Electronic – Kirchhoff’s Laws and Circuits

circuit analysiskirchhoffs-laws

I just started circuit analysis today and learn't about Kirchhoff's Laws.

Its pretty straightforward, but in all the examples my book gives they already specify the direction of the current and the end of the branch with the higher voltage.

For example,

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there arrows everywhere indicating the direction of the current at each node and +/- signs indicating the end of the branch with a higher voltage.

Obviously, when actually designing stuff the direction of the current and the side of the branch with the higher voltage wouldn't be indicated.

So what I'm asking is:

1) Can Kirchhoff's Law be used to figure out the direction of the current and side with higher voltage?

2) If not, do techniques exist to find the directions?

Please note that I'm not talking about convention. (i.e. currents flowing into the node are positive and currents flowing out are negative etc.) You can't just have a convention because you can't differentiate which direction the currents or voltages are going.

Best Answer

When analyzing a circuit, you can put the arrows in either direction according to whim, a flipped coin, or Tarot cards.

After applying Kirchoff's laws to compute all the voltages and currents, you'll find some variables have negative values. Those correspond to arrows you drew backwards. Fix those, and then you know the directions of currents in all branches of the circuit.

It is perfectly normal for an experienced engineer to get a few initially backwards, when multiple different voltage sources are pushing in opposite directions. You can only guess, and let algebra tell you the net result.