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Homework Help: Resitor power problem

  1. Feb 16, 2008 #1
    1. The problem statement, all variables and given/known data

    Three 70.0 W, 120 V lightbulbs are connected across a V = 120 V power
    source, as shown in Figure P18.48. (Assume that the resistance of each bulb is constant even though, in reality, the resistance increases markedly with current.)

    http://i57.photobucket.com/albums/g2...er/physics.gif [Broken]
    Figure P18.48
    (a) Find the total power delivered to the three bulbs.
    wrong check mark
    Your answer differs from the correct answer by 10% to 100%. W
    (b) Find the potential difference across each light bulb.
    VR1 = correct check mark V
    VR2 = correct check mark V
    VR3 = correct check mark

    2. Relevant equations

    3. The attempt at a solution
    I found the potential differences they were 80 and 40.12, 40.12 respectively. However I cannot find the power

    I have Req as being 462.9 so then i use power = 120^2/462.9 and i get 31.1 watts?!?
    I have no idea why this isn't working please give me a clue
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Feb 16, 2008 #2
  4. Feb 17, 2008 #3
    Your Req is wrong. For a single bulb we get p = V^2/R, so R = V^2/70. Then compute Req. R_2 and R_3 are parellel, so replace them with their equivalent resitance etc.
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