1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    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!

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


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

Have something to add?

Similar Discussions: Lamp brightness in circuits
  1. 3 lamp circuit (Replies: 4)