# Help with circuit - Node analysis, how to calculate voltage across R1

#### mrlp823

Joined Dec 12, 2015
3

#### WBahn

Joined Mar 31, 2012
26,398
You might have notice that the name of the forum is "Homework Help" and not "Homework Done For You".

YOU need to post YOUR best attempt to work YOUR homework problem. Then we can look over your work, see how you are approaching the problem and where you might be running into difficulties, and make suggestions and ask questions aimed at helping you get back on the right track.

The first step is to identify and label of the essential nodes and then identify and label the quantity you are trying to solve for. After that you need to set up your equations for each node. Get that much done and post your efforts.

#### shteii01

Joined Feb 19, 2010
4,644
Wait. Does this mean you already found voltages across R2, R3, R4, R5, R6, R7?

#### WBahn

Joined Mar 31, 2012
26,398
Wait. Does this mean you already found voltages across R2, R3, R4, R5, R6, R7?
Probably not -- the question probably only asks them to find a particular voltage instead of all of them. Finding one is generally sufficient to demonstrate the necessary knowledge and skill without requiring a lot of additional time that really doesn't demonstrate much else.

#### shteii01

Joined Feb 19, 2010
4,644
Probably not -- the question probably only asks them to find a particular voltage instead of all of them. Finding one is generally sufficient to demonstrate the necessary knowledge and skill without requiring a lot of additional time that really doesn't demonstrate much else.
Your Sarcasm-O-Meter is off. You should turn it on.

#### mrlp823

Joined Dec 12, 2015
3
apologies i didn't read that @The Electrician, well so far i've labelled the nodes. V1 is where the branches with R6 and R7 connect. V2 is where the branch with R4 connects the middle branch. V3 is the node directly to the left of V2 and V4 is on the top branch between R2 and R5. I have the values for all the components, i just need to find the voltage across R1. I'm not sure if my nodes are correct ?

#### The Electrician

Joined Oct 9, 2007
2,814
apologies i didn't read that @The Electrician, well so far i've labelled the nodes. V1 is where the branches with R6 and R7 connect. V2 is where the branch with R4 connects the middle branch. V3 is the node directly to the left of V2 and V4 is on the top branch between R2 and R5. I have the values for all the components, i just need to find the voltage across R1. I'm not sure if my nodes are correct ?
You already have a couple of voltage sources labeled V1 and V2. It would be confusing to use the same symbols to label the nodes; I would use something like E1, E2, etc., to label the nodes.

Also, it would be helpful if you would post another image with those node labels shown.

Finally, how have you learned to solve networks like this? Are you able to make any progress at all? Show your efforts.

#### shteii01

Joined Feb 19, 2010
4,644
Your nodes seem to be correct.

#### WBahn

Joined Mar 31, 2012
26,398

Remember that Nodal Analysis is merely the systematic application of KCL by writing an equation for KCL at each node in terms of the voltages at other nodes and the intervening devices.

So try to write such a KCL equation for each of the numbered nodes. Do the best you can.

#### WBahn

Joined Mar 31, 2012
26,398
I echo TheElectrician's sentiments. While the diagram I supplied just used the node labels you identified, a far better scheme would be to use letters for the node labels.

#### mrlp823

Joined Dec 12, 2015
3
all the values are labelled now and the nodes are coloured in dark green and labelled with n's. it's homework and had to be done using the node method which i find very difficult. i've made some equations though. n1-n2/6 is the first. then n3-n4/8 is the second. From here i'm confused as what to do.

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#### shteii01

Joined Feb 19, 2010
4,644
all the values are labelled now and the nodes are coloured in dark green and labelled with n's. it's homework and had to be done using the node method which i find very difficult. i've made some equations though. n1-n2/6 is the first. then n3-n4/8 is the second. From here i'm confused as what to do.
n2 and n3 are the same node. so first step is replace those two nodes with one.

#### shteii01

Joined Feb 19, 2010
4,644
pro tip: download the old PSpice 9.1 it was free since 1998 or 1999 and use it to model circuits, or download LTspice it is also free but it is current and is being maintained. This way you can just model the circuit and know all the answers. You still need to do the math to show your instructor that you actually know what you are talking about.

#### WBahn

Joined Mar 31, 2012
26,398
all the values are labelled now and the nodes are coloured in dark green and labelled with n's. it's homework and had to be done using the node method which i find very difficult. i've made some equations though. n1-n2/6 is the first. then n3-n4/8 is the second. From here i'm confused as what to do.
You need to properly track your units. You also need to learn about order of operations. The equation n1-n2/6 is the same as (n1) - (n2/6) which is NOT the same as (n1 - n2)/6. This should be written as (n1 - n2) / 6 Ω. Also, notice that this is NOT an equation. And equation involves an equals sign. All you have are a couple of disembodied expressions.