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Determining value of resistor

  1. Apr 11, 2009 #1
    1. The problem statement, all variables and given/known data

    A flashlight bulb rated at 1.4W and 4.0V is operated by a 9.0V battery. To light the bulb at its rated voltage and power, a resistor is connected in series as shown in the figure. ( i dont know how to paste the picture but its only a resistor and then light bulb in series with a 9v battery attached)

    What value should the resistor have?
    Express your answer using two significant figures.





    2. Relevant equations
    P=IV
    V=IR


    3. The attempt at a solution

    I know that when resistors are in series the current is equal throughout.. so i used P=IV to solve for current, then pluggled that into V=IR using to find R but that is obviously wrong i am going crazy!
     
  2. jcsd
  3. Apr 11, 2009 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi alwaysdazed! Welcome to PF! :smile:
    Hint: the flashlight will only take 4.0V …

    so you need to use up 5.0V on the reisistor.

    use Kirchhoff's rules for potential differences. :wink:
     
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