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Node Voltage Current Direction

  1. Nov 22, 2015 #1
    1. The problem statement, all variables and given/known data
    Hi all, how do I know how the current direction are through each elements after solving a circuit (finding node voltages w.r.t GND)?

    I think my understanding in electricity is rather incomplete. For example, I know that current should flow from a higher potential to a lower potential. However after solving with the node voltage method, I found out that, if you take a look at the bottom left of the image, that the current is flowing from GND to the left node (40V) through the current source. Does this not mean that the current is flowing from a lower potential now?

    Assuming I've no knowledge of the mesh current method, how do I figure out the direction of current flow per circuit element?

    2. Relevant equations

    Kirchoff's Voltage Law & Node Voltage Method.

    3. The attempt at a solution

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    Attached Files:

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  3. Nov 22, 2015 #2

    NascentOxygen

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    Staff: Mentor

    In a resistor, current flows from its higher potential end through that resistor to its low potential end. It is a resistor's potential difference that determines the magnitude of this current, according to Ohm's Law.

    In a current source, current always flows in the direction of the symbol's arrow. For a current source, voltage across it is independent and is determined by the rest of the circuit.

    In a circuit with multiple sources, the current into (or out of) any voltage source could be in either direction; the direction (as well as magnitude) is determined by other elements in the circuit.
     
  4. Nov 22, 2015 #3
    Hi. Thanks for the quick reply. Is there any fixed rule for which I can determine how it will move then?

    I'm asking because I'm interested in knowing how to answer if, for example, the question requires me to determine if power is dissipated or generated through the elements (I do know that resistors always dissipate power, but what about current and voltage sources? From what I understand, if the current enters through the positive side of the voltage source it means a power dissipation? How do I know through which side the current enters i.e. direction?)
     
  5. Nov 22, 2015 #4

    NascentOxygen

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    Staff: Mentor

    When a battery is delivering power to a circuit, current exits the battery from its + terminal.

    If current is instead entering the battery through its + terminal, that represents power coming from the rest of the circuit, so to this voltage source it's sinking power (or, equivalently, sourcing negative power)

    If the source happens to be a lead-acid battery, delivering negative power would mean it is being recharged.
     
  6. Nov 22, 2015 #5
    I have added in current directions and decided if power is dissipated or generated in my picture according to what I've understood from you. Is it correct?
     

    Attached Files:

  7. Nov 22, 2015 #6

    NascentOxygen

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    Staff: Mentor

    Looks okay, but you overlooked one source. I think 'delivered' sounds better than 'generated'. They can be categorized as power sources and power sinks (or energy source vs. energy sink).
     
  8. Nov 22, 2015 #7
    Thank you very much!
     
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