AC Current Flow

• A well-known fact is that electricity flows between oppositely-charged terminals, and by convention, the direction of flow is from the positive to the negative terminals. In a DC circuit, you can visualize the electron flow as a steady stream always moving in the same direction. In an AC circuit, however, the direction of flow alternates, so speaking of positive and negative terminals doesn't make as much sense; instead, say that one terminal is charged, or hot, by virtue of being in contact with the power source while the other is neutral and doesn't become charged until connected to the hot terminal.
  
  

Hot, Neutral and Ground Wires

• The hot terminals in your house that connect to the power company are located in your electrical panel. The colors of the wires leading from these terminals are black and red, although for most circuits, only black ones are needed. When you energize a device by hooking it up with a black wire, it won't operate until you give the electricity a pathway back to the panel. The wire used for this purpose is always white and is called the neutral or return wire. A third wire in household circuits provides a safe path to ground in the event of a wiring problem.
  
  

A Light Circuit

• Besides the bare or green ground wire in a light fixture, there are always two others. Although the colors can vary, the hot wire is black or red and the neutral one white. When you hook up the fixture by connecting the wires to a household circuit, electricity cycles through a filament or energizes a gas enclosed in a bulb to make light. If you want to control the lights so that they'll come on only when you need them, which is the case with most light fixtures, you have to install a switch.
  
  

The Wires in the Switch Box

• A switch works by interrupting the current in the hot wire, and you install one by connecting the ends of the hot wires to the switch terminals. The white wire bypasses the switch altogether. If you were to install a switch in an uninterrupted length of electrical cable, you would only have to cut the hot wire. In practice, though, there are usually two separate cables in a switch box that you have to join together. Electricians join the white wires by twisting them together and screwing on a wire cap to hold them.