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Physics/Lopp rule


oscilloscope diagram
oscilloscope diagram  
Hi, I was just wondering if I could get some help with the number 1? How do you explain that the circuit from the diagram follows the loop rule?
Is it because the red and blue add up to 0?

Hello Bob,

I have to guess what the circuit looked like. Resistor in series with a resistor connected across a voltage source is my guess. The text refers to a generator and then says "battery connected" and later says "no battery". I'm going to guess that the "generator" is a signal generator with output of 0V,6V,0V,6V, .... And the timing must be controlled so the duty cycle is 50/50. I assume the red trace was voltage across the cap and the blue trace was the voltage across the resistor.

My answer will be no better than the accuracy of my guess at what your setup was. At any time, the voltages indicated by the red and blue traces should add up to whatever the output of the generator is. So it should add up to 6V half of the time and should add up to 0V half of the time. That's not what I see on your plot. If the generator is putting out 6V, the sum of the voltage drops across the 2 components should be 6V -- any time that 6V is the input to the circuit. That would be according to Kirchhoff.

It looks to me like you did not have your scope synching correctly (trigger off the generator signal). The relative timing of the 2 traces seem to be skewed from what I would expect. The voltage across the resistor would be proportional to the current through it. Therefore it should be maximum, 6V, when the voltage across the cap is just starting to climb from 0V. Once the cap is fully charged to 6V, the current through the resistor would be zero so the voltage across it should be zero.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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