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Physics/Kirchoff's voltage law

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QUESTION: In solving problems related to Kirchoff's voltage law, that is  to say certain complicated circuits, I'm not being able to understand a small point. How do we find the sign conventions during algebraic sums of products of currents and resistances? To be precise I don't get the ideas related to clockwise and counterclockwise directions of current. How do we know if the current across a resistance is in clockwise or counterclockwise???

AB.

ANSWER: Clockwise and couterclockwise just determines the direction of a current in a loop.  It's a voltage drop if it goes with the current across a resistor, a voltage gain (positive) if it goes against a current across a resistor.  It doesn't matter which you pick to do for your loop, as long as it is consistent.  That is why it helps tremendously to draw an accurate picture.

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QUESTION: Does this mean that I can have my own choice about clockwise or counterclockwise directions and then decide whether the current across the resistor is in my direction or opposing the same?? Please let me know how to decide if the current is flowing in clockwise or counterclockwise direction? If we have all straight line direction of current across resistors in a closed circuit how can I say that the direction of current is clockwise or the way opposite??

AB....

Answer
Yes, your choice is arbitrary.  The circuit has to behave the same way if you flip it over.  Just be consistent with the arrows you draw on your loops in the diagram. Some will go with the loop direction (if you choose clockwise to be positive, those currents are positive), and some will go against it in some cases (changing their sign).  I add little + and - signs to the ends of my resistors in diagrams for thoroughness so that I can check each term of each loop rule equation.

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Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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