Your cell phone charger plugs into an AC electric outlet. A typical charger has a two-prong plug. Either prong can go in the hot side of the outlet; that is, you can reverse the plug and the charger still works. Your cell phone runs on DC current from a battery. If you reverse the battery, the phone won't operate. Explain why you can reverse the polarity of an AC source like an electric outlet, but not a DC source like a battery.