Physics

Alternating Current and Direct Current Electricity



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Direct current and Alternating current are common terms used to describe the two forms of electricity we use every day in countless different situations. These terms are applied to many different types of devices we have come to take for granted in our age of technology. DC devices are sometimes battery operated, although it has become quite common to convert AC to DC by a converter. AC devices, such as household appliances and power tools, use considerably more electricity and are plugged into the familiar AC outlet.

You probably buy many different types of batteries, which supply DC to operate low-voltage devices such as your calculator, cell phone, TV remote control and even to start your car. DC batteries can be quite small and made and named by the different chemicals that make them work. For instance, a 12-volt car battery is a "Lead-Acid" battery, and common dry cell batteries come in sizes ranging from AAA to D. Many low voltage devices use batteries made from Nickel-Cadmium (NiCad) or may be Lithium-Ion batteries. Each type of battery supplies a direct current at a specific voltage and may be rechargeable, recyclable, or disposable.

Alternating Current devices require more electrical power to operate than DC devices. Some examples of AC devices include microwaves, toasters, washers and dryers, power tools and your computer. Electronic devices, such as your television, entertainment system, and computer have internal power supply circuits that convert and regulate household 120V AC to different amounts of DC as required. Other devices, such as your garage door opener, air conditioner and refrigerator use AC to run an AC motor, which drives some mechanical device such as a compressor, or simply perform work for us.

In a conventional explanation of direct current, direct current can be described as the uninterrupted and constant flow of electricity from a point of negative electromotive force to a point of positive electromotive force. There are many different types of batteries used to store electricity and serve as the source of electromotive force. For example, a 1.5 Volt AA battery will supply an electromotive force, or difference of potential, of 1.5 volts when measured across its positive (+) and negative (-) terminals.

When the terminals are connected in a closed loop with an electrical conductor, such as a copper wire, an electrical current will flow between the terminals as long as the difference of potential remains. The battery and the wire, now combined, form a simple electrical circuit. Once the battery exhausts its chemicals, the current flow ceases, we now have a "dead" battery. Useful work is performed by inserting a device, such as a light bulb or any electronic device into the circuit. Electronic devices are designed and built around the size, chemical properties, and intended use of the battery.

Alternating Current is the more efficient child of Direct Current. Early research and development into the properties and practical application of electricity by Thomas Edison (1847-1931) centered on DC. However, DC soon revealed that it has several shortfalls when it comes to maintaining a constant level of current over long distances. After some research, experimentation and invention by George Westinghouse (1846-1914), AC generating devices and voltage controlling components called transformers, made DC impractical as a means to transmit electricity. DC motors and generators do have practical application in a few special situations, but not many when compared to the countless applications of AC alternators, generators, motors and electronic devices.

Alternating current is created when the voltage output of a device rises and falls over a specific period of time, called a cycle. For example, the term "120 volt AC" means that the electrical voltage rises and falls continuously from zero to 120 Volts (positive) and then falls to 120 volts (negative) at a rate of 60 cycles per second, now called "Hertz" after a scientist that performed pioneering work in this area.

If you examine the rise and fall of electricity at 60 Hertz, you would see the characteristic sine-wave pattern created when you slice a 360-degree circle, representing one complete revolution, in half and open the two parts up horizontally. AC devices are not sensitive to the changing voltage levels since they happen at a relatively high rate of speed. Instead, they operate at an "effective" voltage, around 70.7% of its peak of 120V.

AC is generated by spinning tightly wound coils of electrical conducting wire, commonly copper, in the presence of a magnetic field. As electrical current flows through a conductor, it creates a magnetic field, the intensity of which is proportional to the strength of the current flow. The magnetic field surrounding a copper wire will induce a corresponding current flow and magnetic field in another copper wire placed adjacent to it.

If the two wires are coiled tightly around each other, one will form a "primary" and the other a "secondary" in a device called a transformer. When electrical current flows through the primary, it will produce a magnetic field that induces an electrical current into the secondary. The strength of the induced or secondary current depends upon the number of windings and other properties of both. In effect, electricity is stored in the primary winding and controls the electricity induced in the secondary winding of the transformer.

Transformers are at the heart of electrical devices, as the number of windings of the primary and secondary coils result in a way to store and control electricity through their magnetic fields. AC generators and alternators take advantage of this effect by using rotating coils in a magnetic field, to produce an AC output. The primary's magnetic field is the result of a permanent magnet, a coil (primary), or a combination of the two.

For household purposes, a much higher voltage AC is delivered to a nearby transformer, which then "steps down" the electricity to the familiar 120 V and 240V levels. When you plug your radio, lamp, or toaster into the wall outlet, you are consuming electricity generated at a generating station far away.

The purpose of this article was to provide a simple explanation of Alternating Current (AC) and Direct Current (DC). Direct current is a description of a steady state current usually generated by a storage battery of some kind, or the resulting output of an AC DC transformer. AC is produced when both voltage and current rise and fall at a consistent rate, called Hertz. I hope you have found this article useful and informative.

Resources:
deLucenay Leon, George. The Electricity Story: 2500 Years of Experiments and Discoveries. New York: Arco Publishing, Inc. 1983.

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