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    Guineapig

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    Location: Cook Islands
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    #174366   2008-05-15 10:11 GMT      
    I have been able to figure out the resistance for this problem, but I don't know how to get the voltage drop. I missed this class and do not have any formula for the voltage drop.

    You decide to magnetize a steel nail by winding an insulated wire around it and connecting the stripped ends to the terminals of a battery. To limit the current and avoid draining the battery too quickly, you connect a flashlight bulb in series with the battery and wire wrapped around the nail. When the current in the wire is 45mA, the voltage across the coiled wire is 0.25 volts and the voltage across the bulb is 3.12 volts.

    What is the resistance of the bulb? 69.3 ohms
    What is the combined resistance of the bulb and coil? coil is 5.56 ohms = 74.89
    What the combined voltage drop
    What is the power drop across the bulb?

    Limey

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    #174367   2008-05-15 10:20 GMT      
    To figure out the voltage drop, use the formula V=IR again!

    The voltage drop across the coil will be:

    5.56x0.045 = 0.25V

    And the bulb:

    69.3x0.045 = 3.1V

    Combined voltage drop will be 74.89x0.045 = 3.37V

    Power drop can be found using P = I^2 x R

    P = 0.045 x 0.045x 69.3
    = 0.14Watts

    NoName

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    Location: Niger
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    #174368   2008-05-15 10:40 GMT      
    To limit the current and avoid draining the battery too quickly, you connect a flashlight bulb in series with the battery and wire wrapped around the nail. When the current in the wire is 45mA, the voltage across the coiled wire is 0.25 volts and the voltage across the bulb is 3.12 volts.

    What is the resistance of the bulb? 69.3 ohms correct

    What is the combined resistance of the bulb and coil? coil is 5.56 ohms = 74.89 correct

    What the combined voltage drop
    3.12+.25 = 3.37 volts

    What is the power drop across the bulb?
    (more correctly "power in the bulb" or "power lost in the bulb")
    P = EI = 3.12*45 = 140 mW

    RunningFree

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    #174369   2008-05-15 10:46 GMT      
    combined voltage drop:

    V = IR = (45*10^-3)(5.56 + 69.3) = 3.3687 Volts

    P = (V^2)/R = (I^2)*R

    ((45*10^-3)^2)(69.3) = .1403325 Watts
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