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Homework Help: Different ways to check the answers

  1. Mar 16, 2008 #1
    1. The problem statement, all variables and given/known data
    What are different ways to check your solutions for simple Resistive Circuits?
    Or even in general cases, how I can make sure that my answer is right?
    I am making very stupid mistakes (mostly algebra problems and using wrong directions),
    so I want to make sure that I don't repeat these mistakes in the exam.
    I am not very familiar with circuits, so I cannot use reality check (in Physics, it's very simple to guess the answer even without doing detailed calculations)

    2. Relevant equations
    I am using Nodal Analysis and Mesh Analysis to solve them

    3. The attempt at a solution

    Here's a checklist I developed:
    # Reviewing Nodal Analysis Questions:

    1. I should write detailed steps (neatly) leaving space between lines
    2. I should simplify my equations before solving them on calculator
    3. After Calculations:
    1. See if I have used right numbers
    2. See if my equations are right
    3. See if my solutions meet with my equations
    * See if I have right directions of currents as indicated by diagram
    4. If time available, use other techniques to confirm my answers
    5. See if power analysis agree...

  2. jcsd
  3. Mar 16, 2008 #2
    Yes, but it doesn't have to mean starting over, only by substituting your answers. Do a check that the net current into each node is zero. Do a check that the sum of all voltages around each loop is zero. These checks are quick additions of lists of numbers.
    Last edited: Mar 16, 2008
  4. Mar 16, 2008 #3
    Thanks for suggestion; I think it's very efficient.

    I was wondering if my ground node (in nodal analysis) has right net current then does it mean that my solution is good?
    And if I use mesh analysis, doing KVL only at the mesh that was not included in my analysis?

    Because that one node/mesh is dependent upon my set of equations I got from other nodes/meshes, so I guess everything should be fine if I do only one test.
  5. Mar 16, 2008 #4
    I'm not certain but those shortcuts don't sound to me like complete checks.

    If you're talking about checking your work during an exam, you might make the most efficient use of your time in this way: complete all problems, then go back to the beginning and do partial checking of all problems, then go back and do additional checking on all problems, continue returning to the beginning and making your checks more thorough, until you either find an error to fix or until the time has run out.

    If the instructor gives partial credit for "right method, wrong algebra" or "right method, wrong arithmetic" , the first thing to recheck for errors is probably the most basic set-up, with solution steps below that decreasing in importance.

    In fact, I used to solve all problems at once. I would write a set of simultaneous equations, leave a big space, then jump immediately to the next problem. When done, go back and either write determinants (Cramer's rule) or elimination of something by substitution, leave it like that, go on to the next one. I would do the arithmetic for all of the problems last.
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