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Homework Help: Understanding resistors and voltage drop

  1. Aug 28, 2009 #1
    1. The problem statement, all variables and given/known data
    Determine Req, the missing I's and V's?


    2. Relevant equations

    3. The attempt at a solution

    First problem is that I thought I learned that R2 and R3 are in parallel so you have to use the recipercal rule, but looking back at my notes for Interpetation for C.E. I saw that he said that all of these are in series, as you can see this dellima will mess up my Req. So my question is, do I find Req for R2 and R3 then add those to R1 and R4 to get Req, or is it simple R1+R2+R3+R4. And why?

    But besides that I need to find R3 which i thought I could use V3=iR
    i is given as 400mA, so R3 is 10 Ohms. This doesn't look right seeing as all the other resistors are in the thousands(1kOhm=1000Ohms).

    Am i on the right track?
  2. jcsd
  3. Aug 28, 2009 #2


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    Homework Helper

    R1, R2, R3, R4 are all in series because there any electron going from one terminal of the battery to the other must pass through all four. So resistance adds: Req=R1+R2+R3+R4.

    Your calculation for R3 seems right. I've no idea why it's not of a similar order of magnitude as the other resistances.
  4. Aug 28, 2009 #3
    Thanks for that, now what about for the Req?
  5. Aug 28, 2009 #4


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    Science Advisor

    As the last poster said:

    Just add up all the resistances.

    You can get the supply voltage by using I*Req. It is VERY high. You can get all the voltages across the resistors the same way. They should add up to the supply voltage.

    {But please post clearer photos (all digital cameras have a closeup setting on them) and please don't use abbreviations and acronyms without explaining them.}
  6. Aug 29, 2009 #5
    That was taken by phone, I dont have a digital camera with me. It was hard to get a good picture because it was very small on the paper, so the closer i got or zoomed in the blurry it got I tried me best.
    The reason I abbreviated them was because the were already abbreviated in the photo.

    Thanks for your help. It just seemed weird because the R3 was really small compared to everywhere else!
  7. Aug 29, 2009 #6
    Here is what I believe it is right......

    The unknowns are V1,V2,V4,Vs, Req,R3.

    R3=V3/it=4V/400mA=10 Ohms

    Req=15kOhm+22kOhm+10Ohms+33kOhms=70.01 k Ohms


    I repeated the step to solve for V1 for V2V4.


    What do you think?
  8. Aug 29, 2009 #7


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    Science Advisor

    Looks good.

    Did you know that if you hold a magnifying glass in front of your camera lens, it will act as a close-up lens and let you focus closer?
    Focus the sun onto something and measure the distance from the lens to the focussed sun and that is the focal length of the lens.
    Now, put the lens in front of your phone camera and whatever you want to photograph at the distance you measured and it should be in focus. Get good lighting on it (outside, but not in the sun) and you will start to get good photos. Not as good as a real camera, but better anyway.
    Last edited: Aug 29, 2009
  9. Aug 30, 2009 #8
    Thanks for the advice I am sure I will use it when I soon! I am sure ill be posting on here a lot more haha.
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