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Homework Help: Light Bulb Brightness

  1. Apr 14, 2010 #1
    1. The problem statement, all variables and given/known data

    A light bulbs brightness can be measured by its power consumption. This in turn will depend on the I, V and R of the light bulb. The light bulb is rated at a power consumption of 1.0W when placed at a voltage of 6.0V. We change the battery from a 6.0V to a 3.0V one. Will the light bulb shine brighter than the previous case. What will be the current through the light bulb

    2. Relevant equations


    3. The attempt at a solution

    Is the power consumption 1.0W for both cases
    If it is then I1=1/6 and I2 =1/3 Therefore the latter example will shine brighter

    If not how do i take into account the changing power
  2. jcsd
  3. Apr 14, 2010 #2
    It says that the power consumption is of 1 W whan placed at 6 V, not all the time. You might want to calculate, say, resistance of the light bulb (P=IU=U^2/R), what should then allow you to calculate the power consumption of the same bulb placed at 3V.
  4. Apr 14, 2010 #3
    You could also notice that
    [tex]p \alpha V^2[/tex]
    so decreasing [tex]V[/tex] by [tex]\frac{1}{2}[/tex] decreases p by [tex]\frac{1}{4}[/tex]

    or you could multiply out the old [tex]V^2[/tex] and multiply the new one in:
    [tex]p_{3V} = p_{6V}\frac{3^2}{6^2}=p_{6V}\frac{1}{4}[/tex]
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