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AFMC AFMC Solved Paper-2008

  • 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 required to the put in series will be\[R=R'-{{R}_{0}}=\text{ }40-10=30\Omega \]


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