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Homework Help: Resistance question

  1. Feb 11, 2008 #1
    1. The problem statement, all variables and given/known data
    A 60-W lamp is placed in series with a resistor and a 120-V source. If the voltage across the lamp is 35-V, what is the resistance (R) of the resistor?

    2. Relevant equations
    I am pretty sure I am using the right equation, but I am not getting the right answer. What am I doing wrong?

    3. The attempt at a solution
    P=IV --> 60W=I(120V) --> I=-.50 A
    R=V/I --> (120V-35V)/.5A = 170 ohms (this is what I calculated but it isn't the right answer)
  2. jcsd
  3. Feb 11, 2008 #2


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    Homework Helper

    Voltage across the lamp is 35V, so that in P=VI,V=35 for the lamp.

    Current through the lamp = P/V =60/35
  4. Feb 11, 2008 #3


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    Staff Emeritus
    Science Advisor
    Gold Member

    A lamp rated as 60W would dissipate 60W when connected directly to the 120V supply -- when you add a resistor in series with it, it no longer dissipates 60W.

    - Warren
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