Direction of current when performing KCL

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    Current Direction Kcl
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Discussion Overview

The discussion revolves around determining the direction of current flow during nodal analysis in electrical circuits, particularly in the context of applying Kirchhoff's Current Law (KCL) and Ohm's Law. Participants explore the implications of assumed current directions and the potential for errors in calculations.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that one can assume a direction for current flow, and if the actual flow is in the opposite direction, the result will be negative.
  • Others express concern that incorrect assumptions about current direction can lead to wrong results when applying Ohm's Law.
  • One participant argues that if Ohm's Law is used correctly, the direction assumption will not lead to incorrect results, as it indicates the sign of the potential difference based on current direction.
  • Another participant emphasizes the importance of consistency in assumptions, stating that assuming all currents leave the node can still yield correct results, despite knowing that at least one assumption may be incorrect.
  • A participant introduces two formulations of KCL, noting a preference for the version that sums currents to zero, suggesting it leads to fewer mistakes.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to determining current direction and the implications of incorrect assumptions. Multiple competing views remain regarding the application of Ohm's Law and KCL.

Contextual Notes

There are unresolved aspects regarding the assumptions made in current direction and the potential impact on calculations, as well as the varying interpretations of KCL formulations.

Martin V.
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Who do I know the direction of the current flow, when I make nodal analysis?
 
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A priori you do not. You can assume a direction and if the current actually flows in the other direction you will get a negative result.
 
#2: The problem occurs when I user Ohm's law, it can end up in the wrong results if the direction I assumed is wrong. Any solution to that?
 
Martin V. said:
#2: The problem occurs when I user Ohm's law, it can end up in the wrong results if the direction I assumed is wrong. Any solution to that?

No, this problem cannot occur if you use Ohm's law correctly. Ohm's law does not only state the absolute potential difference between two sides of the resistor, it states which sign the potential difference has. The potential is higher on the side the current comes from.
 
Martin V. said:
#2: The problem occurs when I user Ohm's law, it can end up in the wrong results if the direction I assumed is wrong. Any solution to that?

Perhaps post an example problem with your attempt to solve it so we can see where you are going wrong.
 
Martin V. said:
The problem occurs when I user Ohm's law, it can end up in the wrong results if the direction I assumed is wrong.
If you do everything consistently then you will never get wrong results, even when you assume a wrong direction. Because of this, I always assume that every current is leaving the node. I know that at least one of those assumptions is wrong, but by being consistent I still come out with the right answer.
 
There are two ways two write KCL...

https://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws

At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node
or equivalently
The algebraic sum of currents in a network of conductors meeting at a point is zero.

Personally I find I make fewer mistakes if I use the latter of the two definitions and write an equation that sums to zero. eg..

A + B + C... = 0
rather than say
D + E = -F -G
 

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