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Homework Help: Help with simple electric cirquits

  1. Jan 19, 2009 #1
    I have never studied circuits before and in a couple of weeks I'm going to get into the lab to try some things and get an undersanding ofhow they work. I came across two configurations that i'm gonna be trying and I need some help in predicting what will happen.

    the first is as seen in this picture (2 separate figures)
    http://img443.imageshack.us/img443/3995/circuit1xk2.th.jpg [Broken]

    the round things are bulbs that all have the same resistance
    So in the first case with 2 branches:
    What will happen to the brightness of Bulb A if i disconnect the wire leading to bulb B?
    (I would say..since the 2 branches are parallel, the total resistance of the circuit will go up, so the current will go down and bulb A would be less bright ? )
    What will happen to the brightness of Bulb A if the wire is reconnected to bulb B, and you short circuit lamp B?
    (My guess is the same...the total resistance of the circuit will go down but by less than in the 1st part, and bulbs B and A will glow brighter than before (and equally bright).

    Now adding another parallel branch like in the 2nd figure, what will happen to the brightness of A, B, and C.
    (my prediction is that they will become brighter because the total resistance of the circuit descreases)
    In general, when I add more branches like this...all bulbs on each branch should glow equaly bright, and all bulbs on the previous brances should get brighter with each new branch I add, correct?

    The second setup is as seen in this other horrible picture I drew.
    http://img177.imageshack.us/img177/5402/circuit2ks2.th.jpg [Broken]
    This time it uses a constant current instead of a constant voltage supply.
    Would the bulbs all glow with the samebrightess? Hmmm I'm not too sure about this one..help?

    what will happen to bulbs a and C if you disconnect bulb b?
    Well...the total resistance of the circuit should go up and bulbs a and C will glow equally bright I think... If we consider bulbs b and c to form a parallel circuit, removing one will increase te resistance of that part of the circuit and since a and c are now in series, the total resistance would be 2r instead of R + R/2 = 3R/2 so thats why.

    What will happen if you short cirquit bulb B?
    (exactly same thing...resistance goes fom 3r/2 to 2r and bulbs a and c get dimmer)

    What will happen when another set of3 bulbs is added as shown in the second part of the picture?
    The brightness of all the previous bulbs should go down, since we're adding moreresistors in series...

    Can someone tell me if I'm forgetting something, and help me fill in theblanks?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 19, 2009 #2
    The first thing you should do is learn Ohm's law which is E = I*R. I mean really learn it. For instance you might make three graphs.
    Graph 1: Hold voltage constant and graph current versus resistance.
    Graph 2: Hold current constant and graph resistance versus voltage.
    Graph 3: Hold resistance constant and graph current versus voltage.

    With Excel this won't be too difficult.

    Once you get a feel for what happens as one of the variables changes, finding the answer will be much easier.
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