• munna007
In summary, when applying nodal analysis to a circuit with two resistors in parallel, the sum of the currents leaving the node must equal zero. This means that the currents in each branch must be equal and opposite. Therefore, when calculating the current in the left branch, V is equal to 10 minus the voltage across the 6 ohm resistor. In the right branch, the current is flowing downwards, so the polarities add up to V. By solving for V in both branches, we can correctly calculate the voltage and determine the problem with the second method of nodal analysis.
munna007
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 ?

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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

thanks for reply sir...can u please tel me how u have said

V=10 - (voltage across 6 ohm)

and is not V and 10 volt same thing ? n so i think it should be V=10+V6

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munna007 said:
but if we apply Nodal analysis where V is marked then

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

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.

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

and see how things go...

In this diagram,

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

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.

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thanks a lot sir...i understood mistake done by me...

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