Bob S. said, "[Serway] is correct if i is defined as positive when it is
CW (in your previously drawn circuit), and q is the charge on the lower
plate. Note this also defines the relation i = dq/dt."
I agree we can say we are choosing I as positive when positive charges
are moving clockwise. But that is not the figure in the the Serway
text. Serway has the capacitor on the left with positive plate down.
The resistor is on the right with current arrow pointing up (i.e.
current is assumed, by Serway, as CCW). Serway says -q/C - IR = 0.
That means he applied the loop theorem traversing the capacitor from
bottom to top (CW loop) and he traversed the resistor from bottom to top
(CCW loop). He switched his loop direction when he applied the loop
theorem.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu