10. AC-DC Difference

A light bulb with a large curly filament is connected alternately to 110 V AC and DC sources. A magnet is brought near the bulb. The filament under goes a steady deflection in the case of DC, but vibrates impressively in the case of AC.

The difference can be further illustrated by hooking a large inductance or capacitance in series with the bulb. The inductance "passes" DC but "blocks" AC, whereas the capacitance "blocks" DC but "passes" AC. See Capacitors and Inductors for details.

Here is another demonstration of AC: A bicolor LED is connected directly to the 110V AC line. When the instructor swings the LED around her/his head, the light flashes green then red, showing that the LED is lit for only one half of the AC cycle. The same LED can be connected to a DC source, then the LED has one polarity, either red or green.