E-9

Graph
RC = (t₁ - t₂)/ln(V₁/V₂)
At x-axis(horizontal), plot V in logarithmic scale, and at y-axis(vertical), plot t in linear scale. Then find the slope of the graph. The slope is RC.

Answers to the questions

1. The Fluke meter's internal resistance R_f = R_d = 10MW. Since the same current flows through both R_f and R_d, V_f = V_d = 1/2V_supply = 1/2(20V) = 10V.
2. When we plot the graph, we only use ln(V₁/V₂), V_C1/V_C2 = [V_f1(R_d + R_f)/R_f]/[V_f2(R_d + R_f)/R_f] = V_f1/V_f2. Therefore, we use only the ratio of the voltages and need not use the full value across the capacitor.