Why Is I Negative? A Closer Look at the Norton Method

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Discussion Overview

The discussion revolves around the interpretation of current "I" in the context of the Norton method, particularly why it is considered negative in certain scenarios. Participants explore the application of Kirchhoff's Current Law (KCL) and the implications of current directionality at a node.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why "I" is negative, prompting exploration of current direction.
  • Another participant suggests that "I" is negative because it is considered from the negative to positive terminal, referencing a paper.
  • A participant expresses uncertainty about identifying which currents enter or leave the node, indicating a lack of clarity in the application of KCL.
  • Further clarification is provided that the sum of currents entering and leaving a node must be consistent, with an explanation of how to apply the passive sign convention to derive equations based on the chosen direction of current summation.
  • It is noted that if "I" is entering the node, then it follows that -I is leaving it, emphasizing the importance of sign conventions in circuit analysis.

Areas of Agreement / Disagreement

Participants exhibit some agreement on the application of KCL and the passive sign convention, but there remains uncertainty regarding the identification of current directions and the implications of these choices.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about current direction and the definitions used for entering and leaving currents, which are not fully resolved.

Who May Find This Useful

This discussion may be useful for students and practitioners of electrical engineering or circuit analysis who are grappling with the concepts of current directionality and the application of KCL in circuit analysis.

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I is negative because you consider it from - to + (as you show in your paper).
 
i know that KCL says that the sum of the currents that goes in
equals the sum of the currents that goes out

here i don't know what current goes into the node
and what current goes out??
 
transgalactic said:
here i don't know what current goes into the node
and what current goes out??

And you don't need to. The sum of currents entering a node is equal to zero, the sum of currents leaving the node is also zero. Pick one and stay consistent.

If you choose to sum the currents leaving the node, then by the passive sign convention you have (e-v)/R1 + e/R2 - I = 0. If you choose to sum the currents entering the node you have (v-e)/R1 + (0-e)/R2 + I = 0, which is exactly the same as the previous equation once you sort out the signs.

So, to answer your question, you are summing the currents leaving the node. Since I is entering the node it follows that -I is leaving it.
 

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