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Homework Help: Kirchoff's Laws

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


    2. Relevant equations

    Kirchoff's Rules

    3. The attempt at a solution

    hey there!

    i'm having trouble with parts of the question above;

    for the first two
    a: dimmer, b: normal.

    however for c & d i'm confused, I did ask my teacher however he wasn't helpful at all, he told me that c would be normal and d would be "off" however I really don't understand why. I think it might be due to kirchoffs law's however again i find it difficult to understand, could anyone tell me why d wouldn't be on?

    i also encounter the same problem with g & h, i guess i'm not sure how to treat it when there's a line between the batteries.

    any help would be appreciated
  2. jcsd
  3. Oct 4, 2009 #2
    I wouldn't use Kirchhoff's laws to solve this problem. I would use the rules for combining resistors and combining voltage sources into series and parallel combinations. It's really a problem to solve by inspection,with no arithmetic. But if you want to solve it with Kircchhoff's laws, write down the equations.
  4. Oct 5, 2009 #3


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    Science Advisor

    Can you see that in the second diagram the voltage at a point between the two batteries is the same as that between the two lamps, relative to either end of the two battery string?

    So, if you have two places with no voltage between them, you will not get a current to flow if you put a wire between them? And you won't get a lamp to glow either?

    In the last diagram, this is the case, too, but since the wire in the middle can't be carrying a current, you can ignore it or just remove it.
    Last edited: Oct 5, 2009
  5. Oct 5, 2009 #4
    that's a great explanation, much obliged sir
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