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Faulty lamp because it short circuit

  1. May 24, 2012 #1
    1. The problem statement, all variables and given/known data
    http://www.xtremepapers.com/papers/...and AS Level/Physics (9702)/9702_s06_qp_2.pdf

    number 7a


    2. Relevant equations
    None really


    3. The attempt at a solution
    The answer is that C is the faulty lamp because it short circuit. But how do I know it is faulty?

    Im quite sure it has something to do with the last row: open closed closed 15 ohm

    Just realized it was a 1 word title :/
     
  2. jcsd
  3. May 25, 2012 #2

    ehild

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    Re: Shortcircuit

    The ohmmeter reads the resultant resistance between its terminals. Which lamp does it measure in the first case? So what is the resistance of one lamp? Which lamps are connected in series in the third case?

    ehild
     
  4. May 25, 2012 #3
    Re: Shortcircuit

    For the 1st case it measures no lamps?
    The resistance for 1 lamp is 15ohm.
    A and B?>
     
  5. May 25, 2012 #4

    ehild

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    Re: Shortcircuit

    Sorry, I wanted to ask the first case when S1 is closed. But it is right, the resistance of one lamp is 15 ohm. In the third case, A and B are connected in series, the resistances add up, and the meter measures 30 ohm, which is the resultant of two lamps in series. So lamp B is not faulty.

    How are the lamps connected in the fourth case? What is the resistance of C when the resultant is 15 ohm?



    ehild
     
  6. May 25, 2012 #5
    Re: Shortcircuit

    ummm in parallel? Im not sure
    Is it 0 for C?
     
  7. May 25, 2012 #6

    ehild

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    Re: Shortcircuit

    B and C are connected in parallel, and A is in series with the resultant of B and C. Yes, the resistance of C has to be zero.

    ehild
     
  8. May 25, 2012 #7
    Re: Shortcircuit

    Ok, so let me get this right

    the current passes through A but also passes through C only, because it is faulty so it has 0 resistance, so it was shorted?
     
  9. May 25, 2012 #8

    ehild

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    Re: Shortcircuit

    Yes. But a faulty lamp can have either infinite or zero resistance. If it is not shorted, the meter would read a resistance higher than 15 ohm. The measured resistance is 15 ohm, so leaving lamp A, the current goes through a short. Or: The resultant resistance of B and C in parallel has to be zero. That can happen only when C is a short (as B is not faulty).


    ehild
     
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