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Homework Help: Please help me in this question on network ?

  1. Jun 29, 2011 #1
    Hello friends....in this questions the answer is (b) option.....this is from indian engg. services exam.

    if we apply mesh in loop which i have shown , then we find that

    I = (10 - 5 )/10 = 0.5 amp


    V=10 - 6 I = 7 volt (right acc. to answer keys)

    and Rth = 2.4 ohm (i have calculated Rth)


    but if we apply Nodal analysis where V is marked then

    acc to that
    (V-10)/6=(V-5)/4
    V= -5

    so we see that both results are not equal....but in answer keys 7 volt is given....
    please tell me what is the problem in second method of nodal analysis.? is this wrong ?



    thanks in advance.....
     

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    Last edited by a moderator: Jun 29, 2011
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  3. Jun 29, 2011 #2

    vk6kro

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    Science Advisor

    V= 10 - (volts across 6 ohms) because these have opposite polarities.

    so (volts across 6 ohms) = 10 - V

    Current in 6 ohms = (10 - V) / 6

    Then it works out.

    40 - 4 V = 6V - 30

    10 V = 70

    V = 7
     
  4. Jun 29, 2011 #3
    thanks for reply sir.....can u plz tel me how u have said

    V=10 - (voltage across 6 ohm)

    plz explain with detail.....

    and is not V and 10 volt same thing ???? n so i think it should be V=10+V6
     
    Last edited: Jun 29, 2011
  5. Jun 29, 2011 #4

    gneill

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    Staff: Mentor

    You are applying nodal analysis, calculating the current in each branch from the node where V is located. The sum of these currents (leaving the node) must total zero. What you've written equates the currents in each branch -- this is not quite right -- these currents will be equal and opposite if the sum of currents is to be zero.

    So write instead,

    (V-10)/6 + (V - 5)/4 = 0

    and see how things go...
     
  6. Jun 29, 2011 #5

    vk6kro

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    Science Advisor

    In this diagram,

    [PLAIN]http://dl.dropbox.com/u/4222062/loop.PNG [Broken]

    If you look at the polarity of the left branch, then V = 10 - ( voltage across the 6 ohm resistor).

    Since the currents in each resistor must be the same it is valid to work out these currents in terms of V and then solve for V.

    In the right branch, current is flowing downwards so the polarities add up the screen and these must also equal V.
     
    Last edited by a moderator: May 5, 2017
  7. Jun 29, 2011 #6
    thanks a lot sir....i understood mistake done by me....
     
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