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Series Wiring

  1. Oct 4, 2009 #1
    1. The problem statement, all variables and given/known data

    A 60W lamp is placed in series with a resistor and a 120V source. If the voltage across the lamp is 25V, what is the resitance R of the resistor?

    2. Relevant equations

    I derived R= V2/P from ohm's law to solve this problem.

    3. The attempt at a solution

    R= V2/P
    =(25V)2/60W
    =10.4 ohms

    Is this answer correct? Thank you in advance for your time and help.
     
  2. jcsd
  3. Oct 4, 2009 #2

    mgb_phys

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    That's the resistance of the lamp.
    Remember the current in the lamp an the resistor is the same, whats the voltage rop accross the resistor?
     
  4. Oct 4, 2009 #3
    120v - 25v = 95v?
     
  5. Oct 4, 2009 #4
    I guess I just don't understand what voltage to use and why. :frown:
     
  6. Oct 4, 2009 #5

    mgb_phys

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    You have 120V total, 25V accross the lamp and so (120-25) across the resistor
    To work out the resistance you need the current through the resistor
    This is the same as the current through the lamp

    hint: Power = current * voltage difference
     
  7. Oct 4, 2009 #6
    I=P/V
    =60W/(120V-25V)
    =0.63 A

    R=V/I
    =95V/0.63A
    =151 ohms?
     
  8. Oct 4, 2009 #7

    mgb_phys

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    sounds good
     
  9. Oct 4, 2009 #8
    Thank you so much for your help! :approve:
     
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