I also agree with all four of Bob's diagrams. However, please note that
the thing that got me off on this was my son asking about Serway/Jewitt.
The Serway/Jewitt diagram is *exactly* Bob's third diagram. (Same
capacitor placement, same resistor placement, same +/- signs on
capacitor, same current arrow.) But the Serway analysis is -q/C - IR =
0 and I = +dq/dt.
If we agree that Bob's analysis of his third circuit is correct, then
the Serway/Jewitt analysis would certainly seem incorrect. Can anyone
look at Bob's third diagram and legitimize the Serway/Jewitt analysis?
I also think Tim Folkert's comment about the q in dq/dt is helpful.
Students are clearly thinking dq/dt means charge moving past a point in
a wire. They are not thinking of dq/dt as the rate of change of charge
on the capacitor. Making this clear should help them.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu