What is the Difference between DC and AC?

DC stands for Direct Current. A circuit powered by a battery is a DC circuit. Most electronic devices run on DC. Continuing with the analogy of water in a hose, a DC circuit has all the water flow in one direction. The reason why all electrical power isn’t DC, though, is because it can’t be easily transmitted long distances or changed into other voltages. So the early days of electrical power used DC, but required large diameter wiring (expensive) and local power generators (impractical).

Therefore, a more efficient type of electrical power developed…Alternating Current. When thinking about AC, the analogy of the water hose no longer works well. In AC, the current flow reverses direction in a circuit, flowing first in one direction, then in the other. This reversal of flow takes place 60 times in one second for typical electrical AC power in America. Thus the AC power is called 60 cycle (or 60 Hertz). The normal AC power in much of the rest of the world is 50 cycle. The number of cycles is chosen as a mostly arbitrary standard. The map shown in this Wikipedia link http://en.wikipedia.org/wiki/Utility_frequency illustrates the standard voltages and frequencies most countries in the world chose.

As an aside, lights and motors tend to be designed to work at either 50 cycles or 60 cycles. The wrong frequency in lights causes a flicker and in motors more serious problems can occur. Understand that electrical appliances typically are designed for 60 cycle or 50 cycle power and will have problems with efficiency, or even safety, if the correct frequency isn’t used.

AC power became the standard throughout the world, mostly because transformers allow AC power to change voltages. So Utility Companies can produce electricity and send it over high voltage lines (say 11,000 volts), then simply transform the power to 120 volts for normal use. This ability to send high voltage power over transmission lines allows more power to be sent over smaller diameter cable, and with less transmission loss, than DC power would allow.