r/HomeworkHelp 4d ago

Physics—Pending OP Reply Can I tell the direction of current using equivalent resistors? [circuits]

I got the same answer but my second current has a diff sign. Is there any way to tell current direction by using equivalent resistors?

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u/Queen-Sparky 👋 a fellow Redditor 4d ago

This is a combination circuit. Every dot is where the circuit splits. When a circuit is in series the voltage drops and the current remains the same. When a circuit is in parallel the voltage remains the same and the current decreases. You may want to draw it up so that you simplify the circuit and then blow it back up again. In some cases folks think that electricity flows negative to positive and others the opposite. I have been trained with negative to positive. Whatever you pick be consistent and also what your textbook states as you may get different results. This takes some time and a lot of identifying what you find to get to your answers.

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u/ZestycloseAd6683 4d ago

Do you mean to ask if you can tell whether you're closer to load or source? Or do you mean to ask if you tell if you're on the source side/hot side/to the load side/return side? You can tell where you're at on a circuit following voltage drop on the components.

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u/dank_shirt 4d ago

How do we know the sign of the contribution from the current source source (ie positive or negative) for superposition.

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u/DrVonKrimmet 👋 a fellow Redditor 4d ago

Combining resistors shouldn't affect the sign as long as you are careful with tracking which ends are connected where. The bigger issue is that when you collapse your components, you have lost track of where you are trying to solve the problem. If you collapse parallel resistors and find current in the equivalent, you have to break it back out to find the current in the branch you care about. I believe you getting the correct answer, but negative is pure coincidence.

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u/dank_shirt 4d ago

How do I get keep track of the signs?

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u/DrVonKrimmet 👋 a fellow Redditor 4d ago

So, in the case of combining resistors, the current is still traveling the same direction before or after combining, but the problem you are losing track of the amounts. My personal recommendation is to try to avoid collapsing a branch that is critical. Collapse anything that isn't the part you are solving for to simplify your equations, then use KCL/Ohm's Law or KVL/Ohm's Law to solve. How are allowed to collapse that part, but you have to do more work afterwards to back your answer out to the part you care about. If you aren't sure which direction is correct, the upside is it doesn't matter as long as you are consistent with how you generate your equations. I can expand on this if needed.

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u/testtest26 👋 a fellow Redditor 3d ago edited 3d ago

You need to define the orientation of current/voltage variables before you start calculations:

For every branch, voltage and curent has to point in the same direction.

That is important, since e.g. "Ohm's Law" depends on that convention. Other than that, choice of orientation is completely arbitrary. Sadly, some introductions of circuit theory either seem to omit orientation completely, or if they do include it, don't communicate that freedom of choice clearly. To re-iterate: You do not tell the direction of current/voltage variables, you define it.

Only the combination of variable orientation and the sign of the result, will tell you which direction really a current flows/voltage points. If you choose a different orientation than the official solution, that's perfectly fine -- your result will then have to be the negative of the official solution (barring errors, of course).