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CMC Medical CMC-Medical Ludhiana Solved Paper-2009

  • question_answer
    A 30 V, 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 \[i=\frac{30}{10}=3A\] 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 requirec to the put in series will be \[R=R-{{R}_{0}}=40-10=30\,\Omega \]


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