Every external link from this lesson, gathered in one place so nothing gets lost. Grouped by the section it came from; click whatever you missed.
Scattered through this lesson is a handful of optional challenges: actual bonus work, separate from whatever homework lands in your lap. Here they all are in one place, in the order they show up in the lesson. Do as many as you have the appetite for.
Challenge 1 (from "What this course aims"), the bookend. Dig out the definition of a resistor you scribbled on day one, before you knew any of this. Now that you've met resistance, R = ρ × L/A, Ohm's law, and the component itself, write the definition again from scratch. Compare the two. The gap between them IS this whole lesson.
Challenge 2 (from "Resistors: an interesting case"), a real bonus problem. Using R = ρ × L/A, assume the shunt wire is 10 cm long, with the cuts shaving off roughly half the cross-sectional area over a 3 mm stretch (not quite true, but go with it), and work out the resistance of the notched wire versus an un-notched one. The resistivity table is linked in that section, so you've got everything you need.
Challenge 3 (from "Kirchhoff's Current Law"), do this BEFORE the homework. Not really optional, this one. Work through at least two current-law examples (the video linked in that section walks you through the method) so KCL is properly in your hands before you touch anything graded.
Challenge 4 (from "Kirchhoff's Voltage Law", the "for extra points" box), the meatiest one. Prove the series rule (total R = R1 + R2 + ...) and the parallel rule (1 / total R = 1/R1 + 1/R2 + ...) starting from KCL and KVL, instead of just taking them as handed to you on a plate. This is the most genuinely "extra points" challenge on the page.
Challenge 5 (from "Charging, discharging, and the time constant"), exploration. Open the Desmos graph and start yanking the R, C, and Vs sliders around. Confirm with your own eyes that the charging curve crosses 63% at the first tick and is flat against the ceiling by the fifth, no matter what values you throw at it. For bonus satisfaction: pick an R and a C, predict τ in your head, then check it against the graph.