A classroom demonstration is presented showing that the voltage-intensity curve of the filament of an incandescent light bulb is non-linear. For a tungsten filament, it is observed that the electrical resistance increases with the temperature of the filament, which increases with the electrical power consumed, since this is dissipated by radiation according to Stefan's law. For a carbon filament, it is observed that the electrical resistance decreases with temperature, thus showing that it is not a metal (but a semi-metal).... [Leer más...]