Parallel Resistors and Current HW (pics)

Discussion in 'Homework Help' started by breck, Sep 17, 2011.

  1. breck

    Thread Starter New Member

    Sep 17, 2011
    3
    0
    3 Problems I am having on my HW (picures below)

    I have read through the DC chapter which this is all based off of, but there these problems, I can't seem to figure out how to write the correct formulas to solve them.

    3. I assume some type of equation relating the resistors to each other will be used.
    R_tot=R1+R2+...
    or
    R_para=R1R2/(R1+R2)

    That is basically where I am on on the first one. I don't know how to relate R_L to the R_AB to get R.

    5. Some derivation of V=IR where R=R_para

    Not sure how to get around it.

    6. I have no idea how to approach this one. There are no examples of this type of problem in my book and I honestly don't know what to search on the internet to try to find something like this.
    I have absolutely no clue on how to get to the answer, even though my answer was the opposite of the answer, it was a percentage guess from my previous answer and I guess because R1R2-R3R4 is negative it leads to the resultant being negative but I am not sure what so ever.

    Any help would be lovely. Thank-you all.


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  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    For question 3 you have Rab = RL= 515Ω
    So
    Rab = R + R||(R+RL)and you solve for R


    Question 5
    http://en.wikipedia.org/wiki/Current_divider
    Ix = Itot * R2/(R2+R3)

    Itot = V / (R1 + R2||R3)

    Q6
    You can use voltage divider rule

    Vx = (V*R2)/ (R1+R2 ) - (V*R4)/(R4+R3)
     
    Last edited: Sep 17, 2011
    breck likes this.
  3. breck

    Thread Starter New Member

    Sep 17, 2011
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    Thank you for the help, but my math may be a little bit wrong for #3.
    515=R+(515R)/(515+R)
    515=((515+R)R+515R)/(515+R)
    515(515+R)=R^2+1030R
    0=R^2+515R-265225
    quadratic forumla
    (-515+/-sqrt(515^2-4(-265225)))/2
    R=318 and -833
    R is supposed to be 297.
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Well I made the mistake in the equation
    Rab = R + R||(R+RL) this looks good
    So we have
    515 = R + (R*(R + 515))/(R + (R + 515))

    R = 515/√3 = 297.335Ω


    Sorry for the mistake
     
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  5. breck

    Thread Starter New Member

    Sep 17, 2011
    3
    0
    No worries. Thank you for taking the time to help me out.
     
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