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Dissipation, resistance, and power.

  1. Mar 16, 2009 #1
    1. The problem statement, all variables and given/known data
    A string of 19 identical Christmas tree lights
    are connected in series to a 120 V source. The
    string dissipates 60 W.
    What is the equivalent resistance of the
    light string? Answer in units of ohms.
    What is the resistance of a single light? An-
    swer in units of .

    What power is dissipated in a single light?
    Answer in units of W.

    One of the bulbs burns out. The lamp has a
    wire that shorts out the bulb filament when it
    burns out, dropping the resistance of the bulb
    to zero.
    What is the resistance Rnew of the light
    string now? Answer in units of ohms.

    Find the power Pnew dissipated by the string
    now. Answer in units of W.



    2. Relevant equations
    P=IV
    I=V/R
    P=(V/R)V
    R=V2/R


    3. The attempt at a solution
    I'm not sure how to start, since the dissipation of power is in watts. thanks for any hints or help you can give !
     
  2. jcsd
  3. Mar 16, 2009 #2

    LowlyPion

    User Avatar
    Homework Helper

    Welcome to PF.

    Well you have Power dissipated, and you have the Voltage don't you?

    Can't you figure the resistance of the 19 bulbs together?

    And if then the 19 become 18 what happens if figure power again from the same formula?
     
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