Electricity flows in two ways, either in alternating current (AC) or in direct current (DC). The word electricity comes from the fact that current is nothing more than electrons moving along a conductor, like a wire, that have been harnessed for energy. The difference between AC and DC has to do with the direction in which the electrons flow.

In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forwards" and then going "backwards." The power that comes from wall outlets is AC.

Electricity is a very different energy source than heat or light. In nature, electricity only rarely occurs, in some animals or with lightning. In the search to create electrical energy, scientists discovered that electrical and magnetic fields are related. A magnetic field near a wire causes electrons to flow in a single direction along the wire because they are repelled and attracted by the north or south poles. Thus, DC power from a battery was born, primarily attributed to Thomas Edison's work and promotion.

Another scientist, Nikola Tesla, preferred AC because it travels farther without losing energy and could transfer different amounts of power. Instead of applying the magnetism along the wire steadily, he used a magnet that was rotating. When the magnet was oriented in one direction, the electrons flowed towards the positive, but when the magnet's orientation was flipped, the electrons turned as well. AC generators gradually replaced Edison's DC battery system because AC is safer to transfer over the longer distances and can provide more power.

Another difference between AC and DC involves the amount of energy it can carry. Each battery is designed to produce only one voltage, and that voltage of DC cannot travel very far until it begins to lose energy. AC voltage from a generator in a power plant can be bumped up or down in strength by another mechanism, called a transformer.

Transformers are located on the electrical pole on the street, not at the power plant. They change very high voltage into a lower voltage appropriate for home appliances, like lamps and refrigerators. AC can even be changed to DC by an adapter, such as that used to power the battery on a laptop.