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Homework Help: Lamp brightness in circuits

  1. Jan 5, 2009 #1
    I'm looking at a problem where a battery of negligible internal resistance is connected to two lamps L and N in parallel with each other, then through in series to a 3rd lamp M. The question asks if the filament of one of the lamps in parallel N breaks then what happens to the brightness of the remaining lamps L and M.

    The choice was

    Lamp L , Lamp M
    A) stays the same , decreases
    B) increases , stays the same
    C) increases , decreases
    D) decreases , increases

    The markscheme says its C. I understand that L would increase as it no longer shares its current but I thought the answer would be B as I didn't think M would be affected? Where am I going wrong?
  2. jcsd
  3. Jan 5, 2009 #2


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    Homework Helper

    Imagine them all as equal resistors 'R'.
    If you have two resistors in parallel what is the total resistance ?
    If the pair and M are in series what is the voltage drop across the pair and across M?

    Now if one breaks, you have the same resistance L and M in series.
    What happens to the voltage drop across M?
  4. Jan 5, 2009 #3
    Think equivalent resistance before the break and current passing through the resistances, and then again after the break.

    Before (R*R/R+R)+ R

    After R+ R

    substitute a value of your choice for all resistances in the circuit, and see how the current changes in both L and M
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