1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook