A Simple Explanation of the Differences between Alternating Current AC and Direct Current DC

My wife, the retired schoolteacher, would warn you that I’m not capable of giving a simple explanation no matter what the subject. I’ve taught adult continuing education classes in residential electrical repair and I’ve tried to practice the K.I.S.S. (Keep It Simple Stupid) principle in my teaching but some subjects really don’t lend themselves to simplicity, and electricity is one of them. Never the less I will endeavor to explain the differences between alternating current and direct current in as simple a manner as I can.

The first difference that we need to look at is how they flow in a conductor. You could infer from its name that alternating current (AC) alternates in some way and you would have made a correct inference. Alternating current changes the direction it flows in cyclically. The 60-cycle per second AC current flowing in your home’s wiring changes direction 120 times every second. If you were to look at a Sine wave on an oscilloscope you would see that each cycle consists of one positive and one negative alternation; therefore there’re 120 alternations or changes of direction per second. Direct Current (DC), on the other hand, flows continuously in one direction.

The second difference is in the intensity of the current flow. The intensity of an AC current is constantly changing. Looking at the sine wave displayed on the o-scope, you see that the current builds from zero to its maximum positive value during the first 90 degrees of the cycle and then drops back to zero during the 90 to 180 degree portion of the cycle. During the next quarter cycle, 180 to 270 degrees, it builds to its maximum value again but in the opposite polarity. Then from 270 to 360 degrees it decreases to zero again. With AC, the current that an Ammeter indicates is the Root Mean Square (RMS) value, which is actually an average of all its instantaneous values. With DC the current quickly rises to its peak value and remains there for as long as the circuit is complete.

During the early days of electrification, beginning with Thomas Edison’s power plant on Pearl Street in New York City, homes and businesses were wired for DC, and then Nikola Tesla introduced AC and the power wars started. In the end, AC won out because it was safer and easier to transmit and to use. Today the use of DC is pretty much limited to electronic devices, cars, trucks, buses, airplanes, boats and small engine powered equipment.

The third way they differ is in the way they are generated or produced. Rotating machines called dynamos or alternators produce AC; whereas, DC generators, solar panels, photocells, or batteries produce both direct current and alternating current. The alternator on your car, truck, boat, etc actually does produce AC but the AC is converted or rectified to DC by internal power diodes so the DC loads can use it.

The important thing to remember here is that they aren’t interchangeable as power sources with a few exceptions. Some motors will run on either AC or DC and are known as universal motors. Incandescent lights will work on either AC or DC but most loads won’t. Clothes washers, clothes dryers, refrigerators, deep freezers, televisions, computers, etc won’t.

There’s a great deal more to be said about DC and AC but I was supposed to keep this simple, right?