To understand AC or DC power, we need to understand the nature of electricity itself. Electricity is the movement of free electrons between areas of different potential through a conductive medium. Electrons have a negative charge. Free electrons are those that can be forced to move from one atom to another. The more free electrons a material has the better it conducts electricity. Gold, copper and aluminum are the most common materials used to create electrical conductors because they have the highest number of free electrons.
Nature prefers an electrically neutral state. When an excess of free electrons build up in a conductive medium they will try to neutralize the charge by moving to an area of different potential. The most natural form of electricity is lightning. When cloud activity builds up a sufficient static charge, this difference of potential will attempt to neutralize itself by discharging to earth or to another cloud with a significantly different potential. This creates an arc or bolt of lightning. This is nature’s form of DC or direct current.
Direct current is the flow of electrons in one direction between areas of different potential through a conductive medium or electric circuit. Batteries and dry cells are good examples of DC power. Batteries have two poles. When a battery is charged, the free electrons are forced to one pole. This pole is called negative. The pole that gave up the free electrons is called positive. When the battery is connected to a circuit the excess electrons built up on the negative side will travel along the path of least resistance back to the positive side of the battery. The difference in electrical potential is called voltage. Voltage is measured in volts. The volume of electricity flowing in a circuit is called current. Electrical current is measured in amperes. Electrical power is called wattage. Wattage is the product of voltage times amperage. Wattage is also known as VA or volt-amps when applied to the apparent power values in AC electricity.
AC or alternating current is a flow of electrons that is constantly changing direction. This change of direction means a reversal of polarity. This change in polarity is spread out over time. If the voltage and time are plotted on a graph, the result is a waveform pattern. Starting at zero, a rising waveform will rise to its peak positive voltage then begin falling in voltage. The voltage will continue to fall until it passes zero and becomes negative voltage. When the voltage reaches its maximum negative value it will start to rise again. When the voltage reaches the zero point the second time it has completed one cycle. The rate at which the waveform repeats itself is known as frequency. Frequency used to be measured in cycles per second. The unit of frequency measurement has been changed from cycles per second to Hertz.
Alternating current is the form of electricity most commonly used for commercial power distribution around the world. The simplest explanation for why AC is used for power distribution instead of DC is based on their different characteristics. DC voltage and current travel in a single direction from source through the load and back to the source. That means that our poor little electrons have to make the journey from one end of the circuit to the other. All conductors have resistance to current flow because the free electrons in the conductor need a bit of coaxing to move from atom to atom. The longer the conductor is, the higher its resistance. This is called voltage drop. If DC current were used to transmit commercial power customers at the end of the line would receive less voltage than the ones nearer the source. The fact that AC current is constantly changing polarity makes it more suitable for commercial power distribution. The AC voltage is boosted to thousands of volts through the use of step-up transformers. This high voltage is distributed along the power lines. This helps reduce transmission line losses and makes adjusting the user’s voltage possible.
Along the transmission path adjustable step-down transformers convert the voltage to the value required by the customers. Many different combinations of voltage and frequency power are in use around the world. In the U.S., 120 volts at 60 hertz is the standard household current. Most European countries use 230 volts at 50 hertz.
More detailed information regarding the transmission of electrical power can be found in a document entitled primer.pdf at energy.gov.
Many homes and businesses in American are supplied with poly-phase power. Poly-phase power can consist of two or three supply lines and one return line. The voltage is the same on each supply line, but the cycles, or phases, are offset by 120 degrees. Various combinations of phases can be used to provide power for different needs. Here are a few of the most common arrangements for domestic use.
120 volt Single-phase: Used for standard household power. This is obtained across one phase and the neutral return.
208 volt Single-phase: Used for some electric clothes driers and water heaters. 208 volts is obtained by connecting across two phases. Because the two lines of 120 volts are out of phase with each other, the total effective voltage is only 208.
Commercial applications often require higher voltages per phase. Phase voltages supplied can be 240 volts or higher.
This first commercial power was direct current, pioneered by Thomas Edison. He faced stiff competition from George Westinghouse and Nikola Tesla, both of whom advocated alternating current for commercial distribution. The battle between the advocates of AC and DC has been called the War of the Currents. PBS.org has a fascinating article about the War of the Currents. Information concerning the development of AC power can be found at the Edison Tech Center.
In the final analysis, we have and need both AC and DC power to make our world function.
AC electricity is used to distribute power to our home and businesses. It is used in radio and television to transmit electromagnetic waves that carry information through the air and through space. In a world where digital electronics is taking over, we still need AC to carry all manner of information and accomplish other tasks that DC is unsuited for. Dc electricity is used to power digital electronics, which includes our cell phones and computers. Even your desktop computer uses a power supply to convert the AC household current into DC to power the electronics. DC can be stored for later use in batteries and power cells. AC power cannot be stored. The power collected by solar panels is stored in batteries and converted to AC power by an inverter. Computer power back-up units keep batteries charged while the AC power is available. When the power goes out, the units convert the stored energy to AC to give us time to shut them down safely. An interactive explanation of the difference between AC and DC current can be found at pbs.org.
