Engineering Board Exam Reviewer · Series-Parallel Circuits · hard

Three identical lamps are connected so that two of them are in parallel, and this pair is in series with the third lamp, all across a fixed supply. One of the parallel lamps burns out. What happens to the other two lamps?

  1. Both lamps dim
  2. Both lamps glow brighter
  3. The lamp in series dims and the remaining parallel lamp glows brighter
  4. Neither lamp changes brightness

Answer

C. The lamp in series dims and the remaining parallel lamp glows brighter

Solution

With resistance R per lamp, the total resistance rises from R+ R2=1.5R to 2R, so the total current falls from V/1.5R to V/2R. The series lamp carries this smaller current and dims. The remaining parallel lamp now carries the whole current V/2R, which is more than the V/3R it carried before, so it brightens.

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