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Homework Help: 2 Way lightbulb with different power

  1. May 12, 2007 #1
    1. The problem statement, all variables and given/known data
    A 3-way lightbulb can produce 50W, 100W or 150W at 120V. Such a bulb contains 2 filaments that can be connected to the 120V individually or in parallel.

    a) describe how the connections to the 2 filaments are made to give each of the 3 wattages

    b) What can be the resistance of each filament?

    3. The attempt at a solution

    I'm really not sure where to start :confused:
  2. jcsd
  3. May 12, 2007 #2


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    Homework Helper

    equation: P=VI=V^2/R
    obviously, for two filaments (ie. two resistances), you can make the following combo (if R1=R2)

    1. R1//R2 (parallel)
    2. R1 (by itself) or R2 (by itself)
    3. R1+R2 (series)

    with effective resistance of case 1 < case 2 < case 3

    you got it now?

    now if case 3 (in series is not allowed then) you will have to assume that R1 not equal to R2 to get three cases
  4. May 12, 2007 #3
    Ok, so using P=V^2/R

    For the 50w wattage, V is constant at 120V, P=50W.
    Subbing into the equation, R=288ohms

    For the 100W wattage, subbing into the equation, R=144ohms

    For the 150W wattage, subbing into the equation, R=96ohms

    but how do I know if they are connected individually or in parallel?
    I'm a massive physics noob, go easy on me!
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