In this paper we study the transient behaviour of RC circuits with supercapacitors, varying R between 1 and 100 [omega]. We demonstrate that supercapacitors behave as ideal capacitors in series with an internal resistance (r [similar to] 8 [omega] for C = 0.2 F, 5.5 V). This result is important to optimize the demonstration of RC circuits using a supercapacitor in series with a light bulb, because the "r" value is comparable with the effective resistance of the bulb. This means that the bulb brightness is significantly decreased by "r". (Contains 6 figures.)