nodal analysis question

Thread Starter

chrischivers1

Joined Mar 23, 2015
14
Hi,

I am having a few issues with this question about Nodal Analysis (well part (b)).

Can anyone help me out as i am very stuck.

thank you.
 

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Jony130

Joined Feb 17, 2009
5,186
All you have to to is to write a two KCL equation for two node.

First a KCL for node A (Va) next write KCL for node B (Vb). But you need to write all component currents in terms of a its nodal voltages.
For example R1 resistor current is IR1 = Va/R1. The voltage across the 20kΩ resistor is the difference between the voltage on the node on top (Va) of the resistor and the voltage on the node on the bottom of the resistor (Vground).
 

Jony130

Joined Feb 17, 2009
5,186
Hmm, what about R3 current?
What is the voltage across R3?
Also notice that the R4 and R5 are connected in series and when resistors are connected in series the current flowing in each is the same.
 

WBahn

Joined Mar 31, 2012
26,275
You seem to be stuck down in the weeds and not grasping the more general concepts. If it is just a matter of being rusty, then you should be able to go back to a basic Circuits I text and read through it very quickly to blow of the rust and come back up to speed on the various analysis techniques. I'm concerned that the way you are doing it now will only chip away at parts of the rust and take you much longer to end up at a place where the gears are still grinding. So go back and walk yourself through the whole thing to become a well-oiled machine. I really don't think it will take you that long -- a few hours, perhaps.
 

Thread Starter

chrischivers1

Joined Mar 23, 2015
14
Cheers mate, yea it's just reawakening to all the concepts, I'll do some reading up and all being well it'll resink in.

I'm just going to the gym so I'll resume once im home.
 

Thread Starter

chrischivers1

Joined Mar 23, 2015
14
can somebody walk me through this question please? the attached PDF is where i am at so far.. had a family bereavement so my heads not been in it and the deadline is Thursday.

asking for help isn't my thing but im up against it.

thanks
 

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WBahn

Joined Mar 31, 2012
26,275
You use the same labels for different things. It appears that you are trying to color code them, but this isn't a good idea. First, what happens when you make a copy on a black and white copier or your photo gets rendered in grayscale? The point being that color information is too ephemeral to rely on. But even worse, you've got to combine these terms together at some point and it will become impossible to track them properly. Follow the rule that one label has one meaning.

The work in red looks fine other than the fact that you fail to track your units properly.

You last equation should be

\(
V_1 \( \frac{1}{20 \Omega} + \frac{1}{30 \Omega} \) - V_2 \( \frac{1}{30 \Omega} \) \; = \; 3 A
\)

which can be reduced to

\(
V_1 \( \frac{3}{60 \Omega} + \frac{2}{60 \Omega} \) - V_2 \( \frac{1}{30 \Omega} \) \; = \; 3 A
\,
V_1 \( \frac{5}{60 \Omega} \) - V_2 \( \frac{1}{30 \Omega} \) \; = \; 3 A
\,
\( 60 \Omega \)\[ V_1 \( \frac{5}{60 \Omega} \) - V_2 \( \frac{1}{30 \Omega} \) \] \; = \( 60 \Omega \) \; \[ 3 A \]
\,
5 V_1 - 2 V_2 \; = 180 V
\)

See how the units all work out?

Also consider that if I can be bothered to track my units in as inconvenient a format as writing LaTex code into a forum post, why can't you be bothered to do it with pen and paper?

Your work in red goes off the rails almost immediately due to sloppiness.

Your first line is fine, but your second line is

I_A = something = 0

You are explicitly stating that I_A is equal to zero.

Your final line isn't even an equation (it's just an expression), it is dimensionally inconsistent, and it doesn't follow from the expression in the line above it.

Take the time to do it carefully and correctly.
 

Thread Starter

chrischivers1

Joined Mar 23, 2015
14
Thank you for your detailed observations.
Apologies for my sloppiness, the colours seemed a good call but now you mention it, it'll prove troublesome in the future
 
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