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Electric circuit resistor combinations

  1. Dec 9, 2013 #1
    Hello friends, I have problems in simplifying electrical circuits. I've attached 2 (of the many) questions I have trouble with. According to the question, superposition must be used to solve these.

    In the first question, supposing the voltage source doesn't exist, how do you simplify the left hand side of the circuit with the 3 6k resistors and the 2mA source? Can I combine the 6k resistor in the middle with the one on the left because they share a common ground node?

    In the second one, I know one node of the 3k resistor, 6k resistor and 2mA source on the left hand side can be connected to ground (suppose the voltage source there doesn't exist or is short-circuited), and the other node shared by all 3 so they can be put in parallel. But does the 2mA source flow from the upwards direction or the downwards direction? How do you determine that? It looks confusing to me.

    These aren't homework questions but just practice questions I'm doing for my final exam soon. I feel like if I understand these concepts better I would have no problems solving these questions.
     

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  2. jcsd
  3. Dec 9, 2013 #2

    phinds

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    See response directly below
     
  4. Dec 9, 2013 #3

    phinds

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    Hm ... seem to have deleted my own reply. I'll try again:

    Does one common node suffice to make resistors be in parallel? What is the definition of parallel?

    Why would you think that an ideal current source would cause current to flow in anything other than the direction indicated?
     
  5. Dec 9, 2013 #4
    From what I know if 2 elements share both common nodes, they are in parallel. If I'm wrong please correct me.

    Look at the attachment to see what I mean. Drawing the circuit like that makes it a lot easier for me in calculations, but I don't understand why the arrow is pointing downwards and not upwards (the solution says downwards).
     

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  6. Dec 9, 2013 #5

    phinds

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    Yes, you have parallel correct.

    Why do you care which way the current is being driven? That's like saying, the guy in this map problem is walking North but I would prefer that he be walking South. What's going on is just a given part of the problem. You can change it if you like, but then you are solving a different problem.

    EDIT: your redrawing, by the way, is correct.
     
  7. Dec 9, 2013 #6
    If I changed the direction of the current flow my answer would have been completely different. I just don't get how you determine the current direction when you 'rearrange' the circuit like that.. for example, I don't know what the direction of the current source on the right would be when redrawn. I hope you get what I mean.
     
  8. Dec 9, 2013 #7

    phinds

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    An ideal current source pushes current in the direction indicated. All other currents in a circuit can be arbitrarily represented in whatever direction you like. If you've drawn it opposite what the actual flow direction is, then you will get a negative answer for that current.

    Redrawing circuits has nothing to do with any of that, as long as you redraw it correctly, which just means you can do anything you like as long as all the nodes stay connected to the same place. What matters is what is connected to what node, not how they are drawn (although some drawings do make it easier to see what's going on that others do).
     
  9. Dec 9, 2013 #8
    So I can take the direction of that current source any way I want, and then take the opposite direction of the other current source? Would my answer be exactly the same doing this? Or would there be a sign change?
     
  10. Dec 9, 2013 #9

    phinds

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    Which part of
    did you not understand?
     
  11. Dec 9, 2013 #10
    No, I mean in the redrawn circuit I drew the current wasn't facing sideways. It was pointing downwards.

    I actually think I understand it now, the current source is going towards the ground/reference node and the 6k resistor, so it doesn't make sense for it to point the other way. I'm not sure why I was confused earlier.
     
  12. Dec 10, 2013 #11

    phinds

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    Yeah, it CAN be a bit confusing when you are new to it. I would simple re-emphasize three things:

    1) ideal voltage and current sources define their own direction and you can't change that
    2) it doesn't matter how you redraw a circuit as long as you don't change what hooks to what.
    3) it doesn't matter which way you assign a voltage or current (outside of ideal sources) because if you've put it the "wrong" way, you'll just get a negative answer when you solve for it.

    If you can keep those in mind and follow them you can't go wrong. (Well, you CAN go wrong but it's less likely :smile: )
     
  13. Dec 10, 2013 #12
    Yeah step 2 has made it a lot easier for me to redraw circuits. Thanks for your tips. :)
     
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