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Thevenin with two batteries

  1. Feb 22, 2010 #1

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    1. The problem statement, all variables and given/known data

    Hi there, I got this question from an old test. I would say I have limited knowledge of electronics and would ask for guidance into what approach to take with this question.

    2. Relevant equations



    3. The attempt at a solution

    No attempt yet. I thought of breaking it up, in 2, obviously, and then getting the series resistance total, but what then? then I am back at square 1, with a "broken" circuit. What small detail am I missing?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Feb 22, 2010 #2
    1) Change the voltage source into a current source using source conversion.
    i.e current source with current = 12V / 4ohm= 3 A. - lookin at the left voltage source.
    you will get 3A current source with a resistor of 4 ohm connecting parallel to it.

    do for the other side(right) too. 8/3=2.67A

    2) Since you then have resistors in parallel.
    the effective resistance on the left will then be (4*7)/ (4+7)= 2.545 ohm
    right will be 2.25 ohm.

    3) Convert them back to voltage source.
    LEft: 2.545 *3 = 7.635 V
    Right : 2.25 * 2.67= 6V

    The resultant voltage will be 7.635 - 6 = 1.635 V.
    Sum of all the resistance = 10 + 2.545 + 2=14.545 ohm
    Hence without using thevenin,
    I = 1.635 / 14.545 = 0.1124A

    Using thevenin,

    After using source conversion,
    Golden rule: Make current source disappear; make voltage source a short circuit, meaning: treat it just as a piece of connecting wire.

    thevenin voltage = thevenin resistance * short circuit current.
    then calculate the current you wanted by current divider rule.

    hope this helps
     
  4. Feb 22, 2010 #3
    1) Change the voltage source into a current source using source conversion.
    i.e current source with current = 12V / 4ohm= 3 A. - lookin at the left voltage source.
    you will get 3A current source with a resistor of 4 ohm connecting parallel to it.

    do for the other side(right) too. 8/3=2.67A

    2) Since you then have resistors in parallel.
    the effective resistance on the left will then be (4*7)/ (4+7)= 2.545 ohm
    right will be 2.25 ohm.

    3) Convert them back to voltage source.
    LEft: 2.545 *3 = 7.635 V
    Right : 2.25 * 2.67= 6V

    The resultant voltage will be 7.635 - 6 = 1.635 V.
    Sum of all the resistance = 10 + 2.545 + 2=14.545 ohm
    Hence without using thevenin,
    I = 1.635 / 14.545 = 0.1124A

    Using thevenin,

    After using source conversion,
    Golden rule: Make current source disappear; make voltage source a short circuit, meaning: treat it just as a piece of connecting wire.

    thevenin voltage = thevenin resistance * short circuit current.
    then calculate the current you wanted by current divider rule.

    hope this helps
     
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