I can't solve the simplest questions

Discussion in 'Homework Help' started by Dàrk Proféssor, Oct 1, 2016.

  1. Dàrk Proféssor

    Thread Starter New Member

    Oct 1, 2016
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    0
    Hey,
    So I am a first-year Mechatronics college student. I didn't study for the first couple weeks for a certain reason and now I am in hell !

    Basically, when the lecturer explains every thing about mesh and nodal analysis .. etc. I get the point but I can never solve !
    I state the equations and stuff. then stuck. This happens to me even with the simplest equations. Here is an example:

    [​IMG]


    A simple circuit that can be solved by KCL and KVL ( not even "college level" ). I apply the rules, then my idiot mind paralyzes !


    [​IMG]

    I can't proceed after this step !
    I know I should somehow get the values by algebric methods. But simply, I don't how !
    I am always stuck at this very step every single question !

    ( Also regarding the the -ve sign of V2 .. is that correct *-* ?? )

    Thank you,
    Your contribution will help a college student finish his studies xD
     
  2. mjb1972

    New Member

    Sep 18, 2016
    5
    0
    So do either mesh or nodal not both at the same time. I would use nodal since it looks like you trying to find V1, V2, and V3. The current entering the node equals the current leaving the node.
     
  3. WBahn

    Moderator

    Mar 31, 2012
    17,743
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    Aside from playing fast and loose with the units, which will almost certainly come back to haunt you on many occasions, whether you ever realize it or not, you seem to have gotten three valid equations in three unknowns. In other words, the electrical engineering part of the problem is not what is giving you the headaches.

    You have unknowns {i1, i2, i3}

    You have the following equations

    i1 = i2 + i3
    10 V = (2 Ω) · i1 + (8 Ω) · i2
    6 V = (4 Ω) · i3 - (8 Ω) · i2

    At this point the EE stuff is all finished and everything else is math.

    That's three equations in three unknowns. If you are always getting stuck at this point, then your problem doesn't have anything to do with college anything -- your problem is with junior high algebra. Look for tutorials about solving simultaneous linear equations using substitution (typically eighth or ninth grade algebra, though of course that is going to vary depending on the schools where you went).

    Here is a hint -- the first equation directly lets you write i1 in terms of i2 and i3. So use that to replace i1 in the other two equations and now you have two equations in two unknowns (i2 and i3). Now solve one of them for i2 and substitute that into the other one and you will have one equation in one unknown. Solve it and you have i3. Now substitute that value for i3 into the other equation and you can get i2. Now substitute both of the known values for i2 and i3 into the first equation and you have i1.

    But you mentioned mesh and nodal analysis and then proceeded to do neither, but rather just performed an ad hoc mixture of KCL and KVL. While that analysis was completely valid, it was neither nodal nor was it mesh.

    If you are going to do nodal analysis, then do nodal analysis.

    If you are going to do mesh analysis, then do mesh analysis.

    Oh, and before I forget -- thank you very, very much for labeling your diagram so that it is clear exactly what the voltages and currents you are referring to mean. That's something that we seldom see, particularly in a first post.
     
  4. Dàrk Proféssor

    Thread Starter New Member

    Oct 1, 2016
    4
    0
    So basically my problem is with Grade 9 algebra .. that's embarrassing and so messed up *-*

    Anyways, I really appreciate your help !
    Thank you =)
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,143
    1,790
    Besides the explicit methods of solving simultaneous equations, you can look at Cramer's rule and freeware matrix manipulation software like Matlab (not free) or Scilab (still free) or R(free, but steep learning curve).
     
  6. Dàrk Proféssor

    Thread Starter New Member

    Oct 1, 2016
    4
    0
    Yeah I also just found out how to do it on calculator ! So yeah .. thanks to all of you guys !
     
  7. WBahn

    Moderator

    Mar 31, 2012
    17,743
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    Perhaps, but it is what it is. Now that you've identified a problem you are in a much better position to correct it instead of letting it continue to fester.

    The overwhelming majority of us have holes in our basic education somewhere. Mine are concentrated in 7th and 8th grade English.
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,743
    4,795
    Keep in mind that you have not solved the problem, rather you've merely kicked the can down the road so that it can rear its ugly head at a more inopportune time.

    Finding out how to do something on a calculator that you do not comprehend how to do without a calculator is another way of saying that instead of using a tool to do the work for you, you are using it to do the thinking for you. These are fundamentally different things.
     
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