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

Discussion in 'Homework Help' started by mrlp823, Dec 12, 2015.

  1. mrlp823

    Thread Starter New Member

    Dec 12, 2015
    3
    0
  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,284
    328
  3. WBahn

    Moderator

    Mar 31, 2012
    17,775
    4,804
    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.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,397
    497
    Wait. Does this mean you already found voltages across R2, R3, R4, R5, R6, R7?
     
  5. WBahn

    Moderator

    Mar 31, 2012
    17,775
    4,804
    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.
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,397
    497
    Your Sarcasm-O-Meter is off. You should turn it on.
     
  7. mrlp823

    Thread Starter New Member

    Dec 12, 2015
    3
    0
    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 ?
     
  8. The Electrician

    AAC Fanatic!

    Oct 9, 2007
    2,284
    328
    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.
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,397
    497
    Your nodes seem to be correct.
     
  10. WBahn

    Moderator

    Mar 31, 2012
    17,775
    4,804
    An annotated diagram would be very helpful. Remember, when asking strangers for free help, the onus is on you to make it easy for them to help you. We aren't asking for anything overly burdensome here. The following took about 30 seconds using Paint.

    NodeProblem.png

    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.
     
  11. WBahn

    Moderator

    Mar 31, 2012
    17,775
    4,804
    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.
     
  12. mrlp823

    Thread Starter New Member

    Dec 12, 2015
    3
    0
    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.
     
  13. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,397
    497
    n2 and n3 are the same node. so first step is replace those two nodes with one.
     
  14. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,397
    497
    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.
     
  15. WBahn

    Moderator

    Mar 31, 2012
    17,775
    4,804
    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.
     
Loading...