] When lamp is operated on a 120 V, then resistance                 \[R=\frac{V}{i}=\frac{120}{3}=40\,\Omega \] Thus, for proper glow, the resistance required to the put in series will be \[R=R-{{R}_{0}}=40-10=30\,\Omega \]


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