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Because of this, and despite the fact that it is common practice, it is sort of a mistake to find the resistance of a circuit element by plotting V vs I and finding the slope. Obviously, if the circuit element is non-ohmic it doesn’t make any sense to report a single R value. However, even if the circuit element can be nicely approximated by Ohm’s Law, it actually makes more sense to report the value of R as an average of all V/I ratios than it does to calculate the resistance by determining the slope of the best fit line.  This is especially the case if your best fit-line has a non-zero y-intercept.

I explained that we were building the model we’d be using to predict the behaviour of circuits for the  next two years, and that on tests, I would be evaluating whether they used their model in a well-reasoned way (“You’re going to grade us based on what we say??”  They were astounded).  I cautioned them against rejecting things too quickly, since they would need as much structure as they could get. 

After each presentation, we discuss it and voted on it.  For voting, they used the feedback flashcards I’d made in September.  Green means accept; red means reject; yellow means “I have a question or want something clarified”. 

Using a fast-pulsing laser, physicists have recorded the first moments of electrical resistance, the friction that generates heat as electricity travels through circuits.

Physicists knew electrical resistance didn’t kick in the moment a voltage was applied. Electrons experience some freedom before slowing to a crawl and scattering. What wasn’t certain was how quickly they make that transition (illustrated in the animation above, with electrons in blue, “electron holes” left by departing electrons in red, and voltage signified by the green arrow).